Long Fasts: Worth the Risk?

Inline_Long-Term_FastingIntermittent fasting, schmittermittent schmasting. The hot new trend is the extended fast—eating nothing and drinking only non-caloric beverages for no less than three days and often as many as 30-40 days. A mere compressed eating window this isn’t.

If fasting for more than three days sounds riskier than just skipping breakfast, you’re right. Long fasts can get you into trouble. They’re a big commitment. You shouldn’t just stumble into one because it sounds interesting or some guy on your Twitter feed wrote about it.

Skipping a meal or even an entire day of food makes evolutionary sense. We weren’t always successful on the hunt or with foraging. We couldn’t head down to the Trader Joe’s for shrink-wrapped steak, sacks of apples, and jars of honey. Reaching the fed state wasn’t a sure thing. Intermittent fasting—going out of your way to not eat, even though food is available—is a modern contrivance meant to replicate the ancestral metabolic environment.

But long fasts seem more evolutionarily aberrant. The evidence from extant hunter-gatherers, many of whom live on land far more impoverished and limited than our hunter-gatherer ancestors, indicates that outright famine is rare. The Hadza may not eat honey and wildebeest every day, but there’s usually plenty of something to eat.

Are there benefits to the longer fast, though? What’s the purported reasoning behind not eating for days on end?

The Logic of Long-Term Fasting

Weight loss

Back in the 1960s, obesity researchers were quite open to the notion that not eating for long periods of time could combat the results of overeating for long periods of time. The most famous case was of the Scotsman, an obese 27-year-old man clocking in at 456 pounds who, upon asking his doctor for help losing weight, was told to stop eating for a few days. He did it for a week, lost five pounds, and decided to continue the experiment for a total of 382 days. He didn’t do this willy-nilly. He took potassium, sodium, and various vitamins each day. He was under medical supervision for the duration, getting checkups each week.

It worked. After 382 days, he was 180 pounds, having lost 276 pounds. At the five-year checkup, he’d only regained 16. He might have been a bit stocky for the times, but by all accounts this long fast was a huge success. Most dieters nowadays eventually regain most or all of their lost weight.

In another study from the 1960s, 46 obese adults fasted for two weeks. No food, just water and vitamins.

On the good side, they all lost weight—an average of 17.2 pounds (from 7.7 to 31.9 pounds). At the two-year followup, half of them had either kept it all off or regained some of the weight they’d lost. The patients with diabetes enjoyed normal glucose levels throughout the fast and continued to have better glucose control after it had ended.

The bad news is that the other half regained every pound they’d lost or were so embarrassed at their progress that they failed to respond to the followup calls at the two-year mark.


Cancer patients typically lose their appetite, and oncologists often prescribe anti-nausea meds to restore it. What if low appetite is adaptive?According to Valter Longo, a cancer researcher from USC, “normal cells” go into survival mode during starvation and display “extreme resistance to stresses” like chemotherapy. If this is the case, extended fasting could improve normal cells’ resistance to harsh cancer treatments. 

In one of Longo’s more recent studies, fasting for 3 days improved cancer patients’ resistance to chemotherapy. Leukocytes in those fasting for 48 hours before chemotherapy followed by another 24 after had less DNA damage than those who fasted for just 24 hours. saw less evidence of breaks in their leukocyte DNA. The result is preliminary but promising.

In an older case study (also authored by Longo), a woman with breast cancer underwent four rounds of chemo. The first round came during a six day fast. Other than dry mouth, fatigue, and hiccups, she felt well enough to continue working. For the second and third rounds of chemo, she didn’t fast. She felt awful the entire time, couldn’t work, and complained of severe nausea, fatigue, and pain. She decided to fast for the fourth and final round, which went as well as the first round. Fasting also improved her biomarkers, including white blood cell, platelet, and neutrophil counts.

There’s even a recent case study suggesting that fasting itself might combat cancer directly. In a woman with stage IIIa low-grade follicular lymphoma (non-Hodgkin’s), a 21-day water fast greatly reduced lymph size. She followed a plant-based whole foods diet immediately after, and by month 9 her lymph nodes were still of normal size.

If you’ve got cancer and are interested in long fasts, clear everything with your doctor first.


In 2014, researchers used a 3-day fast to protect against damage to the immune system and induce total systemic regeneration of the immune system in mice. Fasting actually triggered the mice’s stem cells to begin production of new blood and immune cells.

A recent report on 6 autoimmune case studies sounds quite promising.

Case 1, rheumatoid arthritis: Symptoms included constant pain in all extremities, extreme fatigue, headaches, and occasional autoimmune conjunctivitis (pink eye). A month after stopping RA meds, the patient fasted for 17 days. Two days in, joint pain had subsided. A week in, all pain was gone and mobility was restored. Electrolytes were stable, and he maintained his progress at follow-up visits.

Case 2, mixed connective tissue disease: Symptoms included severe joint pain, chills, facial edema, weakness, fatigue, myalgia, photosensitivity, and tachycardia. She weaned herself off meds before fasting for 21 days. The first week was rough, but by day 10 she felt better. By 21 days, she had no complaints and remained off her meds. Electrolytes remained stable.

Case 3, fibromyalgia: Symptoms were pain, poor sleep, inability to sustain activity for more than an hour. A 24-day fast cleared them up. Electrolytes were stable.

Case 4, systemic lupus erythematosis: Symptoms were joint pain and skin rashes. Two weeks before the fast, she had weaned completely off her meds. On day 3, she was sleeping poorly and feeling nauseated, but on day 4 she began improving. Joint pain was gone. She cut the fast short after 7 days due to weakness and mild tachycardia, but that was enough—she remained symptom free at one year post-fast.

Cases 5 and 6, rheumatoid arthritis: 12- and 24-day respective fasts fixed symptoms for two patients with RA.


A 2001 study involving 174 patients with hypertension found that a 10-11 day water-only fast led to an average blood pressure reduction of 37 mm HG systolic and 13 mm HG diastolic. Those with severe hypertension (180+ mm HG/110+ mm HG) saw even bigger improvements—a 60/17 mm HG reduction on average.

You might have noticed that many of the cited studies were case studies of single individuals. While it’d be great to have RCTs with placebos and control groups and double-blinding, it’s hard and expensive to get a huge group of people together to fast for 21 days, monitor their vital signs, keep them honest, and ensure their safety. You couldn’t conduct a free-living long fasting study because you’d lose too many to McDonald’s-based attrition. You have to keep people in the facility. That takes a lot of money and manpower.

What Are the Risks?

Loss of lean mass

Any weight loss diet will lead to the loss of lean mass in addition to fat mass. The goal is to minimize the former and maximize the latter. Remember: when most people talk about weight loss, they really mean “fat loss.”

When a slightly overweight, otherwise healthy man drank only water for 44 days, he lost 25.5% of his body mass. A quarter to a third of the loss was body fat, the rest lean mass—mostly muscle.

Nutrient deficiencies

 Nothing’s coming in. You’re going to run out of stuff.

  • In the 44-day fasting study, the subject also developed deficiencies in thiamine, riboflavin, and vitamin K.
  • A man with non-Hodgkin’s lymphoma fasted for 53 days (he intended 60 days, but couldn’t make it) and ended up with a dangerous neurological condition called Wernicke encephalopathy caused by severe thiamine deficiency. 
  • A woman admitted herself to the hospital after a 40-day water-only fast. She had severe sodium deficiency upon admittance and developed severe deficiencies in magnesium, phosphorus, and potassium upon eating.

Refeeding syndrome

Nutrient requirements drop during a fast. Your body isn’t doing nearly as much as it does when you’re fed, so you can get away with less. Serum levels of basic minerals like magnesium, potassium, and phosphorus remain normal, while intracellular levels drop. But upon refeeding, the situation reverses. Your insulin spikes in response to incoming food. You’re suddenly having to store fat, make and store glycogen, and conduct various other metabolic processes that increase intracellular nutrient requirements. To meet the need, electrolytes move from serum to cells, creating a deficiency on the serum level that can be quite dangerous.

Increased susceptibility to infections

A study in famine victims found that starvation increased susceptibility to infections, particularly malaria. Sometimes the infections were suppressed during the fast and only manifested upon refeeding. Fasting isn’t famine, but it’s similar enough that we should heed the story.                 

Tips for doing it safely…

Drink green tea during the fast. Purists will scoff at you for ingesting anything but water. Forget them. A 2003 rat study found that green tea protects against the fasting-induced damage to the intestinal lining during a 3-day fast. Remember: these were rat days. In human days, those 3 days are more like 90.

Take MCTs. Tim Ferriss recommends taking medium chain triglyceride oil in the first couple days as a tool to ease your way into a long fast. If you’re already on a ketogenic diet or count yourself as a fat-burning beast with robust fat-burning mitochondria, you can probably skip this.

Take magnesium, calcium, potassium, and sodium. Long fasts seriously perturb electrolyte homeostasis. I vastly prefer getting my electrolytes through a tall glass of Gerolsteiner mineral water (magnesium and calcium) spiked with sea salt (sodium) and lime juice (potassium).

Take thiamine/B-complex. Many studies indicate that fasting depletes thiamine and other vitamins, so stay on top of that. The Scotsman I discussed earlier took a nutritional yeast tab each day, probably for the B vitamin content.

Take vitamin K. A week of fasting depletes vitamin K, which is incredibly problematic if you’re fasting before major surgery. It’s also not great for general health.

Make mineral bone broth. With all the fat strained out, bone broth is quite low in calories and will have a negligible impact on your fast. Throw in a big handful of leafy greens with the stems. I like collards and beet greens, personally. You can either remove the veggies (all the minerals will have gone into the broth) or eat them.

Refeed with a light, low-carb meal. Don’t come off a week-long fast and immediately tuck into a platter of ribs. Don’t refeed with high-carbs. Large meals are difficult to handle after a long fast, and high-carb meals may lead to dangerous levels of fluid retention.

Take two or three days to ease yourself back into your normal routine. Eat smaller, lighter meals. Don’t train too hard. Refeeding syndrome is a real threat.

Have a good reason for doing it. Long fasts are serious, and you should have a serious reason for embarking on one.

Obese? Sure, a long fast with medical supervision and electrolyte and vitamin supplements can work.

Got non-Hodgkin’s lymphoma? If your doctor is on board, a 10-15 day fast might really improve survivability.

Got rheumatoid arthritis or some other autoimmune disease that just doesn’t respond to anything you’ve tried? Maybe a week-long fast will help.

