Short fasting regimens – less than 24 hours


This post is about shorter fasting periods – up to 24 hours.

There are many different fasting regimens. Let’s be very clear to start, though. There is simply no ‘best’ one. They all work to different degrees for different people. Just as some people prefer steak to chicken, there is no right or wrong answer. One regimen will be perfect for one person but a poor choice for another person.

Fasting is defined as the voluntary act of withholding food for a specific period of time. Non-caloric drinks such as water and tea are permitted. An absolute fast refers to the withholding of both food and drink. This may be done for religious purposes, such as during Ramadan in the Muslim tradition, but is not generally recommended for health purposes because of the accompanying dehydration. In our IDM program, we do not ever use the absolute fast, we always allow non-caloric drinks.

Disclaimer: While intermittent fasting has many proven benefits, it’s still controversial. A potential danger regards medications, especially for diabetes, where doses often need to be adapted. Discuss any changes in medication and relevant lifestyle changes with your doctor. Full disclaimer

This guide is written for adults with health issues, including obesity, that could benefit from intermittent fasting.

People who should NOT fast include those who are underweight or have eating disorders like anorexia, women who are pregnant or breastfeeding, and people under the age of 18.

Different durations

Fasting has no standard duration. Fasts can range from twelve hours to three months or more (usually not recommended!). You can fast once a week or once a month or once a year. Intermittent fasting involves fasting for shorter periods of time on a regular basis. Shorter fasts are generally done more frequently. Longer fasts are typically twenty-four to thirty-six hours, done two to three times per week. Prolonged fasting may range from one week to one month.

I categorized fasting periods with a break point at 24 hours, although this is somewhat arbitrary. In my experience in the IDM program, I generally use shorter regimens for those who are mostly interested in losing weight without much in the way of type 2 diabetes, fatty liver or other metabolic diseases.

For those who have more significant illnesses, I use longer duration regimens as they tend to give faster results. After the initial ‘breaking in’ period, we always work with the patients to find out what they prefer to do. A surprising number of patients prefer longer duration fasts less frequently.

Short fasting regimens

12-hour fasting


There are several regimens of fasting that use a shorter period of fasting but done every day. A 12-hour fasting period done every day used to be ‘normal’. That is, you would eat 3 meals a day from, say 7 am to 7 pm and refrain from eating anything from 7 pm to 7 am.

At that point, you would ‘break your fast’ with a small breakfast. This was pretty standard in the 1950s and 1960s. There wasn’t much obesity back then. However, there were two major changes that happened since then. First was the change to a higher carbohydrate, lower fat diet. This tended to increase insulin and hunger. Second was the increase in meal frequency, as we detail in a previous post, which tended to reduce fasting periods.

If you could eat unprocessed foods, avoid excessive added sugars, and did not have significant insulin resistance to start, this 12-hour daily fasting was probably good enough for most people to avoid obesity. However, it is not really strong enough to reverse decades of insulin resistance.

16-hour fasting


This regimen involves using a daily 16-hour period of fasting and an 8-hour ‘eating window’. For example, this would mean eating from 11 am – 7 pm, and fasting from 7 pm to 11 am. This generally means skipping the morning meal every day. Some people choose to eat 2 meals during that 8-hour window and others will eat 3.

Martin-BerkhanThis regimen was popularized by a bodybuilder by the name of Martin Berkhan who blogged about it on his website, and so the method is sometimes called the LeanGains method. He wrote extensively in the years 2007-2010 but I don’t see much activity on his blog anymore, which is a real shame, because he had some great ideas and original thoughts. There are still some excellent posts there.

He wrote extensively about training in the fasted state and eating predominantly in the post-workout period. These ideas are supported by the science available (albeit not a lot). However, they do make a lot of common sense.

Several years later, an editor at Men’s Health wrote a book called The 8-hour diet, which essentially used the same 8-hour eating window as the LeanGains method. In his entire book, he unfortunately avoided any mention of LeanGains or Berkhans.

The ‘Warrior’ diet (20-hour fasting)


This was one of the first diets to popularize intermittent fasting regimens. Written by Ori Hofmekler in 2002, this diet stressed that timing of meals mattered almost as much as composition of meals. In other words, ‘when you eat makes what you eat important’. Actually, I think both are important, but the ‘when’ question is seriously under-appreciated, and this book was one of the first to really point this out.

