Longer fasting regimens – 24 hours or more


This post is about longer fasting periods – 24 hours or more – and how to do them.

I arbitrarily divide it at 24 hours but there is no physiologic reason to do so, other than for classification purposes. There is no magic dividing line.

We covered fasting regimens using periods less than 24 hours previously. The longer regimens are generally done less frequently. The major determinant of which fasting regimen is right for you is personal preference. Some people find longer fasts easier and some find them harder.

Most people find that hunger increases into day 2. At that point, hunger peaks and then gradually recedes. This is important knowledge if you are attempting a longer fast (3-7 days). It is easier to continue knowing that hunger gradually gets better.

Around one day long fasting periods

24-hour fasts

A 24-hour fast lasts from dinner to dinner, or breakfast to breakfast, whatever you like. For example, you would eat dinner at 7 pm and then fast until the next day’s dinner at 7 pm. In this regimen, you do not actually go a full day without eating since you are still taking one meal on that ‘fasting’ day.

This is very similar to the ‘Warrior’ style of fasting although that allows a 4-hour eating window so is technically a 20-hour fasting period.

This period of fasting has several important advantages. First, as a longer duration fast, it tends to be a little more effective. Because you still eat every day, medications that need to be taken with food can still be taken. For example, metformin, or iron supplements or aspirin should all be taken with food and can be taken with the one meal on the fasting day.

The major advantage of 24-hour fasting is that it is easily incorporated into everyday life. Most people, for example will eat dinner with family every single day. As you still eat dinner every day, it is possible to routinely fast for 24 hours without anybody knowing any different, since it really only means skipping breakfast and lunch on that day.

This is particularly easy during a workday. You simply drink your morning cup of joe, but skip breakfast. You work through lunch and get home in time for dinner, again. This saves both time and money. There is no cleanup or cooking for breakfast. You save an hour at lunchtime where you can work, and be home for dinner without anybody even realizing you had fasted for 24 hrs.

For weight loss, in our Intensive Dietary Management program, we’ll recommend this schedule of 24-hour fasting to be done three times per week. Many people find it so simple they will often increase it to five times per week, and sometimes every day. We also recommend this schedule frequently for those people who are older or taking medications.

One of the main worries of fasting is the loss of lean body mass, or muscle. Many studies have been done on this and these fears are largely misplaced. In one study, fasting every other day did not produce any loss of lean body mass over 22 days, even as body weight steadily decreases.

The 5:2 Diet

untitled-7A related approach is the 5:2 approach championed by Dr. Michael Mosley, a TV producer and physician best known for popularizing this approach. He appeared on a BBC program called Horizon entitled “Eat, Fast, and Live Longer”.

While there had been some fringe interest generated by pioneers such as Martin Berkhan and Brad Pilon, fasting had not really hit the mainstream yet. With the BBC documentary and the book that soon followed, intense interest, especially in the UK followed.

The book, entitled “The Fast Diet” became a best seller in the UK and soon other follow up books were released. The basic diet was not quite a 24-hour fasting period. Instead, the 5:2 diet consisted of 5 days of normal diet. On the other two days, you could eat a total of 500 calories. Those 500 calories could be taken all in a single meal. If, for example, this is taken as dinner, it would be identical to a 24-hour fast. However, you could spread those 500 calories out into multiple meals instead. These two approaches are quite similar and the difference physiologically, is likely quite minimal.

Alternate Daily Fasting (ADF)

krista_facebook-300x300This is the dietary strategy that has the most research behind it. Much of it was done by Dr. Krista Varady, an assistant professor of nutrition with the University of Illinois – Chicago.

She wrote a book about her strategy in The Every Other Day Diet, although this was not the blockbuster success of the 5:2 diet.

Even though it sounds like you only eat every other day, it is not quite true. You can eat up to 500 calories on fasting days, just like in the 5:2 diet. However, fasting days are done on alternate days rather than 2x per week so it is a more intensive regimen.

heilbronn1The major advantage of this regimen is that more research is available on this regimen than any other. We will consider these studies in more detail in later posts.

Risk of complications of fasts >24 hours

As you progressively go longer in fasting, the benefits accrue faster, but there is also more risk of complications. Since I often deal with type 2 diabetics and hard to treat obesity cases, I tend to gravitate towards longer fasting periods, but you must understand that I always monitor very closely their blood pressures, and blood work and progress. I cannot stress enough, that if you do not feel well at any point, you must stop. You can be hungry, but you should not feel sick.