Got invited to a silent meditation/fasting retreat? Go for it.

Does your spiritual practice or religion call for a four day fast? If your faith is important to you and completing this fast is integral to it, you should fast.


If you’re trying to reveal the bottom half of the six-pack you just know is lurking beneath your gut, long fasting is not the answer.

If you’re hoping to shed the last of your baby weight, don’t try a long fast.

If you’re sleeping five hours a night, working twelve hour days, and walk around a frazzled ball of stress, don’t not eat for a week straight.

If you’re fairly healthy and happy and everything’s going well, I’m not sure. Don’t make it a habit. Treat it like a marathon, maybe. A once in a blue moon event you pursue purely for the novelty.

By now, you should have a better grasp of the potential benefits and drawbacks of long fasts. They’re not for everyone or every situation—and I think shorter fasts or compressed eating windows make more sense for most people—but the long fast is an intriguing option that can be safely done if you take the right precautions.

Do you think you’ll try one? Have you tried one? How’d it go?

Let’s hear from you down below. Thanks for reading, everyone.

Primal Kitchen Dijon Mustard

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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173 thoughts on “Long Fasts: Worth the Risk?”

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  1. This is a great post and something I’ve been interested in for a long time. A few years back I attended a lecture on avoiding cancer, and the doctor discussed something like the research you mention above. He said it something like this:

    Imagine a neighborhood filled with normals and one party-er. The normals go about doing their business, doing their job. The party-er, on the other hand, just throws nons-top parties; every time you look the limo is pulling up…

    Then imagine a huge recession hits the area. If you look at the neighborhood a year later, things have changed for the normals–they downsized their house, they traded their Lexus SUV for a Honda Civic, they’ve decided not to have that third baby. But the party-er–wow, he’s not changed at all, he’s still throwing parties! At first blush, it looks like the recession only negatively affected the normals.

    But if you come back three years later, that’s when you really see something interesting. The normals are still hunkered down, but doing their thing,living their lives, just in a smaller way. And the party-er? He’s gone. He burned through all his savings, and because of the recession he had no income…finally when there was nothing left for him to party on, he left town. The problem with the party-er is he cannot save, he cannot hunker down, he just kept party-ing….so the hard times killed him off.

    That normal is the normal cells, the party-er is the cancer cell, and the years are days of a chemo patient on a fast.

    1. If that has more than a shred of true parallel with fasting vs. cancer, it would be a wonderful thing. Since going Paleo, I have been aware that fasting is a powerful tool to employ vs. cancer and general disease and in theory, it seems like it would be.

  2. What are thoughts on taking BCAA while under long term fasting to help maintain lean muscle mass?

    1. yes i second this question ! As a late 40’s male, i’m always concerned with maintaining my muscle mass, and i’m concerned that doing even a 2-3 day fast will cause a measurable loss of some muscle mass, and i’m not sure how re-attainable that muscle mass is at my age….

      1. As I just said in a comment, every time I fasted so far – no matter what kind of fast it was – I gained muscle mass. I’m 49. I do not lift weights or do any serious exercises.

        According to Dr. Fung some, or most, of what people think of as loss of muscle mass is loss of fat in the organs, instead of loss of fat in the more measurable places. He says that how fat your organs are does not register when someone is testing your BMI or your lean muscle mass.

        I go to GNC and weigh and it will tell me that my lean body mass has gotten 5 lbs less and my fat mass has gotten about 1 lb less, but, I can walk better, carry more things longer distances, and, am visibly more muscular than I was before. Therefore, it is not muscle loss.

        However, there are times it tells me I lost 7 or 8 lbs, and, only 1-3 lbs of it is fat, and, I will be noticeably weaker, bloated, bigger, more ill. And, that’s because I was not fasting and my body’s cortisol went wacky again and it ate the muscles of my upper thighs and buttocks, again.

        I am not saying this will happen to you. Just saying, the worrying and stress is worse for retaining muscle than the fast could ever be, as long as you are doing a fast willingly and intelligently.

        Fasting can actually raise your growth hormone. Especially if you do body weight squats while fasting. I’ve done a lot of research on this, as well as simply fasting in various ways.

    2. You can fast any way you want, but taking enough amino acids can stop autophagy, which is where the magic happens. Your body will burn through glycogen for the first 3 or so days, then it will start metabolizing excess body fat. Autophagy kicks in and as the body shrinks, your body will selectively break down unneeded blood vessels and extra skin, skin tags, and whatever isn’t necessary for survival. It really only goes after lean muscle mass once all the fat stores are completely gone. See The Complete Book of Fasting by Dr. Jason Fung and Jimmy Moore for more details on this.

      1. Thanks Meg, good to know! I have a few skin tags and “extra” fat…..
        I will look for the book.

        1. Skin tags are possibly caused by insulin resistance. Fasting will also help lessen your insulin resistance. For me, a low carb diet helped lessen the number of skin tags I was getting, but intermittent fasting (up to 5.5 days) cured those. I now have very few skin tags, and the ones I do have are small.

          When I fast longer than 1 day, I still drink coffee with a small amount of cream, I drink a Gerolsteiner, and I have bone broth/stock with salt and “no-salt” (potassium chloride). I drink these daily. I also do what it takes to make it through the fasting period. For instance, I have some herbal tea of all herbs, but I’ve tested my blood sugar with that, and I think there’s a slight rise in blood sugar due to the herbs (hard to tell, though, as these blood sugar monitors are horribly bad; seems to be about a relatively consistent 5 point rise though). I tried black coffee, only water, etc., but find I can make it longer with fewer troubles allowing myself some amount of calories.

          As for breaking the fast, I have been easing into it slightly, mainly by taking some collagen and other proteins. Then I wait a while and eat a normal meal. For longer fasts (4+ days or so), I will extend this even more, to have another small meal before having a “normal” meal.

          1. I had a skin tag removed by a doctor about 20 years ago. I had two skin tags when I went primal in Oct 2011. I did not even pay attention to them because they were mildly annoying at the worst. A bit over a year into being primal I read an article that involved skin tags. I look (and looked!) to see how both of mine were doing. They were gone. Not even a mark where they had been. I did not ever consider them as a marker of anything wrong – but now that I am a Primal Health Coach I have learned that they can be an indicator of the aptosis/autophagy process not working correctly. Getting rid of T2 diabetes (3 days!), gouty arthritis, 99.5lb weight loss was great but it is always nice to have more indicators of health improvement.

          2. Thank you for your post, may I ask what kind of tea you were drinking? I am in my 1st fast so my plan is to drink 24oz or 2 tea bags once daily even in fasting. I got the non flavored green tea, Is it not a good idea on fasting days?

        2. There’s also a lot of good interviews and such with Dr. Fung that can be accessed on YouTube, wherein he will tell you a lot of information that is in the book and some that isn’t.

          Dr. Berg’s vids are good, too.

      2. I’ve just finished reading Dr. Jason Fung’s book on fasting and it’s fantastic–glad you mentioned it, Meg..

      3. The Complete Book of Fasting book is incredible. I read it from cover to cover (also Keto Reset). I am currently doing Fung’s 40 hour fasting protocol (in the book) where you eat/fast every other day. On the days I do eat, I try to keep my eating strictly between the hours of 1pm and 5pm. Fung allows many drinks on his fasts, which I do incorporate. I also take MagTech (natural stacks), Organic Sulfur and Mega-Mag (a seawater complex with minerals), and lick a bit of sea salt when needed. I have also done longer fasts of 3 days and 5-6 days (which I recommend for autophagy, immune boosting and anti-cancer benefits, but not that often), but I am really enjoying the 40 hr protocol right now. My workout schedule has not suffered and I can do 3 mile trail runs while fasting.

        1. That was very helpful info, Sarah, thanks for sharing. Dr. Fung’s book and videos are great. I’m curious what is your age and how long you’ve been doing IF? I had good results for about 18 months, then gained back all the weight I lost, possibly due hormonal changes. I run trails and lift weights too, and always feel great exercising while fasting.

      4. Three days, approx. and that’s if you are not eating low carb or low carb and high fat already.

        As a side note, I watched a vid the other night by … oh .. .someone who does fasting clinics and such … wherein they said the best way to know it is time to break a fast is by scoping out your tongue.

        When you are detoxing, your tongue can go all kinds of weird- white, black, green – and get really smelly and nasty. When it turns pink again, according to this person, it is time to get yourself some vegetable broth or something else soothing to begin the refeeding process.

        He said this doesn’t necessarily mean you are fully detoxed, but, it does mean your body has reached the end of its reserves of something crucial and it has stopped detoxing for that reason, and, now, it needs you to stock back up before going again.

        It was quite interesting. Oh, how I wish I had a link to it, right now. All I can say is I found it on YouTube after specifically Googling up such things as “How do you know how long to fast”?

      5. Meg, that’s the information I’ve also gleaned from reading Valter Longo’s work. As someone with a mysterious autoimmune disease, I’m interested in this. Longo has developed a commercial diet product that appears to be mostly carb- and protein-free.

        Longo’s research shows a five day fast every month for about six months is what it might take to reset your immune system. Three years ago I did this for two months in a row and felt great; it helps a lot that I was already LCHF. I would put some coconut oil in my tea in the morning and have some broth with salt and coconut oil.

        My current situation is not conducive to fasting like that but I intend to try it again when I can. In the meantime, I pretty much just eat one meal a day.

  3. It all seems to come back to something very simple…listen to your body. When cancer patients don’t have an appetite, respect that instead of forcing food (which is usually starchy junk anyway). Personally, while I wouldn’t mind experimenting a little bit with IF, I can’t imagine going a long stretch of time without food. I’m very active and really enjoy my food! But if I had some major health challenges I would consider it. And the case studies are pretty compelling.

    1. My exact take on this 🙂

      “But if I had some major health challenges I would consider it. And the case studies are pretty compelling”

      1. Join the I recently fasted 35 days on water, herbal tea and herbal broth, mainly to reach autophagy to heal my breast cancer Tumor. Unfortunately it hasn’t worked. The Tumor is still there. Maybe the fast was not Long enough ? I lost a total of 12 Kilos of which I regained 8 within two months. However, spiritually the fast was an amazing experience and after the inital 3-4 days I felt great the whole time. I broke my foot a couple of years ago and the nerves never really healed, I had numb parts on top of my foot. All These nerves have healed during the fast. I also shed a lot of mucus. So many positive things have happened.discussion

        1. I am sorry for your current battles, but very encouraged to hear that you did experience some nerve regeneration. I read somewhere that there have been studies where they found a fasting subject had actually had an increase in stem cell formation. That is really cool. Best of luck, keep your chin up, you seem like a pretty tough cookie.

