14 Scenarios When Fasting Might Be Your Best Approach

There’s a ton of talk about intermittent fasting in the ancestral heath sphere for general health and wellness as well as weight loss, but little indication of specific applications for the practice. Anytime you attempt a “radical” health practice like not eating, it helps to have a good reason to do it. That will not only give you something to aim for, but it will ensure you actually have a physiological justification for your experiment. Never go in blind.

What are some of the specific scenarios and conditions where fasting makes the most sense?

1. You Are Intractably, Morbidly Obese

It used to be that an accepted and well-tested fix for morbid obesity that was unresponsive to other methods was long term fasting.

One experiment was very long term: over a year of not eating anything except for multivitamins. (Disclaimer: I’m not recommending this approach, but it is interesting.)

Back in 1965, an obese Scotsman of 27 years and 456 pounds came to the Department of Medicine in Dundee, Scotland, with a problem. He needed to lose weight. A (1/8 of a) ton of it. The doctors suggested maybe not eating for a few days could help. It was just an offhand recommendation, but the Scotsman really took to it. He stayed at the hospital for several days, taking only water and vitamin pills while undergoing observation to ensure nothing went wrong. When his time was up, he continued the fast back at home, returning to the hospital only for regular monitoring. After a week, he was down five pounds and feeling good. His vitals checked out, blood pressure was normal, and though he had lower blood sugar than most men, he didn’t seem particularly impaired by it. The experiment continued… for 382 days.

Yes, AB fasted for 382 days, drinking only water and taking vitamin, potassium, and sodium supplements. All told, he lost 276 pounds, reaching his target weight of 180 pounds and maintaining the bulk of his weight loss. Over the five following years of observation, AB regained just sixteen pounds, putting him in excellent, but underpopulated territory (at least 80% of dieters eventually regain all the lost weight).

2. You Want the Benefits Of Ketosis Without Having To “Go Keto”

One thing a fast of sufficient length will do is throw you straight into ketosis. Humans are so wired to go into ketosis that a simple overnight “fast,” aka sleeping, will do it.

Then, when you do eat, you have more wiggle room on carbs because you’ve just spent plenty of time in ketosis during the fast. This isn’t the same thing as going keto, but then again, not everyone wants to be in ketosis all the time. Many benefits come from “dipping in and out of ketosis” on a regular basis, and regular intermittent fasting certainly qualifies.

3. You’re Otherwise Quite Lean, Active, and Low-Stress and Just Have a Little Bit To Lose

Fasting can be a stressor. Going without food tends to do that in organisms that rely on food for sustenance. It’s just that in the context of an overall low-stress lifestyle and low-oxidative stress physiology, it can be a positive stressor—a stressor that promotes strength and adaptation.

This is why women, in general, tend to have a tougher time with long term fasting. They are inherently more vulnerable to nutritional stressors since they have to be prepared to carry children to term and nurse them, two functions that require a steady source of calories. Biologically speaking, that is.

4. You Want a Buffer Against Degenerative Diseases

Now, this is mostly speculative. This isn’t medical advice or a guarantee of any kind.  There’s good reason to believe that regular extended fasting (or at least skipping meals/multiple meals on a regular basis) can reduce the risk of degenerative diseases and perhaps even extend life by triggering the autophagy pathway that cleans up damaged cells and keeps pre-cancerous cells suppressed.

Will this ensure you don’t get cancer down the line or die earlier than is your potential? No, not at all. But it’s a relatively easy thing to try with no downside, and it just might help.

5. You Want To Lean Out and Gain Muscle At the Same Time

The classic Leangains-style intermittent fasting with regular strength training is one of the best ways I’ve ever found to gain muscle and lose body fat concurrently. You follow a shortened eating window every day—usually 16 hours fasting, 8 hours eating—and on workout days end the fast with a strength training workout, then eat. Classic Leangains has you eating lower fat, higher carb on workout days and higher fat, lower carb on rest days, with protein kept high throughout. But it should work on whatever macro combination you prefer.