Drawing upon inspiration from ancient warrior tribes such as the Spartans and Romans, the core of the diet consists of eating all meals in the evening during a 4-hour window. The fasting period of 20 hours consisted of most of the day. There was also an emphasis on natural unprocessed foods and high intensity training.

What’s the point of incorporating daily fasting?

The main point of all these fasting regimens is the same. It allows the body to lower insulin levels very low for a specified period of time longer than usual. This is precisely what helps to treat, or prevent the development of insulin resistance. As we’ve mentioned previously, this is the fundamental biologic principle of homeostasis.

The body likes to maintain everything within a relatively narrow range. Any prolonged stimulus leads to resistance as the body tries to resist the change. In this case, prolonged periods of high insulin will lead to insulin resistance, which will lead back to high insulin levels – in other words, insulin can cause insulin resistance.

So, by incorporating daily, or almost daily periods of low insulin, we are able to prevent the development of insulin resistance and even reverse relatively minor levels of resistance. For more established resistance, we recommend longer fasting periods – 24 hours or greater.

One of the key advantages of fasting, from a therapeutic standpoint is the lack of upper limit. What this means is that we can continue to apply fasting in an almost unlimited manner with no upper ceiling. The world record for fasting was 382 days (not recommended!), during which the patient suffered no ill effects (although he was carefully monitored by physicians and did take certain vitamins).

Think about medications for a second. If you take metformin – there is a maximum dose. Same for virtually all meds. Think about low-carb or low-fat diets – you can only go to zero carbs or fat. There is a maximum dose. That is why insulin is so popular with doctors. You can keep increasing the dose with no ceiling. (As an aside, we recently had a lady taking 400 units of insulin per day in our clinic. Her endocrinologist was happy that her sugars were ‘finally’ controlled. I was appalled.)

Fasting, similarly has no ceiling, which offers much more therapeutic flexibility. In other words, I can keep applying fasting until I see the desired effect. The dose can go up indefinitely. If you don’t eat, will you lose weight? Of course. So there is almost no question of efficacy. It is only a question of safety and compliance. So for more complicated or serious cases, we can simply increase the dose. We will consider these longer regimens in an upcoming post.

A big thank you to my friend Dr. Ted Naiman who created these graphs.

Dr. Jason Fung[/text_left]


Longer fasting regimens – 24 hours or more


Intermittent fasting for beginners

Practical tips for fasting

Women and fasting

The 7 practical benefits of fasting

Top videos about fasting

  • How to maximize fat burning
  • The top 5 tips to make fasting easier
  • The top 5 myths of fasting
  • The most common questions

Earlier with Dr. Jason Fung

The common currency in our bodies is not calories – guess what it is?

Why the first law of thermodynamics is utterly irrelevant

How to fix your broken metabolism by doing the exact opposite

More with 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


  1. Vaness
    I want to fast but i am breastfeeding. Any advice?
    Reply: #4
  2. Toni Wray
    Do NOT fast while breastfeeding!! When your baby is weaned, then you can try fasting, but it is not recommended to fast when breastfeeding. And breastfeeding is the best thing you can do for your baby! Just try to eat a LCHF diet from unprocessed, real foods. This will be good for you and provide your baby with an excellent nutritional start. Eat when you're hungry and make sure you drink enough as it's easy to mistake thirst for hunger. Then, when you stop breastfeeding you'll be able to try fasting. Good luck and well done for breastfeeding!
  3. BobM
    While what Toni says is possibly true, what would be wrong with skipping breakfast? Or even breakfast and lunch? (I mean while breast feeding.) I assume our forebearers had to have done that. We couldn't have had food all the time. On the other hand, if you haven't fasted in the past, trying to start fasting while you're breastfeeding probably isn't a good idea. As with anything, there's a transition, and I experienced moodiness in particular until I got used to missing breakfast. (I'd say it was low blood sugar, but I never measured my blood sugar at that time, so I'm not sure.)

    While I do fast daily and am currently doing the 8 hours of eating technique, I also sometimes do the 4 hour eating technique and also try longer fasts. A lot of times, it's easier to fast 3+ days for me than to make it through one day. Perhaps it's psychological, but I can convince myself to make it three days sometimes better than I can convince myself to make one day. I also like to mix things up so that my body never knows what's coming.