    2. I agree. When an animal in the wild falls ill (or when a lion gets bit by venomous snake), it retreats to a secluded place and refrain from eating and drinking, until it get better. Instead of burdening the body with digestion that pulls scarce resources, it best to let it be.

      1. The hurt animal maybe hiding from predators during this time since it’s likely to caught in its weakened state. If you took it food and water, it may eat.

        1. I’ve had multiple pets over the years and they will not eat or drink when ill. They don’t have any predators to worry about so that can’t be the reason.

    3. Except that cancer patients don’t have an appetite after a dose of chemo. For me there was 5 days of fasting after every dose and that was with the meds. To add a day of fasting before that would have been unnecessary torture. The nausea meds are really given to stop vomiting, not prevent fasting. In the old days chemo chairs always had a bucket attached and the whole scene was a barf-a-rama. But, as I’ve written in my blog, we all need a bit of voodoo when dealing with cancer, so if it doesn’t hurt, it might help. https://girlvstamoxifen.wordpress.com/2017/01/24/a-bit-of-voodoo/

      1. The benefits Valter Longo has found for fasting with chemo (regeneration of immune cells, and protection of non-cancerous cells from chemo effects) only occur is the fasting takes place BEFORE the chemo. They haven’t settled on the optimal length, but t seems to be 48-72 hours to get the benefits.

        So, adding fasting days before treatment is not ‘unnecessary torture.’ It makes the chemo more effective, lowers many side effects, and leaves the patients feeling better than without fasting first.

        All this is well-documented.

    4. Luckily for me I dont eat but usually once a day. It is horribly bad food usually so this is a whole new bag for me. I think I could go more days than I have decided on but to start our 36/12 seems like a good start. I look forward to some of these IPA’S, cheeseburgers, and toxins find their way out of my body. So the jury is still out for me, maybe I will shorten or maybe lengthen my initial goal.

  4. Great post! I’m in the middle of my third week-long fast. I do it every six months to a year. It is an awsome experience to know yourself, to get to know you really do not need the food to live and be productive.

    I’m already health and on my weight, but still I lose a little fat and look awesome after the fast!

    I really enjoyed reading this post.

    Most important recomendation, if you fell bad in any way, just eat!

    Second most importante recomendation, always have some broth on the fridge, going to bed with a warm belly makes all the difference!

    1. thanks for the post! very timely for me, as i’m planning to do my first week-long fast in a few days. i’ve eaten in a smallish time-window (~10-5) during the week for a few weeks, and i’ve been psyching myself up for the weeklong fast. i’ve prepared plenty of bone broth for the next few weeks, as well.

      any tips / suggestions for a first-timer?

      thanks in advance!

      1. Close family member has cancer and is advised by the doctor to do long term fasts. I do them with that person as moral support. Google Tim Ferris’ method or check it out in one of his books. In short, MCT powder, KetoCaNa, salt/lime in water and then after day 2 or 3 go to only mineral water. After the 2nd time fasting I could go straight to mineral water and have no problems. When I exercise, i do a gram or so of BCAA in accordance with Tim Ferris, but on days off it’s only water.

    2. Exactly what I’ve been doing for the past 6 days, sipping on some home made broth just before falling asleep.
      Do you end your fasts because you are hungry and need to eat or because you reached your target?

  5. I can’t imaging that a three day fast is problematic except in certain situations. Fasting with fluids gets the job done. Remember Pareto…..even 20% gets 80% benefit. Journal of Translational Medicine just published Dr Valter research in humans on fast mimicking diet. Good results in improving certain biomarkers. Frankly, this has probably too many financial overtones since he has spun off a company that patented the diet. However, for specific medical purposes (like prechemotherapy?), has merit. the rest of us can inexpensively water or tea /coffee fast 24/36 hours.

  6. I have done a 5 day fast and a 4 day. Both were very manageable and helped stabilize my Blood Glucose. I can say I studied Dr Jason Fung prior to fasting long term and I also regularly eat once a day which helps. The best benefit to me was the mental aspect. I realized most “hunger” is in my head and routines vs belly.

  7. “If you’re trying to reveal the bottom half of the six-pack you just know is lurking beneath your gut, long fasting is not the answer.”

    OK … well … nevermind then.

  8. I followed the exact prescribed process outlined in Tim Ferriss’ book, Tools of Titans. Essentially, he recommends a three day fast with tons of water spiked with lemon and sea salt along with up to four table spoons per day of MCT or coconut oil. Coffee and tea are also fine. Along with that he suggests 2-4 hour of brisk walking on day one (and two if you want/need it). No weight or HIIT training on these three days! The whole point of this three day process is to kick your metabolism into ketosis quickly, and then maintain that state through a normal keto based diet from then on.

    It worked perfectly and I was in full blown ketosis by day three and it was easy to maintain or get back to after that even with a cheat here and there (a few high carb treats and some red wine now and again). I wouldn’t go longer than 3 days for now and would also suggest eating a very low carb, high fat/moderate protein diet for at least 2 weeks before trying this. My goal is to do this once a quarter to reset and the move to once a month after I’m comfortable with it. Also looking to get a deep blood analysis to make sure this isn’t silently killing me 😉 Feel good though

    1. thx for sharing the Tim Ferriss info, Kyle…he is very knowledgeable. When u fasted 3 days, did u lose any fat and/or muscle mass ? What was your body composition like when you entered the fast ? My only concern for myself, if i do a 2-3 day fast, is losing muscle mass and then not being able post-fast to regain that mass, since I’m in my late 40’s. Maintaining muscle mass as you age is super important.

      1. I didn’t do any type of body comp test to have real numbers. I did not lose any muscle mass from what I can tell. In fact, I’d argue it’s unlikely to do so in such a short period of time. Re: body fat…hard to say. I definitely looked a lot better but that could be a symptom of water retention. The point in the three day period is not to necessarily lose body mass (fat or otherwise) but to instead get yourself to an optimal metabolism (keto in this case) in order to burn more fat. I quickly went back to a very high fat, moderate protein diet and have continued to make strength gains.

        Give it a shot. You won’t do anything drastic in that three day period but pay attention to hydration, putting salt in your water, using the MCTs and be very careful when you break the fast.

      2. Actual muscle loss would be the very last thing to happen, as someone mentioned before. Your metabolism changes to a more protective mode during a fast if anything. You’d lose fat and dead cells and other waste well before any muscle loss. We wouldn’t be here if we evolved to lose muscle during a period with no food. That’s the reason humans store fat in the first place, to get them through a time with no food. But muscle is actually protected.

    2. thanks for that reference to the book! i have it on my desk but have yet to start reading. 🙁 looks like i have some homework for tomorrow.

  9. How does a person deal with diarrhea when coming off a fast? I often have diarrhea for the 1st day when starting to refeed. Post fasting diarrhea seems to be a pretty common issue on some internet boards. I have yet to find an explanation or a solution. Suggestions?

    1. Hey! That happened to me on my last three-day fast. I thought it was weird. Not an urgent stool, but a pretty loose one. I thought maybe some of Detective Rosewood’s “Five pounds of undigested red meat” had shaken loose, but maybe it’s worth further analysis.

  10. Mark–For the autoimmune case reports, you failed to mention that the patients preceded and followed the fast with a vegan diet, and that the authors conclude the paper by saying a vegan diet appears to be necessary to sustain the results.

    1. Re the autoimmune case histories—Mark’s link has died. Anybody know where the paper can be found, or even just the title?

  11. Tried a long fast of three weeks once. Back in my 20’s, all unfiltered apple juice….at the time I wasn’t working and had little stress in my life. Later realized that was way too much sugar!l I think I ended up with a blood sugar imbalance as I almost passed out (but was quickly revived by eating a prune.) That ended it and I haven’t done a long one since but did try a 3 day fast on green tea and water a few years ago and didn’t notice any apparent ill effects. I would like to try a short one again but pretty much agree with Mark that compressed eating windows and shorter fasts in general are probably just as helpful for most of us, without as many of the side effects of longer periods.

  12. Let’s see, you’re ingesting tea, MCT oil,
    magnesium, calcium, salt and lime juice,
    yeast, vitamin K, and bone broth and
    vegetables. Sounds like what’s called
    “eating” to me.

    Ridiculous fad, promoted by charlatans…

    1. hmmm…That fad has helped me lose another 25 pounds (after losing an initial 30 using low carb). It’s also lowered my blood pressure at least 15 points (systolic), reduced my insulin resistance and blood sugar, asthma, allergies (went from gobbling multiple pills per day to one per YEAR), GERD, overactive bladder, skin tags…too many to list. I stick more towards coffee with small amount of cream, water, some minerals, bone broth.

      1. It may be an effective DIET.

        But it’s stretching it to call it a FAST.

  13. I occasionally do a fast for 24 hours. For an appetite suppressant, I drink ginger tea made from boiling chopped ginger root. Do any of you have suggestions for other appetite suppressants?

    1. I’ve been doing 36-hr fasts about every 7 – 10 days and feel GREAT! That includes during the fast. For me, no appetite suppressant is needed or even wanted, it feels GOOD to fast sometimes. But I believe I am usually or always in ketosis to start with as I follow a very low-carb, high-fat diet. In fact, every morning I wait until at least 12 hours after yesterday’s last food before I eat. Being in a fat-based metabolism rather than glucose-based metabolism is a natural appetite suppressant.

    2. Apart from staying well hydrated, coffee is a good appetite suppressant. It’s touted as such, and works well in my experience too, often delaying hunger for a few hours. Not sure if decaf has same effect, but if caffeine is not an issue….I’ll try your ginger tea next time too, sounds great. 🙂

    3. When on a LC diet, hunger usually lasts seconds to minutes. When hunger hits me harder (days 3-5 of the fast) I add butter to my coffee. Going to sleep works, too.

  14. Personally, I think long-term fasts of anything more than a week (and even that’s too much for some) are a really lame idea. In starvation mode (long-term fasting), the body will literally cannibalize itself in an effort to stay alive. That doesn’t sound like a worthwhile ambition if you value your health.

    Other nasty things can happen as well. Years ago I read about someone who fasted as a protest. After several weeks he lost his eyesight. It didn’t come back. He also lost the ability to eat and derive nutrition from food–even when he desperately wanted to. I guess none of that mattered because he died shortly thereafter. Obviously his body didn’t have the wherewithal to tolerate a long fast.

