The most important thing – practicing fasting safely


As we start the new year, many people are looking to lose weight. Fasting, the voluntary abstention from eating is an ancient weight-loss method with a long track record of success. However, many people forget the cardinal rule of fasting, or indeed, of any kind of dietary change – always make sure you are doing it safely.

I highly recommend a great new documentary now available for purchase/rent online called Fasting – The Movie. It presents the science of fasting and how people can lose weight and even reverse many metabolic diseases such as type 2 diabetes and its related conditions. Given the importance of these diseases in modern-day medicine, this is nothing short of revolutionary. However, taken to an extreme, fasting can also have its dangers.

This is true not just for fasting but for anything. If you take veganism to an extreme, you may put yourself in danger, for example, of vitamin B12 deficiency. If you take the low fat diet to an extreme, you are at danger of vitamin D deficiency. If you take salt restriction to an extreme, you may be at danger of volume depletion. If you take exercise to an extreme, you may put yourself at danger of rhabdomyolysis (muscle breakdown). Everything must be done responsibly, with knowledge and with common sense.

Fasting is no different. Because fasting is already more intensive than most diets, taking fasting to an extreme can be problematic. The movie goes into some of the dangers of fasting and explores many of the variants of fasting that are popular and may be beneficial for people. Simply put, fasting is a weapon to be used in the fight against obesity-related conditions and perhaps some aging-related conditions.

But, like any weapon, it has two edges. It has real power, and that power can be used constructively and it can also, in the wrong hands, be used destructively. It is all a matter of context and applicability. Much of the resurgence of interest in fasting as a therapeutic option centers around intermittent fasting – generally of shorter duration done consistently and frequently. The 5:2 diet, popularized by Dr. Michael Mosley is 2 days of fasting per week, but those ‘fasting’ days still allows 500 calories per day. Time-restricted eating such as a 16:8 schedule, allows you to eat during only 8 hours of the day, so 16 hours are spent fasting. Many of the patients in my Intensive Dietary Management program use 24 hour to 36 hour fasts 2-3 times per week, and this is done under medical supervision with their physician.

Certainly I use extended fasting as well, but generally limited to 7-14 days, only in the appropriate person and with supervision. Clients are always instructed to stop immediately if not feeling well, and we also check in with them regularly. Longer fasts have more power, but more risk. To me, there is no reason to fast for 30 consecutive days just for the sake of argument. Why not do 4 separate 7-day fasts instead? It will have roughly the same beneficial health effects with far less risk.

Can extended fasts get you into trouble?

By contrast, in Fasting – the movie, director Doug Orchard tells the story of a young lady who decided to join a 30 day water-only fasting retreat. As far as I can tell, there was no medical supervision, and there was no bloodwork being checked and there was no professional who even determined whether this was appropriate. One of my primary rules for fasting is that if somebody is underweight or there is concern about malnutrition, then they should not fast. Underweight is defined by Body Mass Index < 18.5, but for a margin of safety, I don’t recommend that anybody fast longer than 24 hours if they have a BMI< 20. The reasoning seems rather obvious. During the fasting period, the body must survive on the nutrients and energy stored. If you have lots and lots of body fat (stored food energy), then you should be fine. If you do NOT have lots and lots of body fat, then it’s NOT fine.


People get into trouble with extended fasts because they don’t follow common sense. Many of these fasting retreats offer 30 day water only fasts. If you become depleted of sodium (quite common), there are no doctors there to monitor for warning signs. If you become very weak and unable to get out of bed, there is very clearly something wrong, and you should not continue fasting. This is common sense. In my IDM program, clients know that they may feel hungry, maybe a little irritable, constipated perhaps, but they should not feel UNWELL. If you are really feeling poorly, you must stop. There is no reason to continue, because fasting is free. It is far better to stop and try it again (if you want) in a few days when you are feeling better. The problem with these fasting retreats is that people have paid money to be there and therefore they push on far beyond the limits of good safety practice and far beyond the limits of common sense.

Further, people undertake extreme fasting without any kind of preparation. Instead of undertaking shorter fasts and gradually extending it, they immediately opt for a full on water-only extended fast. This is like a rookie mountaineer that decides that he/she will tackle Mount Everest, without oxygen and push on to the summit irregardless of weather. The experience mountaineer would immediately recognize this as a death wish, but the rookie has no inkling of the dangers and may come home in a body bag. Yet fasting clinics promote this very same idea. Taking the most extreme fast (water-only fasting, as opposed to allowing some bone broth or some caloric intake), to an extended period of time (30 days instead of 1-2 days), in anybody irregardless of whether this is medically appropriate, without any adequate medical supervision or access to bloodwork? I can tell you right now, that’s not a smart idea.

A recent article in the New York Post “Is this the most dangerous diet ever?” tells of a man who, in an attempt to lose weight, decided that he would fast for 47 days. At day 5 he was feeling great. At day 28, he was so weak that he could hardly get out of bed. NOT smart. This is NOT something that I would advise. The Daily Mail shared similar concerns in its own article.

Consider the story of the marathon. According to legend, in 490 BC the Greek soldier Pheidippides ran approximately 26 miles from the battlefield near the town of Marathon to Athens to deliver the news of the defeat of the Persians. He shouted Niki! (Victory) and then promptly keeled over and died.