You won’t gain as much muscle as quickly as if you ate enormous meals all the time, but the gains you make will generally be leaner.

6. You’re Recovering From Major Gut Issues

A friend of mine just did a 5-day water fast to reset his gut biome after SIBO and/or a parasitic invasion. It fixed him right up. And whenever my dogs have ever had digestive upset, like diarrhea or something, I’ll throw them on a two-day fast and they bounce right back.

I think the gut needs periodic “resets” to stay in top shape. Give it a rest, have nothing go through demanding its attention for a couple days, and allow things to balance out. Just like someone who trains all the time can really benefit from a deload week, a digestive system that’s constantly digesting and processing food can benefit from a day or two of rest.

7. You Want To Control Blood Glucose Levels

In men with an elevated risk of getting type 2 diabetes, intermittent fasting without consciously changing what or how much they ate improved blood glucose levels. They either ate from noon to 9 PM or from 8 AM to 5 PM, so a solid nine hour eating window was enough to trigger improvements. That they didn’t change what they ate suggests that irrespective of the quality or quantity of the diet, simply not eating for 15 hours a day will improve your metabolic health.

Dr. Jason Fung consistently uses intermittent fasting in his patients with type 2 diabetes, so the potential for powerfully therapeutic effects for even full blown type 2 diabetes is quite high.

8. You Only Have Access To Terrible Food

When I travel for business, which is quite often, I tend to fast. Airports are getting better, but it’s still a sad state of culinary affairs. I usually have a few choices: I can pick at a wilted Caesar salad with flaccid chicken breast. I can eat some congealed beef patties from whatever fast food joint has set up shop in the terminal. I can drop $30 for a mediocre steak. Or I can just fast.

I usually choose the last option. At this point in my life, I refuse to put substandard food into my body, especially if it isn’t even very delicious. I’d rather just skip the food entirely and have a great meal when I arrive.

9. You Can’t Stop Snacking

Total freedom is hard for some people to manage. Even if the food is high quality and Primal or keto or whatever, constant access to eternal amounts of it is hard to turn down. Snacking happens. Again and again. Sometimes, we need to put up barriers to manage that freedom, to make it work. After all, paradise is a walled garden, and erecting the artificial eating barrier of a full-on fasting day (or two) or a compressed eating window will allow you to overcome this. If this describes you, a fasting regimen just might be the trick to work.

Plus, many people find that forcing yourself to not eat for an extended amount of time on a regular basis upregulates fat burning machinery and allows better eating habits and reduced snacking when you do go back to normal eating.

10. You’re Willing To Try an Unconventional Recovery Technique

When Dude Spellings was on the podcast, he relayed a wild story about racing 50 miles through the Grand Canyon in a (mostly) fasted state, being greeted at the finish line with a stack of pizzas, and instead of wolfing down with all the other competitors, continuing the fast through till the next day—theorizing that in his exhausted, inflamed state he could use the benefits of cell repair and anti-inflammatory processes enhanced by fasting. He woke feeling less stiff and sore than his previous crossing 13 years prior.

11. You’re Trying To Avoid Jet Lag

Another reason I often fast when traveling is to establish a new circadian rhythm aligned with my destination. By waiting until the morning after my arrival to eat, I take advantage of one of the most powerful stimuli, or zeitgebers, for establishing a new circadian rhythm: food. Eat a big meal in the morning, and your body “knows” it’s morning—biologically speaking.

How this looks:

I arrive at noon in the new location, which feels like nighttime for me. Instead of eating a big “lunch” and collapsing into bed, I spend all day staying active and fasting. I skip dinner. I walk everywhere. Then, in the morning, I get a workout in, preferably outdoors to get natural light exposure, and follow up with a big breakfast. That combo—the light, the workout, and the breakfast eaten at my desired breakfast time in the new place—sets my internal clock and minimizes jet lag.