    In other words, have a damn good medical reason before embarking on a lengthy fast. Don’t do it as a frivolous experiment.

    1. Shary, that old myth about the body canabalizing itself has been thoroughly debunked for decades, actually. If you go to Intensive Dietary Management and read Jason Fung’s remarks on this, you’ll see it explained extremely well. The only ones who want us to believe that we’ll waste away from fasting is Big Food and Big Pharma.

      1. And if you check the reference with the person who fasted for 44 days and lost of his muscle mass, you will see that it was David Blaine. He locked him self up in a small box for 44 days und just drank water. But this also means, that he didn’t move. And if you don’t move, you lose your muscles no matter how much you eat. Happens to everyone, who had an accident and had to stay in hospital for some time.

        1. And then there was the Scottish man who fasted for 382 days and came out of it quite well, indeed. Blaine’s inactivity certainly didn’t do him any favours. Just standing up and puttering around one’s house is better exercise than he got. They didn’t do any good tests like a dexa scan afterward; just the old, inaccurate pinch test and muscle loss was determined by creatinine excretion, so it’s not the best example for me to use. There are very inactive elderly people who maintain more muscle when they take in more protein than they had previously consumed, so we can’t really make sweeping statements about fasting and muscle loss.

  15. I had a good experience on my one 3-day fast. Most critical was staying well hydrated, electrolyzed, and rested–no strenuous or tiring activity of any kind.

    One additional benefit I might add to this list: giving the GI a little break and a gentle “cleaning”. Maybe TMI, but what I found fascinating was that after days of no food, substantial bowel movements persisted.

    1. It would be interesting to take a strong probiotic during the fast to see if the “cleansing” was enhanced. The one I am currently taking had me in the bathroom 6 times in a single night (next morning I was almost 3 pounds lighter than the previous morning lol).

      Not sure if it matters but I was recovering from anti-biotics and switched from SAD to Keto after finishing the anti-biotics. My first time on Keto and it is amazing.

  16. Thanks for this post it was exactly what I was looking into, I want to implement short fasts into my personal prayer life and am also on a strict workout and nutrition regime for health and weight loss and didn’t want to interfere with it, it’s good to read there are ways to do it carefully that won’t disrupt ur regime but also can have benefits health wise not to mention the clarity u gain spiritually from denying urself. Thanks again!

  17. I’ve been eating primal for 5+ years and decided to try my first fast a few months ago. Just a n=1 experiment. I had no predetermined length of fast – I just winged it. I drank only water and some cold tea and took my daily vitamins. After 10.5 says I stopped the fast because I was concerned about the health of my gut biome. After gradually returning to eating I had issues with solid waste frequency and lack of firmness. Most of my bacterial enzymes are from Greek yogurt and aged cheese. This was not enough to return me to normal. The issue did not get better over the next few weeks. I then started a daily probiotic for the first time in my life – after two weeks everything is now back to normal. My typical routine involves fasting from 8pm to noon. Good to have had this experience. . . but without a need for extended fasting I don’t plan on going more than 1.5 days without food for my gut bugs. .

  18. Mark, thanks again for another expert blog. I’m 57 year old male, pretty strict paleo to keto, healthy, work out a lot, about 15-20 lbs overweight versus my adult weight goal, and am already incorporating 24-40 hour intermittent fasts into it once or twice weekly with only bone broth/water/coffee during fasts. Read Jimmy Moore’s fasting book – good info, but longer fasts pretty radical. My only desires on fasting are to add it into the mix to mildly shock/challenge my system, definitely be in ketosis during fasts, and increase body fat loss a little beyond paleo/keto. Your post confirmed my sense – periodic (weekly) fasts of 18-40 hours at a time probably helpful if carefully done, longer fasts more risky. Thanks again!

  19. What is considered a long fast. My usual is 36-45ish, that still be considered short?

    1. If you’re seeking the answer regarding fasting as the ancient practice of convalescing on water alone (or dry fasting) for therapeutic intentions, then historically a long fast is 40 days or more. This is based on the thousand years practice of fasting as a proven health injunction.

      If you are asking in regards to the current fads of caloric restriction, intermittent fasting, and subsisting on broths and teas as dietary modification options (these are not fasting, but something else entirely different) as regards a search for other positive physiological and emotional consequences due to such practices, I couldn’t say… might have to pay those gurus to share their opinions and see if it works in practice.

  20. I did a 40 day fast with broths and some juiced fruits and vegetables plus some added medicinal protein at my doctor’s recommendation. I can honestly say I had more energy during the fasting period than I did before. I followed it with clean eating afterward and lost a lot of weight and felt excellent. Unfortunately, i went back to my old way of eating and gained it all back plus. In October of 2017, I discovered Paleo and Primal living and my wife and I have embraced it full on. Did my first sprint today as part of the PBF program and have had great success and feel this will be our lifestyle for the rest of our lives. Thanks Mark for the information you bring to the table.

    1. Oct 2017? The big news is you are from the future! In our timeline it is february 2017.

  21. What’s the impact of extended fasting on gut flora (the microbiome)? Those lifeforms are obviously going to be stressed, perhaps to net benefit, but that would be conjecture.

    And if there are hazards there, could they be mitigated by supplementing with probiotics and daily non-caloric prebiotic fiber to keep the critters happy?

    1. This is a great question.

      I’ve been wanting to take gut cultures prior to, during and after a long fast to get them tested by UBiome or some such service, to see if there was any delta… but I was too cheap about forking over the dough to buy the multiple testing packages and missed my opportunity recently when I did a 21 day fast… sigh, oh well. It was more of a curiosity impulse of a theorist than an actionable necessity for a practitioner.

      My sense from the literature I’ve read so far is that the microflora and fauna that would suffer most are those that depend upon sugars… which are both the most voracious and potentially most pathogenic. But nobody really knows even how many critters we contain, let alone the ecosystem variables that dictate our epigenetic fate due to these multitudes within us.

      Maybe I’ll probe them in the Fall on my next fast…

  22. Unfortunately, even though I’ve a good deal of respect for Mark’s work, I’m compelled to share that this post is NOT good information about fasting. This may perhaps be for several reasons, but the most visceral is the wildly misleading and very common conflation that many believe regarding fasting as starvation–these two conditions are NOWHERE near the same thing. The fallacy is like comparing bicycles to military nuclear rockets and saying they’re both are dangerous because they move, but in reality one is highly useful and common to the masses and the other is not, unless there’s been a big problem nobody solved in time.

    The other common error we must suffer is that most people who are critical, opinionated and share presumed facts about fasting have never fasted themselves, have never guided or observed someone else fasting, or even bothered to do much research into fasting based on those with such direct experience. There has been so much click-bait, cherry-picking lab studies offered up on any variety of fronts that really don’t give anybody any practical insights other than comforting assurances to resume their flawed opinions and beliefs because they can now presume there is legitimate support in theory. Meh.

    Fasting is thousands of years old. It’s proven and far safer than the real faddishness of intermittent fasting, mono-diets and caloric restriction diets It’s practiced in almost all cultures as an INTENTIONAL therapeutic, not one imposed by famines… (geez!) and even more importantly it still works today for almost 100% of the world’s population (contraindicated for pregnant and nursing women, the malnourished/starving and those very few with an extremely rare genetic trait that prohibits metabolizing of ketones… that’s it).

    That’s it.

    1. So…at what point does fasting become starvation, Rob? Saying there’s a big difference between intentional fasting versus famine is playing games with semantics since the end result is essentially the same if carried on long enough. I would think an obese person has enough bodily reserves to fast for a long time, whereas a fairly thin person might be on the ropes rather quickly.

      I routinely fast between 6 p.m. and 11 a.m. or noon the next day. For me this means nothing at all after dinner (other than water) and a single cup of tea in the morning. I don’t fill up on various drinks and juices and call it “fasting.” This system comes naturally for me. It might not for everyone. I see no point whatsoever in fasting longer than that, other than for medical reasons. You say “it works for almost 100% of the world’s population…” Works to do what? Is there a purpose?

    2. Also, FWIW I just wanted to add to the “contraindicated for pregnant and nursing women” sentence that it appears most religions that fast (ie Ramadan) contraindicate menstruating women… This raises questions regarding women of childbearing age fasting for longer than say 28 days (I’d like to hear Mark’s opinion regarding this 🙂

      1. In both sexes, it is not uncommon for hormonal levels related to sexual reproduction (e.g. testosterone and estrogen) to become reduced during long fasts (i.e. 40 days or more).

        As well, many religions contraindicate women participating in ritual practices for a variety of reasons. Especially menstruating women…

        Regardless your faith, you can rest assured from a health and wellness practitioner standpoint that menstruation is not a contraindicating factor for women seeking to fast for longer than 28 days (i.e. or any menstruation cycle period).

        As to fertility increasing or decreasing during long fasts, that’s another question… but after any long fast is successfully concluded and a reconstituting diet commenced with discipline, one could confidently expect fertility levels to be improved over baseline experience prior to fasting (along with most every other physiological attribute.) And this is due not just to the deep and comprehensive cellular autophagy, but also the complete and holistic rest that attends a properly conducted, therapeutic fast.

        1. Thanks for taking the time to reply Rob. Your point
          “And this is due not just to the deep and comprehensive cellular autophagy, but also the complete and holistic rest that attends a properly conducted, therapeutic fast.” is something I hadn’t considered before … fasting being a way to give the body rest and a deep clean, I’ll have to ponder this some more 🙂

    3. In no way, shape, or form is intermittent fasting a fad. It’s a healthy, sustainable way to live.

      I also, don’t understand the rest of your post. You seem to be agreeing with Mark.

    4. Thanks for that, Rob. I’m also very tired of click-bait headlines and comments in the area of fasting. But the other sentence that’s been said to death is: “Most dieters nowadays eventually regain most or all of their lost weight.” I enjoy this site and have learned quite a lot, but when I read that sentence, yet again, I felt a little deflated. That remark has caused more people to wonder, “why bother?” than any other I can think of for those needing to lose weight for health reasons. At the very least, it should explain that only those who return to their ‘previous’ eating habits tend to gain back the weight they lost. That’s part of the reason The National Weight Loss Registry was created, to show that lasting weight loss is more than possible with a change in eating habits and other healthy and easy lifestyle changes.

