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Let me introduce myself. My name is Mark Sisson. I’m 63 years young. I live and work in Malibu, California. In a past life I was a professional marathoner and triathlete. Now my life goal is to help 100 million people get healthy. I started this blog in 2006 to empower people to take full responsibility for their own health and enjoyment of life by investigating, discussing, and critically rethinking everything we’ve assumed to be true about health and wellness...

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April 25 2019

Dry Fasting: Is It Worth It?

By Mark Sisson

Today’s post is about dry fasting. I’ve covered plenty of other aspects of intermittent fasting, including recommendations around longer fasts, but lately I’ve gotten enough questions about this particular angle that I thought I’d address it.

Dry fasting is going without both food and fluid. That means no coffee, no tea, no broth, and no water or liquid of any kind (except the saliva you manage to produce). It’s an extreme type of fast whose fans and practitioners are adamant that it can resolve serious health issues. But does it? Is it safe? And what kind of research is available on it?

Where Does the Idea of Therapeutic Dry Fasting Come From?

The main proponent of dry fasting is a Russian doctor named Sergei Filonov. Filonov is still practicing from what I can tell, somewhere in the Altai mountains that span Central Asia. I found a very rough English translation of his bookDry Medical Fasting: Myths and Realities. Difficult to read in full because it’s not a professional translation, but manageable in small chunks.

His basic thesis is that dry fasting creates a competitive environment between healthy cells, unhealthy cells, and pathogens for a scarce resource: water. The dry fast acts as a powerful selective pressure, allowing the strong cells to survive and the weak and dangerous cells to die off. The end result, according to Filonov, is that the immune system burns through the weak cells for energy and to conserve water for the viable cells, leading to a stronger organism overall. He points to how animals in nature will hole up in a safe, comfortable spot and take neither food nor water when recovering from serious conditions, illness, or injuries that prevent them from moving around. But when they’re able to move while recovering from more minor issues, they’ll drink water and abstain from food. I’m partial to this naturalistic line of thought, but I don’t know if the claims about animal behavior during sickness are true.

Another claim is that dry fasting speeds up fat loss relative to fasts that include water. There may be something to this, as body fat is actually a source of “metabolic water”—internal water the body can turn to when exogenous water is limited. Burning 100 grams of fat produces 110 grams of water, whereas burning the same amount of carbohydrate produces just 50 grams of water.

Are There Any Dry Fasting Studies?

Unfortunately, we don’t have many long term dry fasting studies. In fact, we have one 5-day study in healthy adults. For five days, ten healthy adults refrained from eating food or drinking water. Multiple physiological parameters were tracked daily, including bodyweight, kidney function, heart rate, electrolyte status, and circumference of the waist, hip, neck, and chest.

Participants lost weight (over 2 pounds a day) and inches off of various circumferences, including waist, hip, neck, and chest. The drop in waist circumference was particularly large—about eight centimeters by day five. Blood pressure, heart rate, oxygen saturation, sodium and potassium levels, creatinine, and urea all remained stable throughout the study. Creatinine clearance—which can be a marker of muscle breakdown but also a normal artifact of fasting—increased by up to 167%.

The most voluminous research we have on dry fasting is the Ramadan literature. During the month of Ramadan, practicing Muslims complete a daily dry fast—from sunup to sundown—every single day. They eat no food and drink no fluids during daylight hours, which, in the countries where Islam originally arose, run about 15-16 hours. These are shorter dry fasts than the 5-day fast detailed above.

What happens to health markers during Ramadan? Mostly good things.

A 15- or 16- hour dry fast isn’t very extreme, even in the hot climates of the Near East. Two or three day-long dry fasts, particularly in hot weather, is another thing entirely. What works and is safe across 16 hours might not be safe or effective over three or four days.

I wonder if there’s a genetic component to dry fasting tolerance, too. Have populations who’ve spent thousands of years in hot, dry, desert-like climates developed greater genetic tolerance of periods without water? I find it likely, though I haven’t seen any genetic data one way or the other. It’s an interesting thing to ponder.

Is Dry Fasting Safe?

Obviously, skipping water can be dangerous. While we’ve seen people go without food for as long as a year (provided you have enough adipose tissue to burn, take vitamins and minerals, and are under medical supervision), going without water is a riskier proposal. The number I’ve always heard was three weeks without food, three days without water, though I’ve never really seen it substantiated or sourced.