  4. Johan

    Fasting while breastfeeding is not recommended:

  5. Mario Castro
    How often should I fast?
    I'm 26 years old and I am going to start the warrior fasting and keto diet. My questions are can I fast everyday at the same times and can I or should I take supplements while fasting.
  6. Danny
    Is it OK for the 8 hour fast to keep breakfast and lunch (as day is when I'm most active) and skip dinner? Say eat between 6:30 am and 2:30 pm and not eat from 2:30 pm until the next morning?... I don't need fuel at night if all I'll do is sleep, right?
  7. Anneleen
    I did a 22h fast (only water and black coffee), I had my energy skyrocketing througout the day, but I was glad to eat something in the evening. I wanted to do this because I am at a weightloss plateau. I am 1m62 and stuck on 60kg. I don't think this is my natural weight. The morning after the 22h fast, I only weighed 100gr less! How is this possible? Do I need to fast longer? Or will my body adapt when I do this more often?
  8. Lisa S
    Please someone answer this for me.
    I am type 2 diabetic and ha e been dong the 16:8 for a week now and my fasting numbers are high (180-200) . Is this normal ? When should it lower? I’m confused
    Reply: #9
  9. Vic
    I'm going to start a warrior diet. Is there any possibility, that a meal which i will eat after fasting, will be too less for a day? Through if I'm planning to do a warrior diet on a daily basis should i eat more than one meal, or will it be just enough?
    Reply: #11
  10. Kristin Parker Team Diet Doctor
    I am not familiar with that way of eating. Generally speaking, results will vary from person to person and you can experiment with what works best for you.
  11. Vic
    Thank you
  12. Tracie
    When fasting can you drink water or is it nothing at all?
    Reply: #14
  13. Kristin Parker Team Diet Doctor

    When fasting can you drink water or is it nothing at all?

    Dr Fung recommends bone broth, water, coffee or tea.

  14. 1 comment removed
  15. Natasha Grundy
    Hi all I am interested in the 8hr eating and 16 hour fasting but am asking the same as comment 6 can we do this from say 6 am in the morning to 2 pm and the fast till 6 am the next morning. Please somebody advise?
    Reply: #17
  16. Kristin Parker Team Diet Doctor

    Hi all I am interested in the 8hr eating and 16 hour fasting but am asking the same as comment 6 can we do this from say 6 am in the morning to 2 pm and the fast till 6 am the next morning. Please somebody advise?

    You can arrange the time windows to best suit your schedule and preference.

  17. aDJA
    I would like to know if i do 16:8 everyday will I loose weight? Or should I do regular eating everyday other day?
    Reply: #20
  18. Sowmya
    I am 24year old. I am willing to start 16:8fasting .
    I would like to know how many calories should I consume during meals at 2pm
    Reply: #21
  19. Kristin Parker Team Diet Doctor

    I would like to know if i do 16:8 everyday will I loose weight? Or should I do regular eating everyday other day?

    Different people are going to respond differently to the various fasting regimens.

  20. Kristin Parker Team Diet Doctor

    I am 24year old. I am willing to start 16:8fasting .
    I would like to know how many calories should I consume during meals at 2pm

    We recommend eating to hunger rather than specific caloric intake.

    Reply: #27
  21. Tina Harris
    I would like to see Every ones questions get answered😣
    Reply: #23
  22. Kristin Parker Team Diet Doctor

    I would like to see Every ones questions get answered😣

    I have been responding to questions since I joined the Diet Doctor team last fall. I did not go back and reply to old questions but would be happy to help you if you have a question.

  23. Dania Rodriguez
    Question how many times can I do the 20/4 fast per week? Or does it matter ?
  24. Ben Middleton
    Question, is there any point of doing a 16-20 hour daily fast whilst on a ketogenic diet, as during the ketogenic diet you are predominantly in a fat fuel burning state?
  25. osman
    can i fast with gaps means if i fast from 10am till 4pm and then i fast from 2100 HRS till 0900 hrs
  26. Scott Slay
    I do 18:6 and usually at least a 42 once a week. I eat enough to be full but if I check it’s only 1100-1200 calories.
    Is it enough? I haven’t learned to eat higher fat, yet.
    Your thoughts?
  27. Leanne
    Hi, I am eating in less than an 8 hour window, fasting until 11am each day and usually having dinner before 5pm. I tend to eat 2 meals. Sometimes, I don't feel like eating the second meal but am worried about getting enough nutrients and fat for my muscles and health. When fasting like this, are we supposed to make sure we eat the right amount of macros and quantities of food each day?
    Thank you

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