  23. I faced similar problems a few months back as because of my daily schedule, I was not able to eat properly which impacted on my health.
    Thanks for sharing the tips for doing it safely.

  24. I dry fast once a week for 24-40 hours. I am fighting a rare, progressive muscle disease. I have tried autoimmune and Paleo diets with no luck for the past 5 years. I’ve been eating raw as well. I feel fantastic after a long fast. I totally feel rejuvenated! I am presently on a personal herbal protocol to protect my kidneys and the rest of my body. I understand that while being dry the cells will squeeze the interstitial fluids out to hydrate and take some bad cells with it.

    1. Hi Lisa, I’m glad it’s working for you. As I’ve mentioned above, I came across several websites that cover eating raw meat (I’m sure you know which one) and was very impressed with the info. I wish I could make the switch….

  25. I lost 100 pounds last year from a long term low I couldn’t break with KETO using long fasts. (In fact my thyroid went hypo by TSH over the year I did KETO and went back to normal with fasting.) I mainly used 5-7 day fasts. I didn’t really even watch what I ate when not fasting to a great extent more than just generally being aware.

    The 1960’s study sounds bad but 50% having measurable long term improvement after a single 10 day fast is quite remarkable given the figures being bandied about for diet failure exceeding 90%. (I think there might be a large file drawer issue with successful dieters not making it to studies, but once someone signs up for an academic study it’s very unlikely they’ll lose the weight).

    I expect I’ll have to make it a habit for life, but for me fasting for weight loss save my life basically.

    Admittedly the first couple months kinda sucked, but it just gets easier and easier. My appetite when not fasting started crashing after about 4 months.

    For me at least this is completely different then any diet I’ve ever tried which start off fairly easy and just made me cold sick and miserable with no energy.

    1. David, were/are these water-only fasts, and how often were/are you doing them? Did you take supplements of any kind? I’ve done a couple water-only fasts recently but only lasted 2 to 3 days each time (I was aiming for 4). Actually, it’s not accurate to say water-only; I was having a cup of coffee in the morning and green tea (straight up loose leaf, no flavorings or additives) in the afternoon. I’ve read (from Dr. Jason Fung, mentioned elsewhere in this comment thread) that even small amounts of protein can break the “fasted” state and stop autophagy so I was avoiding any supplements, including fish oil, which come in gelatin soft-gel capsules. I don’t know if that small amount gelatin is enough to boost one out of a fasted state but fasting is difficult enough for me that I didn’t want to risk losing the autophagy benefits. I’m hoping it will get easier over time.

      1. Mix of water only and some calling sub 500 calorie a day fasts. Over 5 days I do water lite salt multivitamin, also camu-camu powder (basically vitamin c).

        It’s gotten so much easier I basically almost don’t believe it. I can be somewhat full for a couple days now. Makes starting a fast really easy since first couple days were the hardest. Biggest enemy mainly boredom now.

        1. Thanks so much for that info, and congrats on coming up with something that works so well for you and sticking with it. Boredom and habit were really, really tough for me during the fasts I did (1 at the beginning of March and 1 at the beginning of April).

          Breaking the routine/habit and finding a new way to cope with stress and boredom seems to be a lot harder than the actual not eating, which was kind of a surprising thing to learn. Your experience is so inspiring — thank you for sharing those details.

      2. “I’ve read (from Dr. Jason Fung, mentioned elsewhere in this comment thread) that even small amounts of protein can break the “fasted” state and stop autophagy”

        That’s why I would think bone broth is a bad idea for autophagy. Weston A. Price Foundation says “Although gelatin is not a complete protein, containing only the amino acids arginine and glycine in large amounts, it acts as a protein sparer, helping the poor stretch a few morsels of meat into a complete meal.”

        I do love bone broth and make it frequently. But when fasting to encourage autophagy, I think I will avoid it.

  26. Thanks for the great post Mark, it’s just what I wanted to read about today. I’ve been researching this topic for a few weeks now on Dr. Jason Fung’s website. I’ve started doing a 42 hour fast one or two days a week, to reverse insulin resistance and gain more insulin sensitivity, to reap the metabolic benefits of up-regulating/down-regulating this or that process, and to burn off any risky fat that may be lurking around my liver and pancreas. I’ve followed the PB for the last five years, and strict LCHF/keto since December, but when I did the fasts I quickly lost inches off my waist and hips and am still shrinking. With LCHF/keto I gained 6 lbs, but my pants are falling off, just like Butter Bob said they would! I feel confident that I’m safely reversing and preventing possible health issues with this protocol. Thanks for the mineral water tip too!

  27. Anyone interested in fasting should read Dr. Jason Fung’s Complete Guide to Fasting. It goes into detail about topics in this blog article and corrects a few things written above. It’s an awesome book.

  28. This is great! Interestingly, I am at the end of my first water fast (supplemented with a carefully chosen multivitamin). I feel great, but I have decided to start the refeeding process tonight after 5 days just because I am not an experienced faster and feel that I am already pushing the envelope. After a ton of reading and research, I decided it would be something good to try. I did not do it for weight loss (but did lose about 5 pounds), but because I was interested in some of the proposed health benefits – namely because it is supposed to give the digestive system a break, and I had been having some digestive issues that I couldn’t figure the source of. I have been fat adapted for a while and am otherwise healthy but wasn’t sure I had cracked full ketosis, so I figured a short fast would be a good way to give things a reboot and check out full on nutritional ketosis. The first 2 days were rough in terms of cravings, but since then I have felt great. I also monitored blood ketones once daily – Day 1-2: 1 mmol/L, day 3: 2.6 mmol/L, day 4: 2.1 mmol/L (haven’t yet checked on final day), and had a noticeable difference in sustained energy and alertness on the days that were in the nutritional ketosis range. Of course, I don’t mean to recommend this, I am more of the “treat it like a marathon” category, but it has been a good experience overall. Will post later about the refeeding process and how I feel in the coming days!

  29. One thing worth considering with longer fasts is the effect (or possible effect) on mood/personality… I saw Rachel Hunter (on her tv series Tour of Beauty) do a week or 2 fast and she admitted that she got very angry/aggressive/ easily annoyed etc… However she wasn’t primal/paleo to begin with so maybe someone who is already a fat-burning-beast wouldn’t have such side effects, and it was a “TV Series” so I’m not sure how much faith we can put in that, but she did seem genuinely irritable (maybe try meditation, yoga etc when doing longer faster for your family’s sake 😉

  30. Hi Mark, I’ve been on the mend after an impact injury to my knee (fell playing pond hockey in jeans ie no protection) and I am getting back to the gym and have heard a lot lately about simply fasting for a day a week every week and I am intrigued about a longer fast. My Question is about the use of green tea Kombucha during an extended fast – would this be something you recommend as I have been using it for a while now and love it…?

    1. Kombucha still has an appreciable amount of sugar, even if you make your own, as I do. Unless you let it go all the way to vinegar 😉

  31. At age 57 I’ve done many 3 day fast about once every 3 weeks. I usually do 2 hours of aerobic exercise (biking/jogging/hiking) and one hour of weight lifting during those 3 days. I follow a Primal diet before and after with good protein sources and I have not noticed any problems with strength, endurance or muscle loss.

  32. I did a 9 day water fast but I had so much pain in my lower back I had to stop. I wondered if the pain was caused by a deficiency of some sort. But they said it’s retracing but I don’t buy that to this day. The thought of reliving that pain keeps me from trying another fast. It helped my SIBO so much though, I really want to do another.

    1. I have experienced the same in a Fast Mimicking Diet, at day three the back pain started to the point that I would woke during the night for the pain. At day 5 I had to stop the protocol, few hours from finishing it. I could not even breathe for the pain…

      1. Continuing on this post, I share your impression that it might be some deficiency accumulated during the fast. In my case I suspect an electrolyte imbalance, in fact after 24 hours from the end of the fast the pain was gone. I think my diaphragm muscle was very tight, as if it was cramped, the problem by breathing fully was similar to what I experience when I overeat and my stomach has stretched too much. I suspect that some muscles on the back were maybe too tight or cramped as well, pressing on some vertebra or nerves, inducing the back-pain.
        For me it is a lesson learned. it was my mistake to enter the fasting the day after I have run a very hard half-marathon, I was most likely depleted of many electrolytes, and the post-race lunch and the dinner were probably not enough to replenish my body.

  33. Hi Mark, thanks for this interesting and thought-provoking post. My question is, what impact does fasting, for any length of time, have on gut flora. It seems to be positive, given all the benefits, but what exactly is going on in there when we deprive the critters of their food?

  34. @Mark, thanks for this post; I’m sharing widely. I’ve done a couple 36-hour fasts when I’ve been sick, so I want to encourage you all to try this (same idea as the cancer), and I think it hastens my recovery. I only drink green tea and water when I fast. When I get the flu or a bad virus, I starve it until I start feeling better. If I had a serious illness, I’d totally do longer fasts and will probably try longer ones just for fun. I IF often and find that hunger is oft driven by habit rather than true hunger, so I skip meals frequently. Much of the need for food is psychological I think.

  35. I had a dexa scan a few months ago and discovered that instead of the 16% body fat the machines at the gym told me I had it was 21%. I ditched most alcohol ( glass of wine once it twice a week ). Dialled down my carbs whilst increasing the amount of vegetables. I have been on a 23 hour fast every other day ( meaning just one meal before bed then two meals the following day then just dinner the day after and so on ). I have a lot of muscle mass ( 95 percentile of guys my age ( 41) according to the dexa results.
    I’ve been strict with sprints once a week..3 times heavy lifting ( 5×5 protocol ) and one to two 80 min MAF sessions.
    Its been really easy. Especially the fasting. And I’ve been in ketosis for over a month.
    I’m having a repeat dexa scan tomorrow and it will be interesting to see how much fat/muscle mass I’ve lost …if any.
    The hardest part has been cooking massive steaks and only being able to eat half of it. No supplements except magnesium and we have a special tanning shop with vitamin D sun beds which I do once a week ( not much tan but I’m not interested in tanning )
    I can see fat loss. Six pack is showing and veins popping so something is happening.
    The best part is that I wouldn’t mind doing this for ever. Having every other day to eat as much as I want is nice but I’m just not that hungry. I weigh 230lbs and 6’2 so I guess those 20% of body fat account to quite a lot of calories. Once that goes down I might have to eat more I guess.
    Time will tell.
    Oh and blood pressure went down from 130/80 to 114/60 for over a month and my HRV is through the roof. I’ve never had issues with blood sugar. Always been around the 4-5 mark except after really heavy workouts when it spikes to 6.5 ( despite training in fasted state ). But I know the biological mechanism behind it so not worried.