One reason I’m skeptical of “three days” as a hard and fast rule is that most cases of people dying of dehydration occur in dire circumstances. People are lost out in the wilderness, hiking around in vain trying to find their way back to the trailhead. They’re thrown in jail after a night out drinking and forgotten by the guards for three days. They’re spending 24 hours dancing in a tent in the desert on multiple psychoactive drugs. These are extreme situations that really increase the need for water. Your water requirements will be much higher if you’re hiking around in hot weather bathing in stress-induced cortisol and adrenaline, or dancing hard for hours on end. Very rarely do we hear of people setting out to abstain from water on purpose for medical benefits, water on hand in case things go south, and ending up dehydrated. Part of the reason is that very few people are dry fasting, so the pool of potential evidence is miniscule. I imagine this last group will have more leeway.

Still, if you’re going to try dry fasting, you have to take some basic precautions.

6 Precautions To Take When Dry Fasting

1. Get Your Doctor’s Okay

Sure, most will be skeptical at best, but I’d still advise not skipping this step—particularly if you have a health condition or take any kind of medication. Diuretics (often used for blood pressure management), for one example, add another layer to this picture.

2. No Exercise

Avoid anything more intense than walking. For one, the hypohydration will predispose you to middling results, increasing cortisol and reducing testosterone. Two, the hypohydration may progress rapidly to dehydration. If you’re going to exercise during a dry fast, “break” the fast with water first and then train.

3. Keep It Brief

Yes, there was the 5-day study, but those people were being monitored by doctors every single day. I’d say 16-24 hours is a safe upper limit and probably provides most of the benefits (as Ramadan literature shows). Any longer, buyer beware. (And, of course, make sure you get fully hydrated in between any dry fasts you might do.)

4. Fast While You Sleep

Ramadan-style probably isn’t ideal from a pure physiological standpoint. The length (16 hours) is great, but the eating schedule is not. Those who observe Ramadan fasting ritual often wake up before sunrise to fit in food. They may stay up late to eat more. They go to sleep in a well-fed state, never quite taking advantage of the 8 hours of “free” fasting time sleep usually provides (and, of course, that’s not what their fasting practice is about). For a health-motivated dry fast, on the other hand, you should take advantage of it.

5. Take Weather Into Account

Hot, humid weather will generally cause the most water loss. Cold, dry weather will cause the least. Adjust your dry fasting duration accordingly.

6. Listen To Your Body

I’ve said this a million times, but it’s especially worth saying here. If you’re not feeling well during the dry fast, listen to your instinct rather than your agenda. (And don’t begin a dry fast when you’re ill. That should go without saying.) This is an optional tool. There are hundreds of other ways to serve your health and well-being. Don’t lose the forest through the trees because you’re drawn to a practice that feels more radical. Approach it smartly, but let your body’s intuition be the final arbiter.

That’s it for me. I haven’t done any dry fasting, not on purpose at least, and I’m not particularly interested in it for myself, but I am interested in your experiences. Do any of you do dry fasting? What have you noticed? What do you recommend?

As always, if you have any questions, direct them down below. Thanks for reading!


Mascioli SR, Bantle JP, Freier EF, Hoogwerf BJ. Artifactual elevation of serum creatinine level due to fasting. Arch Intern Med. 1984;144(8):1575-6.

Fernando HA, Zibellini J, Harris RA, Seimon RV, Sainsbury A. Effect of Ramadan Fasting on Weight and Body Composition in Healthy Non-Athlete Adults: A Systematic Review and Meta-Analysis. Nutrients. 2019;11(2)

Fahrial syam A, Suryani sobur C, Abdullah M, Makmun D. Ramadan Fasting Decreases Body Fat but Not Protein Mass. Int J Endocrinol Metab. 2016;14(1):e29687.

Aliasghari F, Izadi A, Gargari BP, Ebrahimi S. The Effects of Ramadan Fasting on Body Composition, Blood Pressure, Glucose Metabolism, and Markers of Inflammation in NAFLD Patients: An Observational Trial. J Am Coll Nutr. 2017;36(8):640-645.

Unalacak M, Kara IH, Baltaci D, Erdem O, Bucaktepe PG. Effects of Ramadan fasting on biochemical and hematological parameters and cytokines in healthy and obese individuals. Metab Syndr Relat Disord. 2011;9(2):157-61.

Saleh SA, El-kemery TA, Farrag KA, et al. Ramadan fasting: relation to atherogenic risk among obese Muslims. J Egypt Public Health Assoc. 2004;79(5-6):461-83.

Gueldich H, Zghal F, Borji R, Chtourou H, Sahli S, Rebai H. The effects of Ramadan intermittent fasting on the underlying mechanisms of force production capacity during maximal isometric voluntary contraction. Chronobiol Int. 2019;36(5):698-708.

Shephard RJ. Ramadan and sport: minimizing effects upon the observant athlete. Sports Med. 2013;43(12):1217-41.

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24 thoughts on “Dry Fasting: Is It Worth It?”