Suppose a sedentary, middle-aged, out-of-shape man decided to run a full 26 miles at maximum speed tomorrow, without any type of preparation or knowledge. He might very well keel over and die, too. Indeed, in 2014, a 42-year old man died after the London Marathon, the event’s second death in 3 years. Later that year, a 31-year old man and a 35-year old man died in a North Carolina event. Since marathon is a relatively extreme event for most people, it takes some preparation to do safely. That’s easy to understand so you don’t see hysterical headlines saying “Running, the most dangerous thing ever”. If you want to run a few minutes, that probably won’t kill you. Running a marathon in an untrained state could very well do so.

So the bottom line is that fasting, done properly and with knowledge and experience is a powerful tool in the fight against metabolic disease and obesity. But tools can cut both ways and can sometimes harm the user. A chainsaw is a powerful tool for cutting trees. It may also kill you if used improperly. But the proper lesson is not to abandon the chainsaw. Instead, we need to learn how to properly use the tool. Fasting, used responsibly can be a powerful force for health. Fasting, used inappropriately can hurt or kill you. Fasting by starting out skipping a meal here and there – good idea. Fasting by starting out 30-day water-only fast come hell or high water – bad idea. You can read more about fasting in Diet Doctor’s evidence-based guide. Be safe, everybody.

Dr. Jason Fung

Do you want to read more by Dr. Fung? Here are his most popular posts:

  1. Longer fasting regimens – 24 hours or more
  2. How to renew your body: Fasting and autophagy
  3. My single best weight loss tip

Intermittent fasting

More with Dr. Fung

All posts by Dr. Fung

Dr. Fung has his own blog at He is also active on Twitter.

His book The Obesity Code is available on Amazon.

The Obesity Code

His new book, The Complete Guide to Fasting is also available on Amazon.



  1. Eileen Mynes
    Thank-you, I no longer feel bad about not fasting for as long as I thought I should have been able to do. When you put it in terms of training for a marathon, it suddenly makes sense that I have to build up to longer fasts. Currently, I love the 16:8 routine I have, and shall work towards combining it with a 5:2 schedule.
  2. Kathryn
    Good luck renting the movie. At this point they may have charged me 3-4 times and I still can’t access it. Hope they refund my money as I requested.

    For my diabetes, I try to get in a 36-48 hr fast every week. I actually aim for 72 hrs in my mind, but I keep remembering to listen to my body which has had enough at about 48 hrs. I think the bottom line continues to be to listen to your body, you shouldn’t feel sick. Combined with common sense. I’m not over weight and only have so much fat to draw upon, making long fasts not a good choice for me.

  3. Marion
    Would it be okay to have bone broth on a fasting day?
  4. Asheea
    Loved this article! Very clear with great metaphors to get the point across. Thx for your research.
  5. Elizabeth
    irregardless isn't a word :)
  6. Tara Dantouze
    Thank-you for the caution and thank you for alleviating my fears. I am 5'2 at 202 lbs and I am on a 4 day water/coffee/salt/tea fast. I have been doing intermittent fasting for 1 year, and dabbled in alternate day fasting (36 hours) for 7 months, so I feel experienced when it comes to my fasting muscles. However, since I can't get rid of my dang sweet tooth, i can't get rid of the weight either (i need to think of it like megan ramos "in my past life, i would eat.."). I am excited to finish my 4 day fast, but also dread the idea of not eating for another 2 full days, but at this point, its mostly psychological. But at the same time, i am excited to do more extended fasting in the coming weeks. Sorry if this reads like a journal but you are inspiring, Megan, Jason and Jimmy.
  7. Doug Fishback
    One of the moderators on the Facebook group claimed you should do a 7-day fast no more than twice a year. When I quoted page 260 of The Complete Guide to Fasting, she said the book was "dated" and made some other dubious unsourced claims about extended fasting. When I respectfully said I didn't buy it, she deleted my comment. So I left the group. If you can find the time, you might want to clean up your moderator situation over there.
    Reply: #8
  8. Kristin Parker Team Diet Doctor

    One of the moderators on the Facebook group claimed you should do a 7-day fast no more than twice a year. When I quoted page 260 of The Complete Guide to Fasting, she said the book was "dated" and made some other dubious unsourced claims about extended fasting. When I respectfully said I didn't buy it, she deleted my comment. So I left the group. If you can find the time, you might want to clean up your moderator situation over there.

    In the Diet Doctor community group, we can not offer support for fasts longer than 24 hours. In this article, Dr Fung even specifically mentions that he recommends medical supervisions for the longer fasts which is out of the scope of the Diet Doctor community group. I am not showing an interaction with you in the Facebook group; if you do have concerns about a moderator please feel free to reach out to

    Reply: #9
  9. Doug Fishback
    Thank you, Kristin. I need to correct myself and apologize. My bad Facebook experience was with a moderator of The Fasting Method Network group, which I don't believe is affiliated with DietDoctor. If you like, it might be helpful to delete my earlier post. I appreciate your quick response. Keto + IF is a remarkable tool; I'm happy you're here as a resource.

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