  36. After doing two 24hr fasts a week for some time, my friend and I decided to try for 72hrs. Strangely at some point the feeling of hunger went away… but then I started actually dreaming about food. Not sure if I didn’t drink enough water or what but the only negative effect I had was a killer headache on the third day.

  37. And then there are those who resort to eating meat only (from head to tail organs included) and drink water, who do dry fast with ill effect. This stem from the idea that hunter gatherers in the true sense and prior to the agriculture revolution, weren’t always successful in their hunt and sometime didn’t eat for days. There are several websites on the subject and I find their experience and results vis.a.vis dealing with health issues fascinating. I seriously thought about trying it out as a way to deal with autoimmune disease, if not for the fact the Meat is outrageously expensive over here and as it is I suffer from high ferritins

    1. I know the site you’re referring to with the raw meat diet and dry fasting. I’m not quite ready for that, yet, but it’s very insightful. Have you considered giving blood/plasma for high iron levels? Sometimes women in menopause develop this and find that giving blood can help.

      1. Thanks for your comment. I gave it a shot for a few days and it wasn’t bad but an all meat diet isn’t for me. I would love to give blood but I am also slightly anemic and it all stems from a g6pd enzyme deficiency (look it up). I is frustrating and depressing as it has ntoing to do with how wel I eat, sleep or workout. 🙁

  38. I just wanted to share the effect of my 6 day water and green tea only fast. I had been having some allergy/autoimmune issues for months resulting in a constant sniffly nose and sneezing. Also I had eczema on my arms. After a few days my sinus problems went away and the skin on my arms was a lot better. My sinus problems are only now creeping back, despite being on a low inflammatory diet for months. (Obviously I’m consuming something which my body doesnt agree with but I havent been able to eliminate it yet)

    Its not really a fast if you consume bone broth, as your body has to metabolize it. Things are more complicated than stating (AS A FACT) that bone broth will have no effect on your fast. Im sure caffeine might affect a fast too….

  39. I started following the Fung Protocol in 2015 after eating LC since 1972. The reason was that I had not lost fat for 6 years and wanted to shake things up. IF allowed me to break that plateau. At age 72 I walk 2 miles every morning before dawn, take no prescription meds and have no illness other than recurring facial dermatitis which doctors can’t diagnose. I’ve begun taking foods out of my diet and have it successfully controlled. Taking out cow’s milk dairy got me losing fat again, and that was a total surprise. My IF usually consisted of stringing together 23 hr fasts and I think the longest string I put together was 5 days. Oh, and I haven’t had a cold in at least 5 years.

    1. That’s so inspiring, Reluctantexan. I’m 60 now and it can get so discouraging reading about ‘all the things that slow down’ and the ‘sleep problems’, weight gain etc, if I’m not careful about what I read these days. I eat one low carb meal per day at 1pm and it’s much easier than I thought it would be. I’d like to work my way up to 48 hours and then maybe 72 once in a while..Thanks for sharing your experience–all the best!

  40. I’ve been interesting in trying fasts after listening to Dom D’Agostino’s podcasts on ketosis. He theorizes that a 5 day fast 3x a year could eliminate cancer cells in the body, and I was thinking of trying a 1 day fast, then later a 3, and moving to 5 if 3 was ok for my body. I’ve drank a LOT of alcohol in my life and feel like it might be a good way to wipe out some of the damage I’ve done to my body. Just a theory but think I’ll try it.

  41. Great post. Thank you, Mark, for covering this. Very interesting as well.

  42. I underwent chemo after seeing your previous information about fasting into treatment. 48 hours in and 24 out and I had very few food related side effects. When I didn’t fast things were horrible. I want to thank you for sharing all the information in your blog. It’s so inspiring and informative.

  43. I pretty much fast every single day until 1-2 pm. I normally wake up around 6:30 every morning. I drink a cup of black coffee and water throughout the day. Sometimes I will have a Bai 5 cal drink that has some green tea in it for the caffeine effect.

    I’ve been super focused at wok ever since incorporating intermittent fasting into my daily routine.

  44. Just read Dr. Jason Fong’s book on fasting. He is a nephrologist and works with a lot of diabetic patients with some really fantastic results. I feel the book is really good as he addresses all kinds of fasting and the results he’s gotten over many years with different kinds of patients. People with medical conditions should work with their doctors especially anyone on medications. Otherwise, most fairly healthy people can try one that appeals to them. He explains what to watch out for, has people continue with exercise has recipes for people doing intermittent fasting and some advice on taking supplements though I would have liked more info on that. I’m in the midst of a fast now using bone broth I made and some supplements. I feel great, have plenty of energy and am actually enjoying not making food or spending money on it for a while. I’d recommend the book for anyone having any questions about fasting.

    1. I think that people who are calling IF a fad or not a real fast because some people have the bone broth, cream in their coffee, fat, etc. need to realize that Dr. Fung makes it clear that those additions are ‘only’ to help some people comply, so they’ll get ‘some’ of the benefits of a water-only fast. He would rather that people take only water during short fasts up to 3 or 4 days.

  45. Great topic! I just read in Scientific American about Longo’s recent study about 5-day semi-fasting (1,100 cal first day, 700 each subsequent day) to delay aging processes; published in Science Translational Medicine. Seemingly terrific results for relatively short periods of calorie restriction, and quite importantly as you mention, very low incidence of lean muscle reduction.

  46. Personally, this doesn’t appeal to me. I believe if you eat right and get exercise, your body pretty much heals itself. There are exceptions, some may be noted in this article. But if you are eating right and moving, I doubt you’d need this extreme of a regimen.

  47. I did my first long fast as a freshman in seminary. Then I had my first baby, and 10 years went by without any more fasting. Starting three years ago, I got back into fasting and I have done perhaps 5 to 7 one week long fasts. I have found them to be strangely quite easy, and all but one of them I have done while working. My reasons for doing so are more spiritual than health-related, but I feel that they have improved my health as well. Additionally, I lift weights, and during the same period of time of doing two biannual long fasts I have managed to add several kilograms of muscle mass to my body. Would I have added more if I had not fasted? Perhaps. But I’m very happy with where I’m at now.

    1. I do IF routinely and have done a longer fast as well, and never once after completing a fast did I feel compelled to binge; actually it was the opposite. When it was time to eat again, I had no desire to do so. I started fasting for health reasons but kept on because of how good I felt while doing it.

      PS — anyone who has completed more than one fast and jumped back into eating too quickly will tell you that they rather promptly learned why bingeing after a fast is a very bad idea. Most of the time, your GI tract will not tolerate it, and either you’ll never fast again or you’ll smarten up for the next round.

    2. I can’t help but wonder why you would make such a statement when all evidence points to the contrary. Do you have any evidence to back up your comment that bingeing is an ‘extremely common by-product of fasting?’ If not, then you are simply causing people to doubt fasting’s many benefits and that’s just wrong on so many levels. Do you work for Big Food or Big Pharma by any chance?

  48. Every year for Lent I perform a fast for the entire season, which is 46 days. I either completely fast, except for fluids, or fast on just fruits, vegetables, nuts and seeds. After the first couple days it is really quite easy; I even sit down with my family each night as they eat their regular dinner. I take a full complement of vitamins each (most) days, and get a few extra calories from the occasional beer. One year I didn’t pay enough attention to supplements and got slightly anemic, but that was easily corrected. The most I’ve ever lost was 14 pounds and each year I’ve felt fully energized during the fast, unless I decided to engage in too much activity on some days, at which time I was, naturally, easily fatigued. I’ve never been ill during a fast. Blame it on good genes or the inherent goodness of fasting. Either way, I will keep it up and also encourage many of my patients (I am a Family Nurse Practitioner) to do the same; although I have a zero success rate at convincing any of them!

  49. Boy, the people who are just so happy and pleasant in these posts are unrecognizable to me. You’re telling me you have no mood change during fasts. I have experimented with a number of different fasting methods and never did not have a resulting mood change. Other practitioners I know have said the same. If you can leave your spouse/partner, kids, friends and work colleagues for the fast, knock yourself out. But it’s a disservice otherwise to people reading these posts to pretend there is no effect on mood

    1. I am currently on day 3 of my water only fast with no change in mood or side effects whatsoever. I do believe that it matters if you are used to being in ketosis (and switching back and forth), since I do occasionally have mood issues when fasting or switching back to ketosis. But in general I always end up feeling fantastic and side effects have lessened over the years.
      I am also over side effects at this point in my life and will take Advil as a favor to my family if I feel crappy enough. I don’t feel like it messes with my fast and haven’t needed them in a long time anyway.

    2. Magnesium and B vitamin deficiencies contribute to feelings of depression, irritability, anxiety, and mood swings, regardless whether or not a person is fasting. Many common drugs (oral contraceptives, statins, and others) deplete the body of these and many other nutrients. STRESS also depletes B vitamins and magnesium. Anyone who fasts without adequate amounts of these would likely experience mood changes. Also, in my opinion folks should be really clear on WHY they are fasting. Mood changes are not necessarily “bad”. Repressed or ignored emotions could surface during a fast, and it helps to realize this before fasting.

  50. Sorry to disagree Mark but any fast longer than 3 days requires only distilled water otherwise your body will not switch to endogenous mode. If you even drink tea/broth etc it will be just diet and not fast. All this has been documented by scientists and Doctors around the world. There is a HUGE difference when you fast and when you fast properly. Anyone doing more than just IF should really do their research.

    1. Commenting on this old post! You are right. Even juicing is an entirely different dynamic than water fasting. This article is typically misleading on true fasting.

    2. Can you have minerals while fasting? (Magnesium, potassium, etc.)??

  51. I’ve done multiple 3, 5, and 7 day fasts. The 3 and 5 were water and the 7 had green tea with stevia and supplements. I found that much easier. Every successive fast got easier.

    The purpose was for rebuilding the immune system. Studies show at least 3 days is needed. I believe that long fasts to lose weight is a mistake. In every case, whatever weight I lost, I put back on. Once again, not the goal.

    Day one was the hardest hunger wise. On day 2, my energy was low and I felt I needed to rest. By day 3 I usually felt OK. By day 4/5, you feel wonderful. I broke at 7 just because the immune rebuilding effects only needed at least 3/4.