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  1. Dry fasting is super hard for me, whenever I don’t eat I get reeeeeaally thirsty.
    All of your mention of fasting with water that I’ve seen also includes coffee and other noncaloric liquids. What’s your take on the difference between liquid fasting and pure water fasting, with regards to their benefits/effects?
    I’ve listened to podcasts with Dr. Rhonda Patrick and she is a proponent of anything except water breaks the fast because the body has to metabolize it/non water consumption starts the body’s clock, etc. I agree with her work, it makes sense, and I’d like to see your thoughts on the topic of water only fasting.
    Also, when I try to intermittent fast, skipping breakfast, I can’t do it if I consume coffee because it makes me jittery on an empty stomach. But I did do it successfully for a while and actually felt energetic when I tried it with just water.

  2. The Yom Kippur fast is a 25-hour dry fast, which has been observed annually by Jews for millennia. I’ve done it many times. Other than a mild headache in the afternoon (more likely from caffeine withdrawal than dehydration), I have never heard of healthy people having adverse reactions.

  3. You know what other cells hold lots of water my beautiful brain not saying I wouldn’t try shorter dry fasts if I thought it might improve a health problem but right now I’ll pass but it would probably be 24-48hr fasts.

  4. The idea that animals retire to a safe place while recovering from illness or injury is a solid one. The question is, if it’s right to assume that the animal internationally stop eating to aid her in recovery. After all, by confining itself, the animal is unable to forage for food and water. What’s overlooked (or not mentioned in what I have read so far) however, is the fact that if an injured animal will continue to roam around instead of hiding while sick, it will make itself an easy target to predators- even if the sick animal happened to be a loin. Humans on the other hand needn’t be concerned about falling victim to predators when sick. Still, I can see why dry fasting is alluring to some and I experienced it in the past, in a religious context no more (: This days, I eat in a small window and I am fine with that.

      1. I think “internationally” was supposed to be Intentionally, lol.

    1. My cat retired to a safe, quiet place and died a horrible death. I wouldn’t take animals isolating and not eating or drinking as health advice.

  5. I was led to believe that there was a connection between insufficient hydration and kidney stones. Many pilots – whose anability to just stop, step out and relieve themselves on long flights leads them to severely restrict their drinking, consider kidney stones an occupational hazard.

    I’m also cautious when it comes to accepting comparisons with animals. Ruminants, with their high-volume digestive system, can take on more water at a single drink than we can, and maintain hydration comfortably over a 24hr period, even in warm climates.

  6. Hi Mark,
    keep in mind that severe dehydration can cause permanent damage to the kidneys.

  7. A timely article, Mark, especially seeing as Ramadan is right around the corner for me. I’ve always felt in need of your opinion on dry fasting specifically and how it fits into a primal lifestyle, trying to imagine how you yourself would do it!

    I dry fast annually during Ramadan, and sometimes during the year. I’m preparing for a month of 16-17 hr long dry fasts during the spring/summer. It’s more mental than physical to be sure, as it gets easier as one’s body adapts. It’s different this time, however, as I’ve done the keto reset earlier this year and am more metabolically flexible and fat-adapted.

    The challenge for me has been time management with feeding times and exercise. My practice in previous years has been to strength train hard (e.g. bodyweight and kettlebell training) an hour or so before breaking the fast at sunset with water and a moderate-sized meal, rehydrating during the night, and having a pre-dawn meal of mainly fat and protein, with some napping during the day to help stay awake and alert. Fortunately, I haven’t experienced much difficulty managing the above and it seemed to work out well, so it’s admittedly difficult for me to settle for mere walking, but I can appreciate your general advice and rationale from a safety and performance standpoint. I can also personally attest to the many pleasantly surprising benefits it confers which you’ve touched upon.

  8. I also observe the Yom Kippur fast, although I “cheat” and take a dose of Excedrin around 10 in the morning so that I don’t get the hideous caffeine withdrawal headache I did the first year. For me, it is actually easier to fast without water. I hear so many people say that drinking water inhibits hunger, but it’s exactly opposite for me. The water seems to wake up my stomach and remind it that it’s hungry!

  9. This is very interesting! I am always grateful to be exposed to new ideas and information from this blog…
    This makes clear sense to me. And the fact that water is released by fat breakdown is great to have, and explains a lot. This would enable the recovery of severely wounded animals in the wild – which we know can happen.
    I would like to point out that a Ramadan fast involves doing your normal activities while dry fasting – though only between sunrise and sunset. I think that if I try dry fasting, that would be the model I would try.
    And – for those concerned with the kidney stone risk – certainly you would not want to dry fast if that could be an issue for you. But since your body is not processing food during a dry fast, I would suspect that there would be less calcium or other minerals going through your kidneys at the time.