12. You’re a Shift Worker

Shift workers are at an increased risk of many diseases, like diabetes and breast cancer, and a lot of this comes down to the disordered eating they’re often forced to engage in. They eat in the middle of the night, when their body wants to be sleeping, and in doing so throw their circadian rhythm out of wack more than it already is going to be.

If you’re primarily awake in the middle of the night but want to maintain a semblance of circadian rhythm, it makes sense to eat at normal dinner time and at the end of your shift, but not during. Fasting during your shift might just be the big breakthrough.

13. You’re Undergoing Chemotherapy

It’s common knowledge that calorie restriction can improve the response to chemotherapy while reducing the negatives. Fasting is just a more reliable, arguably easier version of calorie restriction. There’s even evidence that fasting can improve your healthy cells’ resistance to chemotherapy while reducing the cancer cells’ resistance while reducing negative symptoms like nausea and vomiting.

Don’t consider this medical advice, but do discuss it with your doctor. This info is resonating across the oncology world; it’s getting harder to deny that many patients can benefit from intermittent fasting.

14. You’ve Got a Massive Feast Coming Up

If you have a history of eating disorders, this is probably unwise. The feast/fast method can be taken to unhealthy levels, especially if it’s couched in feelings of body dissatisfaction or deep childhood trauma. But if these aren’t an issue and you have a one-off feast (like a holiday dinner) you simply want to really dig into, fasting for a day before the big feast can enhance the effects of the feast.

Whenever I hit the Brazilian all-you-can-eat BBQ joint, I’ll fast for at least a day—just to get my money’s worth and really develop that insatiable, salivating, Primal urge to eat meat. Hunger is the best spice.

These aren’t even all the scenarios where fasting helps or makes sense. There are others, which is where you come in. What have been your reasons for fasting? Has it worked?

Thanks for reading, everyone. Take care!

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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27 thoughts on “14 Scenarios When Fasting Might Be Your Best Approach”

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  1. Hi Mark, I’ve enjoyed reading your article about fasting. My wife and I started intermittent fasting on a plant based keto diet…yes…extreme and maybe overkill. Our journey began Jan 5, 2019. First 30 days were 16/8 and then went to 18/6 until a few weeks ago. We wanted to give our bodies a couple weeks to reset. I lost over 70lbs and wear a 32” waist in pants (high school days) and my wife lost 40lbs and has not looked this fit in years. Besides the benefits of losing weight and fat… we feel better than we EVER have. We both have the energy to work out 5-6x a week, our blood work impresses our doctor and our mental clarity and focus have been amazing. I’m 55 and my wife is 52 and we feel like kids. You’re an inspiration to me because I can always learn something of value from you and incorporate it into my own life. I appreciate all that you give and do for us.

    Best regards
    John n JoJo

  2. Mark, this post isn’t showing up on the blog. I could only access it through the weekly email.

    1. TGJ, I’m not sure if it was a temporary glitch, but it appears to be showing up. Thanks for your note. Best — M

  3. Last October 2018 I went 16/8 IF with my window 10am to 6pm. Just so happens that was the most pumping surf we’ve ever had in October. Chest to head high plus for 26 days straight.

    I get up at 4am and surf in the dark until the crowds drive me batty. I was doing 3 hour plus sessions every day. I’d get done, then maybe follow up with stair sprint or pushups, planking and pull-ups at home. Have some coffee, work a couple hours, then feast.

    The result was unlimited stamina and the fastest recovery time I’ve ever experienced. Usually on the 4th or 5th day you really start to feel all the hours of hard surfing but doing IF it took eleven days before I felt the fatigue and soreness. And I’m 53 years old!

    That was enough to convince me that all the science was correct.

  4. I would add that fasting is perhaps helpful when you need to be at the top of your game mentally. There’s increased mental clarity and focus.

  5. The jet lag advice is brilliant, and I will put that to use. I tend to travel in spurts and jet lag kicks my rear, killing my sleep patterns and such. This just might be the perfect remedy. Thanks.