    Coming out of the fast is the most critical. After the 7 day fast, I made the mistake of thinking well if I also want to rebuild my gut after healed, I’ll take strong probiotics. I had bad diarrhea that instantly stopped when I cut the probiotics a couple days later.. I think it was too much and my system wasn’t ready to handle that. I instead spent a few days of gradually introducing veggie/fruit smoothies, lighter fish, etc… back up to meats.

    In every case, my auto immune issues (mouth sores, skin rashes etc…) improved or completely went away.

    I second many folks opinion that you should have a health reason for trying these. Losing weight is not one. Eat better, move more and possibly try ketosis if weight loss is your goal

  52. Ray Cronise is all about the long fasts now. Well, he’s still about cold, and really all about “Our Broken Plate,” his Kickstarter project to discuss our cultural perspective on food. But, he likes them long fasts.

  53. I’ve found this while on day six of my fast and I’m very pleased I did. I’ve been very low carb for ten years (T2 and the only carbs I get are from dairy and the small amount of greens I eat) and your article has cleared up some questions I had.
    So far, with home made bone broth and clear fluids, its been a walk in the park. Today has been the hardest but I still feel eating would be a choice, rather than a necessity.
    My current feeling is that I will want to over eat when I come off my fast, I’m planning my menu but eggs are top of the list 🙂
    One reason for doing the fast is to really deplete my glycogen stores and become utterly fat adapted but could any of you advise on fat/protein ratio as although I lost a huge amount of weight ten years ago, kept it off and hold non-diabetic BS, I’d like to improve my (really bad) insulin resistance by getting rid of more weight that has hung on.

  54. I’m on day 10 of my prolonged fast. I have tried everything to lose weight. This is the first time I have found something that seems to be working. It’s hard but simple. When I feel weak or hungry, I drink the bone broth, liquid minerals, and kombucha. I have added AlkaLime and Amino Acids from Young Living Essential Oils. So far, I have gone from a size 16 to a size 12, even though I’ve only lost 5 lbs. I do not have a specific goal, but am taking it one day at a time. The most amazing things are happening to me. I’m still eliminating junk on the 10th day of my fast, even though I have not had food. I’ve still been able to function as a house cleaner, walk 4 miles each morning, and do yard work. Urine strips have been especially motivating–it’s all about ketosis! Also, I find great encouragement from Mark’s Daily Apple, Jimmy Moore’s fasting podcast, Jason Fung’s YouTube videos, and the other YouTube postings from random people. I’ve been amazed with what I’ve been able to accomplish. I will make another post in a few weeks. This fast is helping to reset my body but especially my mind/cravings. This experience has helped me realize that I can be the master and not the slave to my body.

  55. I smell a bias study sited as the evidence for protein and mineral loss.. look at the the estimated results.. no actual measurements listed after the initial ones.. come on Mark .. no need to site bogus studies that only estimate the results.. read Dr Fungs book.. he sites plenty of read results .. protein loss spike for less than day.. can you say Autoghagy?

    Macro- and micronutrient losses and nutritional status resulting from 44 days of total fasting in a non-obese man.
    Jackson JM1, Blaine D, Powell-Tuck J, Korbonits M, Carey A, Elia M.
    Author information

    We wanted to establish and understand how the fractional losses of fat, fat-free tissues, and selected nutrients compare with that of body mass during a 44-d voluntary starvation (water only) and measurements of nutrient status.

    We used anthropometry, sequential measurements of urinary substances during the fast, and blood analytes at the end of the fast.

    At the start of the fast, body weight was 96.0 kg (20% fat) and body mass index was 28.36 kg/m(2). The changes in body mass and arm anthropometry and in the pattern of urinary excretion of creatinine, ammonia, sodium, and ketone bodies during the study were consistent with starvation. At the end of the fast, body mass had decreased by 25.5%, of which a quarter to a third was due to loss of fat and the remainder to fat-free mass, predominantly muscle. There was an estimated loss of 20% of total body protein, 20-25% of fat-free mass, and a greater fractional loss of fat. Total energy expenditure was estimated to be 1638-2155 kcal/d of which 13.0-17.1% was from protein oxidation. Differential losses of minerals in urine ranged from 1.2% of estimated initial body content for manganese to 17.3% for selenium and 40.5% for zinc. At the end of the study, plasma concentrations of zinc and vitamin B12 were increased, those of copper, selenium, and manganese were normal, and there was biochemical evidence of deficiency in thiamine, riboflavin, and vitamin K (prothrombin time).

    The data confirm and extend the available information on prolonged fasting in lean individuals and have relevance to the understanding of the physiologic responses to starvation and the associated homeostatic mechanisms.


    [Indexed for MEDLINE]

  56. I am not saying this to be argumentative, but, just to point out something that, maybe, you didn’t know. You said “When a slightly overweight, otherwise healthy man drank only water for 44 days, he lost 25.5% of his body mass. A quarter to a third of the loss was body fat, the rest lean mass—mostly muscle.”

    According to Dr. Jason Fung (and I know I heard it other places, too, from people’s personal experiences) there may be SOME muscle mass lost. But, when fat is lost from, say, a fatty liver, it generally registers as lost muscle mass.

    He said, and I believe, that many people actually gain a lot of muscle. But, when they get their results, it may show that they lost 30 lbs. and only 15 lbs. of it was what would be registering as fat. Because, a fatty liver and all those other fat deposits inside of us do not register as fat or muscle or bone. So, it could make someone think they lost 15 lbs. of fat and 15 lbs. of muscle, when maybe what they did was lose 30 lbs. of fat, or, 10 lbs. of water and 20 lbs. of fat, or, 1 lb of muscle and 9.5 lbs. of water and 14.5 lbs. of fat, or ….

    Well, he didn’t say ALL of that. Some of that was just by way of explaining things he was talking about, without quoting him, as I am not about to look it up.

    When I was purposely intermittent fasting, I got quite a bit stronger. And, even though it’s been a long time since I’ve done it, that strength has stayed with me. But, when I would get my results back, it would say that I lost muscle mass.

    I’m not someone who has so little muscle or spends so little time using it that I can’t tell when I’ve gained it or lost it. If you can pick up a 42 lb. bag of cat litter for a friend, walk it around their building and carry it up a flight of stairs or two, and, you couldn’t do that before you allegedly lost this muscle mass, then, you didn’t really lose the muscle. That’s just common sense.

    Nevertheless, right t first, when I got the results and it said I had lost a lot of muscle and barely lost any fat, I almost freaked. Fortunately, I had already heard this piece of advice about internal fat loss- oh and water loss, too, won’t show up as fat loss so it is often assumed to be muscle loss – and I considered both how my muscles felt and how much less tight my clothing was and realized something didn’t add up.

  57. I don’t understand why people keep saying there is always muscle loss during a fast. Whether it was intermittent OMAD, soft dry, hard dry or just plain water fasting, every time I have fasted I have increased my muscle mass.

    It’s when I eat food and then I get ill and can’t do much that I lose muscle. Especially when the cortisol issues kick back in and I can’t sleep.

  58. I did an involuntary fast of about 3-4 months during chemo for Stage 4 nonhodgkins lymphoma. The lymphoma was so rampant in my body that it squeezed my intestines so much that food could barely leave my stomach. I had an 8 cm mass in my chest and two 4 or 5 centimeter masses elsewhere, plus the radiologist later told me he’d never seen kidneys so coated. For months I was only able to get down two of the smallest jars of infant baby food – everything but blueberry sucks, by the way – and the same amount of water. About 5 ounces of food and 5 ounces of water per day. Every three weeks they’d bring me in for chemo and give me saline solution. A couple times I came in between to get some additional saline. That was it, aside from the many meds. The monoclonal rituxin was given most of the credit for my complete recovery, but the radiologist, who hadn’t been able to see my x-rays after the 6-session treatment, told me not to bother with radiation, because there would be too many targets – too many tight masses inside and studies showed no advantage in that case. But it turned out the radiologist was wrong. There were no masses inside. I think my oncologist considered it something of a miracle. I later saw that there have been mouse studies showing similarly amazing effects. The cancer had no sugar to feed on and it wasn’t until the end of the 5th treatment that I started to be able to consume a bit more – the best food I’ve ever tasted was dinty moore from a blender.

  59. I had very high blood pressure (245/156) as well as bad cholesterol. After a lot of research I decided on a long water fast. I did miss the comfort of food the first few days, but by the 20th day I felt fine, needed less sleep, my BP went down to 121/81, lost my belly bump and 18 lb.,the psoriasis that I had under my hairline on the back of my head – disappeared. I realized that so much of what I ate before was habit/comfort/desire rather than actual need. I now eat less – and my BP has remained normal.

  60. Wonderful advice on the supplements! Too many people are jumping into very extended “water only” fasts and getting in trouble (resulting in Er visits) due to electrolyte imbalance.

    Breaking an extended fast incorrectly is just asking for refeeding issues – anything from binge eating and refeeding edema to the life threatening refeeding edema.

    I completed a successful fast in excess of 120 days last year, supplementing with electrolytes and vitamins as well as some other supplements. Also drank coffee and herbal teas as well as lots of water.

    The breaking period went well with no issues as I followed a strict ketogenic protocol and also kept protein capped for the first 10 days.

    Again, wonderful advice – a long fast is not something that should be undertaken unless you have an experienced coach or are willing to educate yourself on how to fast safely.

  61. My first time water fasting I lasted two weeks and still worked full time. It made me feel amazing.

  62. I am in the 18 th day of a 21 day fast.
    Likely losing some muscle judging by the lack of fat on my body. My hope is that these final days will be used for autophagy and taking out the old irreparable muscle and other damaged tissue cell cells.

    FWIW My prostate seems to be much less swollen and I think this fast is the main reason why. That alone would be a good reason to continue extended fasts IF this is what is happening.

    This is likley a one time thing, That is unless some good science convinces me to tray a long one again.

    I definitely will supplement next time after reading this article!! Thanks Mark!