  10. I’ve been keto and fat adapted for awhile, along with IF. Great benefits. So when I learned about dry fasting it piqued my curiosity 🙂

    I’ve now done two 12-hour dry fasts a week apart, 6pm-6am. After the first one, I woke up and my chronic sinus congestion was gone. It’s Spring! Yet I can breathe easily, and am delighting to once again have a functional sense of smell. Hallelujah!

    Now, was it the dry fast or coincidence? I dunno, but the chronic congestion hasn’t returned so I’m happy.

    Also, this week I did like you recommend: broke the fast with water and re-hydrated before my workout. Felt absolutely fine for the hard workout.

    There are some claims that dry fasting is 3x more effective than water fasting, but for now I take them with several grains of salt. (Himalayan Pink Salt, of course 😉 )

    1. This is interesting. I do IF and I close my eating window, generally, by 6:00 and stop drinking any liquids by 7 or 7:30 (not for reasons of dry fasting, but so I can stay sleep all night and not have to wake up to pee). I then drink a cup of black coffee in the a.m. around…7 or 7:30. I just noticed today that I have not been taking my allergy meds – I forgot about them completely – and I am symptom free.

  11. I passed out after not drinking for a few hours in Hawaii. There’s no way I’ll not drink water for a long time on purpose. So many people die every summer in my country from dehydration! (They forgot to drink, or forgot to bring water with them outside).
    Just don’t do it, it will be a very stupid way to die

  12. I have dry fasted at least 6 times. for 24, 36, 48 and 72 hrs. I am a regular water faster and didn’t find it differcult. If you are interested there is a lot of information about the subject on You Tube or go to

  13. I do 3-day fasts every couple of months where I only drink minimal water. Once every year I do a 14 day fast where I drink only water. I intermittent fast every day where I don’t drink water for 18 – 19 hours and eat once per day some times maybe 2 meals but that is rare. This is pretty much every day except when I am doing a prolonged fast. I have done many dry fasts where I don’t drink water or eat for over 24 hours and I have never experienced any side effects. I am 47 years old. I have never had a headache in my entire life. I never have had back problems etc. I am fortunate in that regard. I have worked outside in the heat all day without drinking water and sweating a lot and have never suffered from severe thirst etc. The longest dry fast I did was 3 days with no water or food. I was thirsty after that but I never suffered from side effects. The thirst was minor. My body is accustomed to fasting I suppose. I have been intermittent fasting my whole adult life even before I ever heard the term intermittent fasting. Never any problems. As for losing muscle mass, I never have lost noticeable muscle mass. Fat yes but that’s it. Do I recommend this for others? I can’t say that. It’s just the way I have eaten most of my life and it hasn’t been a problem for me.

  14. I think dry fasting is superior to intermittent water fasting as it is easier to do once you get use to it. My longest dry fast was 63 hours, as soon as my lower back ached I broke the fast. I then began doing 1 24 hr dry fast per week and felt like I had plateaued. I now do 23hr Intermittent water fast with 16 hrs of it dry, 5 days a week. The dry fasting has defined my muscles and faded my wrinkles and strech marks. It takes a lot of mental strength going into the dry fast so the almost daily feels more comfortable. I started adding dry fasting into my regular intermittent water fasting 6 months ago and the physically results are astounding, I look 15years younger and my mental clarity and artistic ability have increased to levels I didn’t know existed in me. I recommend dry fasting but not without gathering as much information as possible first. Thank you for your post.

  15. Yet more crap written by people who don’t know a damn thing about dry fasting. I’m dry fasting RIGHT NOW. You don’t even get to “real” dryfasting until you hit the 72 hour mark when your body begins to break down your bodyfat and make metabolic water. 16 HOURS IS THE UPPER LIMIT? I’ve dryfasted 48 hours and walked 5 miles a day with no ill effects. Stop promoting weakass habits in people.

  16. I tied dry fasting the last couple of days. It was maybe a bit cheated, as I had my usual small cup of coffee in the morning. But basically last drank liquids around 8pm, had coffee at 6am and then meal at 4pm, consisting of fatty meat and steamed veggies and a beer (yes, I am a big cheater 😉 ). Otherwise no liquids or food inbetween.

    The observations:
    – Surprisingly I did not feel thirsty as I usually feel during water fast
    – My skin feels nourished and flexible, opposite to feeling of dryness on water fast
    – No constipation, which occurs often on water fast
    – No headaches

    – Little lightheaded but not as bad as on water fast
    – no cold feet. I actually feel my body to be warm inside, even thou the weather chilly (which in itself is a big help)