  6. Meditation. Fasting is good for most types of meditation and breathing exercises.

  7. – To save money could be another scenario…
    – To get clarity and focus when you have a very important task…
    – To practice self-control when you are going through life storms…
    – To prevent yourself from overeating due to emotional disorders or life struggles…

  8. How does this relate to women and some of the negative physiological effects of fasting you’ve written about?

  9. I love this post, thanks Mark. Great to be reminded of all the benefits of fasting.

    I sadly can’t fast much these days. I’m so painfully thin after a number of crohn’s flares that my body responds badly to anything longer than an overnight fast of about 10-12 hours. My BMI is dangerously low (15) so my aim now is actually to get as much primal fuel in without discomfort as I can.

    I do remember the benefits I felt when I occasionally fasted (usually 16 hours overnight style) And felt it helped my digestion and I know it’s such a great tool when travelling especially. Hope to get a little bit of spare weight on so I can have the option again.

    On this note, perhaps you could do a balancing post of when it’s not beneficial to fast? Would be curious of other situations or health factors that might influence this.

    Merci beaucoup?

    1. When not to fast.
      When it is zero degree weather and you have to work outside. I caught a chill that cold not be stopped until I broke out the emergency canned stew, nuked it and ate it.

      1. I notice this too. I don’t have to work outside, but I often find myself really chilled while I’m cooking dinner (the only meal of the day for me), and I warm up a ton after I’ve eaten. I’m someone who’s chronically ran hot over my lifetime, so the chill is most welcome. I can wear sweaters!

  10. I would love to see a study on shift workers who are genetic night owls. Wondering if the same negatives regarding health would apply.

  11. Mark,

    That was a great short read on fasting. What are your thoughts on intermittent fasting during post-op recovery? In this case it was a total hip replacement.

  12. This is very interesting about shift workers. As a shift worker I have tried several different eating routines and I have read previously about a shortened eating window being beneficial.
    I typically follow eating no food until a small meal/snack about 4 hours into my shift (midnight) and then at 4-5am eat my main meal plus breakfast in am once I am home before bed and after exercise (10am)
    I often struggle more about when to get my exercise in as prior to work I can’t consistently wake up early enough and some mornings after a 12 hour shift I can feel quite drained and only get in my 3 mile walk as a minimum.
    (I have worked as a shift worker for > 10 years and don’t think I function or feel any worse than day shifters – only that some days off I struggle not to nap too long)

  13. I’d add that, beyond degenerative disease, for anyway I’ve experienced many fewer colds, minor illnesses since I began fasting three years ago. Maybe once a year, where before it seemed every couple of months or so I’d develop a cold. A single case study of one, but I’d say it’s been notable for me.

  14. “Yes, AB fasted for 382 days, drinking only water and taking vitamin, potassium, and sodium supplements”

    This is fat shaming, the way it’s presented. And it’s dangerous advice.

    According to Joel Fuhrman MD a person runs the risk of exhausting the energy of their heart muscle during a fast that lasts longer than 3 months. Dr Fuhrman uses fasting as part of his medical practice and wrote “Fasting and Eating for Health” (a low fat diet guide but the fasting section is extensive and well referenced).

    Even “aaroncohen” the Youtuber who fasted for 2x40days had a trip to the hospital with congestive heart issues and she ate a steak in between fast 1 and fast 2. I like the way she did her fast but many would say that if this is the only way to be thin again, they’ll live with the weight.

    About the fat shaming aspect… It’s implying that anyone who hasn’t done this hasn’t tried hard enough. The message is something like this:

    1. You’re obese and that’s BAD. (Puts the person off balance, makes them defensive)

    2. You need my advice because obviously you’re not smart enough or dedicated enough to win the battle by yourself. (Count the assumptions.. Also every learned helplessness lesson begins with you can’t do this without my help.)