  63. I’m the biggest “alternate therapies” skeptic alive but did a ten day strictly water only fast a couple years ago to reset my weight and eating behaviour (studying and meditating on healthy mindful eating during that time). To my utter amazement and without having read about it before hand my rosacea disappeared, my chronic hip pain totally disappeared and I felt ten years younger for months. Days 1/4 were hard with fatigue and that vile mouth but no nausea or headaches. Drank tap water (rainwater tank) and about 2-4 litres per day . Stresses in my life returned and over two years the weight and the other symptoms slowly crept back in. Another four day water fast early this year did some improvements but unfortunately a heavy period stalled the fast (was intending 10 days) as I became weak and suspected anemia which I’ve struggled with in the past. Symptoms returned so I’m currently on day six of another ten day fast. Because of my previous fasts (in my opinion) day 1-4 of this fast were super easy (only very slight fatigue) but day five I crashed. Spent the whole day in bed, nausea on and off (5/10 intensity) and terrible leg cramps tonight. I have enough experience of the massive benefits of fasting to hang in there but at 47 years of age I doubt that I’ll do another 10 day water fast. My family are intermittent fasters and I intend to decrease my animal proteins a bit, do daily 18-6 fasting (eat only light lunch and healthy dinner) for 6 days per week. As usual I’m journaling and meditating and studying mindfulness and healthy habits this time. Feeling positive.

  64. I am diagnosed with complete 21 hydroxylase deficiency, and was creeping toward diabetes. I decided to fast a week before my last blood work. 7 days of water, vitamins and broth, because I have salt deficiency issues. My labs normalized, electrolytes normal. Weirdest of all, I made normal cortisol (I’m not supposed to be capable of making ANY) and my disease markers dropped more than half. It wasn’t bad, and the best labs I’ve ever had. If medically monitored, I’d recommend making the effort.

  65. You really should fact check your statement on loss of muscle during a fast. It is not in keeping with modern scientific studies that prove muscle loss is negligable. Your fat is what is being burned.

    Refer to Dr Jason Fung’s book on Fasting. He dispells old myths like this with real data.

  66. I think you need to read Jason Fung. This isn’t abherrent and you don’t lose muscle mass, growth hormone increases through fasting. You make it sound risky, when the science is there and it doesn’t harm people.

  67. Hi, I’ve done several 3 day casts and a few 7-8 day fasts over the past year for autoimmune issues. I love the taste and feel fab ( except for day 3 which is generally difficult). I feel 20 years younger during the fast and the autoimmune symptoms resolve usually for up to a couple of weeks after the fast and then start creeping back. I tend to eat a little on the fast in the first three days ( lingos FMD) but after that I’m not hungry.
    Down sides of the fast – I have lost significant muses mass . People can tell by looking st me. I have done a bit less excercise ( I get lost excercise fatigue ) but not a lot. I have also gained a little more fat. I’m a 47 year old female. I need to look at ways of restoring ithe mussel mass if I want to continue.the fasting.

  68. I do extended fasts every other week. 95hours – one cup of coffee a day, then only green tea. I haven’t felt weak or lightheaded at all. My RHR is normally in the low-to mid 60s. I could go much longer if my schedule permitted

    But 3 days AFTER I end my fast, my pulse starts RACING. Like 127 RHR. I am not having extra caffeine or any other stimulant that would impact it. I felt like I was going to pass out going up a flight of stairs 3 days after fast ended. I have tried adding in Na/K/P + trace minerals during the fast, wondering if it is a delayed reaction. No difference. It lasts about a week. I am not dehydrated – I am drinking over 150oz of green tea/day. My blood pressure is fine. No physical issues with refeeding. I take a fair amount of supplements when not fasting – so I should be good in that department not taking them for a few days. I have never had this happen otherwise. Heart has checked out. I feel great during fasting, sleep better, energy better, mental clarity better… as well as weight loss. But the backlash afterwards is concerning. Thoughts?

    1. jane what you had is called refeeding syndrome, it can be fatal, I had it too, I had made alternate fast for years, so I thought a 3 day fast would be safe, the problem is that after the fast, I binged, my heart raced to 110, and I ended in the hospital where they didn’t understand what i had, before I had influenza, that left me with a weakened inmunitary system, that is why I made the 3 day fast

  69. I hear a lot about 5 day fasts. What is so important about 5 days?

  70. I’m on my second day of what I hope will
    Be a four or five day fast. My first time to try it.
    My father has alzheimer’s Disease so I am very interested in getting autophagy as well as ramped up stem cells. I’m at 46 hours and though I may be a little tired and I slept only four hours last night, my only major symptom is a head ache which seems to get better when I add exogenous ketones to my water.

  71. Forty eight hours is my personal limit thus far. I just finished my second two day fast and my blood sugar is responding beautifully.

    After reading all I could from Dr. Jason Fung, I was hyped to work toward an eleven to twenty one day fast but my experience with the shorter fasts shows me that they are beneficial and certainly easier. I’m all about the easy way which is why I have metabolic problems now.

    I’m going to keep trying to fast longer while watching my blood sugar, weight, and blood pressure. I’ll limit myself to one try per month, and if I work up to more than a week, I’ll decrease the frequency to three or four times per year.

    I can see the benefits of the shorter fasts now so if two days is all the further I get, I’ll be happy.

    I doubt I would would have ever tried this without the internet or Mark’s Daily Apple. I truly believe the internet (and Mark) have, if not saved my life, greatly improved my quality of life – thanks, Mark.

  72. I did try a long fast this year in June. I fasted for five days. The reason was that I had heard from a doctor that my kidneys’ ability to reduce creatinine wasn’t as good as it should be. Then I read about this study, where a group of men fasted for five days, and creatinine clearance increased with up to 167%.


    Two weeks after the fast I saw another doctor due to kidneys’ reduced ability, and the value was fine again. But she thought such a great reduction couldn’t be because of a fast, and her guess was that they had gotten the blood test wrong the first time. Anyhow, all other values were also fine, after the fast. (I fasted on my own initiative, not because any of the two doctors had told me.)

    I also read that creatinine can temporarily be increased right after you have had meat, and I had had a large steak just two hours before I had the first blood sample taken. (The first doctor didn’t even ask about that. Or about any possible medication that can increase creatinine.)

    Then some time after that I talked to a third doctor, who was very clear on the point that high creatinine doesn’t matter anyway. So I had worried for nothing. (And been worried by the first doctor for nothing.) This third doctor remembered this point specifically from a seminar about kidney disease.

    So there you have the reason for my five-day fast, and the result. Or maybe, as the female doctor thought, they had measured wrong after the first blood test. As for the fast itself, it was tough. I kept thinking about food throughout the day. I had fasted for three days before, several times, but this was much tougher.

    When it was time to eat again, I noticed that the taste was more bitter than usual. It was the same for anything I ate – I had both meat and eggs and candy, I recall. It was a very odd sensation.

    I also lost four kilo. But if you fast for longer than about two days, much of the weight loss will be muscles. Still, it was fun to get down to a weight that was much lower than I’d been the whole year.

  73. Thank you for this post. I literally just started my first fast 24 hours ago. I decided to hit my system with a 48 hour fast to start the program off. From now on my goal is to maintain 36 hrs of fasting followed by 12 hrs wherein I will ingest around 1500 calories. I am almost 300 pounds and my BMI is practically off the charts at almost 43% I have to take this very seriously, have researched quite a bit, have gotten all the appropriate supplements, food, as well as a GC/Ketone testing kit, a scale, and for at least a month subscribed to a great app I found. I feel very strongly in the goals I have set for myself, just a bit worried that I missed something or it wont work properly because of too much of this or not enough that. Anyone out there attempted a schedule like this? Am I totally nuts or is it feasible? If you use blood testing supplies, what procedure did you use, how often do you test yourself? I just want to be on top of my progress and give myself the best chance. Thanks for your time and BEST of luck to you!

  74. The whole issue of muscle loss from prolonged fasts is confusing.

    1) There is a lot of evidence that some muscle loss occurs in the first 48 hours, and then drops off to very low levels–or reverses.

    2) Everything else held equal, however, large bodyweight loss results in some muscle loss just because the muscles are carrying around less weight. (There are possible countermeasures.)

    3) Before and after body comp measurements with most tools are deceptive. Visceral fat and fat in organs often measures as ‘lean mass’ and hugely overestimates loss of lean body mass.

  75. Tim Ferriss is a farce. Distance yourself from him. If Ferris said the sun is going too rise tomorrow I would not believe it until it did.

    1. I’d love to get an idea of why you think this? While I’m not a massive fanboy of his, he seems to be very thorough and honest in what he tries and puts out to the public.

  76. I have microscopic colitis and am in the middle of a flare up – diarrhea going on 4 weeks – not about to stop – would a fast of mineral water or water with potassium citrate and sodium in water help to reset my gut and help to end these bouts of diarrhea

  77. Hi Mark,
    I’ve done a number of water only fast over the last year. The longest I’ve done is 12 days. I’m currently five days into a fast that I may go as long as three or four weeks. Every fast I’ve done has just been a little different each time. This one that I’m in right now is probably the most comfortable one I’ve done. At five days I have very little hunger. What is there is easily dismissed. I probably spent 30 hours of reading on various people who promote water fasting. True north in Santa Rosa was an option I was looking at but couldn’t afford. So I struck out on my own and tried it. My goal when I’m done is to be a whole foods plant-based vegan. I’ve been a vegan for eight years but unfortunately I have a fair amount of processed food just because I love to eat. Once I’m done water only fasting I’ll switch to a intermittent fasting regimen. I liked your article it was good! Thank you.

  78. Having read this I believed it was rather informative.
    I appreciate you finding the time and effort to put this content together.
    I once again find myself spending a lot of time both reading and
    commenting. But so what, it was still worthwhile!

  79. Great article thanks! There is so much content put there that is either ‘fasting is bad’ or ‘fasting is the best’ with a whole heap of points to justify their position. This is such a well balanced article on the benefits and possible risk factors to be aware of.
    I really liked the last section, it is really important to question why we are doing something and whether it is the best thing for us at that time.
    I am mostly interested in reasonably regular shorter 4-5 day fasts so I don’t think the longer term nutrient deficiencies are a huge issue but there are a lot of good takeaways from this info.
    I’m about to undertake my 3rd 4 day fast (1 per year so far) but would like to work up to doing it every couple of months.

  80. I’m 6’-2”, 67 year old male. Fasted for five days. Wanted to fast for longer but was obsessing so much about eating I broke the fast.
    Went from 311 lbs. to 291 lbs. Felt terrific. Almost no hunger pangs until the “wall”. Swelling in feet and ankles almost completely disappeared. Hadn’t seen my real feet in years. Belly was on the run.
    When I broke the fast, within I’d say an hour, has a bout of liquid out the bottom, with some initial solids. A little concerning but it was short lived. Within two weeks I was back up to 305 lbs. Swollen feet are back, belly is back. Seem to have lost all the positive gain. Am about to take another run at it. At the peak weight I’ve become almost debilitated.