Another major consideration is that medication must be carefully monitored by a physician. The major problem are diabetic medications because if you take the same dose of medication and do not eat, you will become hypoglycemic and that is very dangerous.

Blood sugars going low is not a complication per se, because that is generally the point of fasting. We want the sugars to go low. However, it does mean that you are overmedicated for that day. You must work very carefully with a physician to adjust medications and monitor sugars. Also, there are certain medications that may cause stomach upset on an empty stomach. NSAIDS, ASA, iron supplements and metformin are the major drugs here.

In general, diabetic medicates and insulin MUST be reduced on the fasting day to avoid hypoglycemia. Exactly how much to reduce it should be overseen by your physician.

I do not recommend anybody who is taking medication to try longer fasts without clearing it with their doctor.

36-hour fasts

A 36-hour fast means that you fast one entire day. You finish dinner on day 1 at 7 pm for instance, and you would skip all meals on day 2, and not eat again until breakfast at 7 am on day 3. So that is a total of 36 hours of fasting.

In our clinic, we will often recommend 36-hours fasts 2-3 times per week for type 2 diabetes. From experience, this longer fasting period produces quicker results and still has good compliance. Since type 2 diabetics have more insulin resistance, the longer fasting period is more effective than more frequent shorter fasting periods, although we have had good results with that too.

42-hour fasts and beyond

We often advise our clients to make a routine out of skipping the morning meal and break their fast around noon hour. This makes it easy to follow a 16:8 fasting period on regular days. After a few days, most people start to feel quite normal just starting their day with a glass of water and their usual cup of coffee.

When you combine that with a 36-hour fast, you get a 42-hour fasting period. For example, you would eat dinner at 6 pm on day 1. You skip all meals on day 2 and eat your regular ‘break fast’ meal at 12:00. This is a total of 42 hours.

For longer duration fasts, we often try NOT to calorie restrict during that eating period. Often, as people get used to fasting, we hear very often that their appetite starts to seriously go down. Not up. Down. They should eat to satiation on their eating day.

There’s a very good reason for this decrease in appetite. As you start to break the insulin resistance cycle, insulin levels start to decrease. Since insulin is the major regulator of the body set weight (BSW) your body now ‘wants’ to go lower. In response, hunger is suppressed and total energy expenditure is maintained. So – appetite goes down and TEE stays same or goes up. Remember that standard Caloric Reduction as Primary (CRaP) strategies produce the opposite. Appetite goes up and TEE goes down. Which do you think will work in the long run?

You can extends fasts much longer. The world record was 382 days, but there are many people who can fast 7-14 days without difficulty. Indeed the Master Cleanse used by Beyonce is simply a variation of the 7-day fast which allows some concoction of maple syrup, cayenne pepper and lemonade.

There are some theoretical benefits of stimulating autophagy, a cellular cleaning process which often requires 48 hours of fasting or more. A state of ketosis may require over 36 hours of fasting to enter. There are many theoretical benefits, including appetite suppression and greater mental clarity. For cancer prevention, some recommend a 7-day fast. Many of these benefits are theoretical and unproven, however. Nevertheless, many have found the 7-day fast much less difficult than one initially imagines.


Short Fasting Regimens – Less Than 24 Hours


Intermittent Fasting for Beginners

Get started video course

In these two short video courses, Dr. Jason Fung guides you into a 24 hour and 7 day fast, respectively. Videos with membership or free trial.

How to Get Started with Fasting – Dr. Jason Fung
The Power of a 7–Day Fast – Dr. Jason Fung

Top videos about fasting

  • How to maximize fat burning
  • The top 5 tips to make fasting easier

Full IF Course

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 intensivedietarymanagement.com. He is also active on Twitter.

His book The Obesity Code is available on Amazon.

The Obesity Code


  1. Ada
    Can someone look at this and explain WHEN autophagy can become dangerous? This guy won a nobel prize for studying it after all...


  2. Larisa
    Could you tell me if I am drinking bullet coffee in a morning is it still fasting or not? Or I need to drink just coffee without butter/oil in it?
    Replies: #4, #14, #35, #50
  3. Nate
    Thanks, Dr. Fung.
  4. BobM
    Larisa, I use only black coffee, "black" tea, "black" herbal teas and water when fasting.
  5. Larisa
    Okay, I got it. Thank you, BobM.
  6. gbl
    I am ravenous after I fast longer than 8/16.
    Reply: #7
  7. Lonneke
    Me too! In saying that when I first started this WOE I was able to do the longer fast once or twice. Now that I'm nearly down to goal weight it's not so easy and I get very cold.
  8. gbl
    I guess our hunger means we need to eat more fat?
  9. Joseph
    What about the whole notion of the body going into "starvation mode" when fasting and then rapidly storing your next meal as fat? Is this reasoning still valid. I know it's quite a big deal with ppl eating 5 to 6 meals per day.
    Replies: #12, #44
  10. Cassandra Greensmith
    Joseph it is a myth, read Dr Jason Fungs blogs, it explains all about how starvation mode only really happens with calorie reduction but not fasting!
  11. Victoria
    Hi I know this was a while ago, so I hope it is being watched, I usually take resistant starch and inulin. Can I take it on fast days
  12. Susan