    3. Even if you thought you tried hard before, have you tried THIS yet? (Preys on the desperation of a person who feels shame and guilt because you’ve set them up already to feel like that.) Where does it end? Next will it be… have you tried moving to the Yukon and living on elk and whale?

    “AB” is not in underpopulated territory, he’s had an unusual experience and everyone should be glad that he’s well now and he didn’t suffer some bad effect or other. But that doesn’t make it OK to imply that everyone who is obese should do like he did. Does AB even recommend it to others? Is he aware he’s being used as an example?

    An obese person constantly feels uncomfortable unless they actively work to accept their situation and love their body. I appreciate the people in the body positive movement, especially women like this:


    She uses her body just the same as anyone else while being obese. I’m very impressed by her book which pinpoints the social problems with obesity, and I’m impressed with her ambition.

    A large part of the problem is that everyone thinks they can solve obesity by just giving people the right advice. Maybe obese people can give you some good advice. Read what she has to say and then re-evaluate. Fat shaming is a more complex issue than it seems.

    1. I didn’t get a sense of fat shaming at all from the sentence you quoted, and I doubt that Mark intended it as such. AB’s choice of a weight loss method merely struck me as extreme and, yes, potentially dangerous. I didn’t construe it, even remotely, as advice.

      May I respectfully suggest that a good percentage of so-called fat shaming is, in actuality, more often a case of hyper-sensitivity on the part of the “shamed” individual? In other words, you can’t control what people say or think, or what you think they might really mean. So why let it bother you?

    2. “Fat shaming”?


      Mark is simply recounting an option for those who qualify as “morbidly obese”….. and THAT is a medical term.
      Nor does he recommend extremely long fasts, merely recounting an experiment which showed that it can be effective if done carefully.

      It’s worth noting that people have remained healthy for years while deriving their nutritional requirements from nothing more than animal body tissue and minor mineral supplements. Which is exactly what the subject of that experiment did. Except that he used his own.

    3. AB died in 1990. He holds a record for the longest fast, I believe. I did not get a sense of fat-shaming here at all, simply a recounting of the event,

  15. I used to really enjoy fasting before I had my son who is almost 1. Obviously I didn’t fast through pregnancy and I’m still breastfeeding, and assume it wouldn’t be sensible to fast whilst still feeding. Is there any way to find a balance and get some fasting in without it having a detrimental affect?

    1. I would wait if you’re still breastfeeding a lot. The general advice is not to do it while you’re breastfeeding, though I’ve read about women who did it. I did one or two 24 hour fasts due to a stomach bug, and was incredibly weak and shaky throughout, though I was sick. I couldn’t even manage a day without carbs while breastfeeding, I’d wake up extremely shaky and weak (though now I look back on it and it could have been blood sugar issues, as I was a sugar addict).

    2. I was able to lose some baby weight while still nursing by doing what I considered a “modified fasting.” Rather than eating just because it was mealtime, I would wait until I was truly hungry. Then I would sit down for a meal and eat until satisfied. Then no snacking. Once I was truly hungry again, I’d have another real meal. Stuck to a strict primal diet, so I didn’t have any wild blood sugar fluctuations. This gradually and natural evolved into having “brunch” around 10:30 or 11am and then dinner around 6. I felt great and lost about 15lbs.

  16. Thank you. Would you be able to differentiate between ‘fasting’ and ‘skipping meals’ and is there benefit to fasting ‘as needed’ or does fasting need to be intentional and consistent to be effective?

  17. One of my favorite article you have written. Thank you!

  18. Hey Mark – thanks for the tips. Very helpful as always. Quick clarification on #14 – so is it OK to eat more than usual the day after a fast? I wanted to make sure I wasn’t doing my body any harm by over feeding the day after fasting. Should the first meal be something light, and than go for it?? Thanks for your time!

  19. #15 You are supposed to drink “clear fluids” on the day before a colonoscopy and so either you load up on sugar–laden fruit juice and sports drinks OR you just fast while drinking plenty of water