    What about the whole notion of the body going into "starvation mode" when fasting and then rapidly storing your next meal as fat? Is this reasoning still valid. I know it's quite a big deal with ppl eating 5 to 6 meals per day.

    Joseph, I suggest you check this out. https://www.dietdoctor.com/?s=survival+mode
    There are many things "they" have told us that just don't pan out.

  13. Keto.MD
    I've been on LCHF diet for 5 weeks. Got busy this Thursday morning - skipped breakfast by accident. Now it's Sunday dinner time and I haven't anything other than some bovril packs in water, water by itself, and a bit of cream in my coffee. I'm not hungry at all. Zero fatigue. In fact, I cleaned the garage and gardened all day. I guess I have had a few Nestea Ice Tea Zero and a few diet coke.
    Reply: #16
  14. Andrew
    Yes, Larisa, you will break your fast with a bulletproof coffee. Fasting is broken when you consume calories. 1 tsp of butter in a bulletproof coffee is about 34 calories.
    I used to do a this every morning, but I'm on a 24 fast and now have black coffee, herbal infusions and water. Though if I'm getting really tired (often at lunchtime) I've had a glug of MCT oil to get me through to Dinner time (Dr Fung has said MCT oil will technically break a fast but the impact is very minimal).
    Reply: #49
  15. Peter Phelps
    Observations on my fasting sessions show that I tend to gain weight after fasting. My longest fast of 3 days showed a 2.4kg drop at the end and a 0.5kg gain 4 days later. I deliberately kept my meals normal and weighed out potions to do so. I have seen this tendency after shorter (24hr) fasts also. Is this just me ?
  16. Andrew
    BOVRIL ! !
    OMG, I never thought of that. Bouillon or Bone Broth is always mentioned, but Bovril sounds like a great choice.

    1 tbsp of Bovril
    Calories 1
    Total Fat 0 g
    Saturated 0 g
    Polyunsaturated 0 g
    Monounsaturated 0 g
    Trans 0 g
    Cholesterol 0 mg
    Sodium 77 mg
    Potassium 0 mg
    Total Carbs 3 g
    Dietary Fiber 0 g
    Sugars 0 g
    Protein 0 g

    Reply: #17
  17. Peter Phelps
    I use OxO too, Mum fed us that as kids.
  18. Julie
    Many people don't fit on the BMI chart if they have higher muscle mass. Make sure you know your lean weight and go by fat percentage goals, not BMI goals, or you could become underweight. My local hospital has a dunk tank and does tests for about $50. I found my lean weight was about double that of most women, so if I went by the BMI charts I would be severely under weight at the "goal weight" the BMI charts mention https://upload.wikimedia.org/wikipedia/commons/b/b7/Correlation_betwe...
  19. Julie
    Here is a good explanation of how BMI can be way off for women of both high and low Fat Free Mass Index (large and small muscled or framed) : http://bodyfatprcent.com/body-positive-math-fat-percent-total-fat-ffmi/
  20. Jeff
    Thanks for clearly summarizing everything. Very informative one pager.
  21. Laura
    This is really cool and interesting stuff. I've been trying to look into research involving these mid-ranged periods of fasting, in the 24-36 hour window. Do you have any research articles or scientific data covering this topic that you would suggest?
  22. 1 comment removed
  23. Steven Cruz
    Your Book is a God send I been on the Obesity Code Fasting for 12 weeks and I lost 65 Lbs. I was 374 and now I am at 309 I have 59 more Lbs. to lose and I can get my Knee Replacements thanks to you I seen a different lifestyle and some of my co-workers were trying to side track me but I was very discipline.I started 2 days a week then increase it to 3 days a week then I increase it to 5 days a week for four weeks its great my A1C is down from a 5.6 to a 5.0 even my good and bad cholesterol are great the food I eat is no more bread or instant oatmeal or cereal or any Carbs or sugar I am very happy but I still have a long way to go 59 more Lbs. and I can get my knee replacements.Thank you Dr.Fung its a great book and it change me.
    Reply: #40
  24. 1 comment removed
  25. Cari
    I am not diabetic. I do take 600 mg of gabapentin nightly for a sleep disorder. I would like to lose 15 to 20 pounds. I have started a daily 24 hour fast schedule eating only dinner. Can this IF plan counteract any insulin spike that may be caused by the gabapentin? I also try to eat Keto
  26. Dispenser
    When did the definition of fasting change from having nothing but water? I am not being snarky but I thought fasting was no calories.
    Reply: #45
  27. Marta
    Ante todo, muchas gracias x toda la información aquí expuesta, ya que personalmente es motivadora y alentadora para realizar alimentaciones adaptadas con más consciencia. El máximo ayuno, realizado en mi caso es de 11 días, únicamente tomando café con miel e infusiones de gengibre y canela, en ocasiones zumo de limón. El resultado, desde mi punto de vista es único y la experiencia sanadora. En los 2 primeros días se puede notar como aquí bien explica ,como el hambre se mitiga y se comienza a experimentar una sensación de ligereza mental,así como un estado de paz mezclado con positividad, ya que al realizarlo es muy satisfactorio, siempre y cuando te sientes revitalizado. Durante estos 11 días, las duchas de agua caliente por la noche son en mi experiencia ideales, pues el momento se convierte en una introspección sobre la experiencia diaria y un análisis de los resultados diarios. Desinflamacion, mejora en el aspecto de la piel, buen humor, claridad mental, pleno descanso en las horas de sueño, sensación de vigor físico y mental, aumento de la energia y la fuerza...estos son sólo, algunos de los beneficios . En los últimos días, tras notar algun mareo ocasional y tras estar muy satisfecha con los resultados, decido volver a una dieta de alimentación diaria. Ahora, estoy en el reto de incorporar a mi dieta diaria un ayuno de 24 horas, los 7 días de la semana, igualmente alternado con 1 o 2 días de ayuno semanales, aún estoy decidiendome!. Por lo leído aquí,creo estar en lo cierto, para obtener los resultados que pretendo. Por cierto,el guaraná puede ser, creo yo , también un buen aliado. Muchas gracias!
  28. Jerry W Wilson
    what should you eat after a 36 to 42 hour fast
  29. Dan
    What happens if I break my fast and an hour or two later I feel like my mind is cloudy diarrhea and anxiety?
  30. Jolie
    Unsweetened almond milk is just as good. Fasting gets easier the more you do it. You also binge less after. The first time was torture, but the second time not so much. Then the third time went 48 hours without feeling a thing. No binge cravings.

    48 hour fasts twice per week. No food Monday /Tuesday/Thursday/Friday(although I sometimes go out and drink so I cheat on Fridays a lot, lol.)

    I'm 68 inches/ 5ft 8Started at 168lbs
    Goal weight: 145

    First week: 8 lbs down(160) with a 3lb(163) rebound Monday after I binged
    Second week: 5 lbs(158/159lbs) down with no rebound Monday after I binged (I suspect I was in full ketosis by this point).
    Third week: Stalled - This kind of water fast does slow down metabolism. Ate a sausage everyday to pick up metabolism (only one frozen sausage patty). Down 3 more pounds(155).

    However, I thinking fasting is going to be a regular thing for me. I can't do the calorie counting thing anymore. Too exhausting.
    I hear that if you have more weight to lose, the results are even more dramatic than this. Some like 20 or 30lbs in two weeks. No joke.

  31. Mel
    What exactly can do drink during a fast without breaking it?
  32. Deb
  33. Betty
    Water, green tea, black tea, black coffee, bone broth (strained ),add himalayan salt to some of the water. It's worthwhile to read Dr. Fung''s book "The Complete Guide to Fasting". The information you get there will be more complete than what any of us try and explain.
  34. M Rod
    If I squeeze lemon in my water, would that break my fast?
  35. Yuranny
    lemon doesn't break a fast, bulletproof coffee absolutely will
  36. h
    Will a 24 hour fast induce autophagy?
    Reply: #37
  37. Kristin Parker Team Diet Doctor

    Will a 24 hour fast induce autophagy?

    That's going to vary from person to person!

  38. Michele
    Bariatric physician and keto expert, Adam Nally suggests that black tea can cause an insulin spike. For this reason, I only drink water and black coffee when fasting.
  39. Gregory
    Need help with a painless way to break my fast! I'm closing in on a 14 day fast. I only consumed water, black coffee and fresh brewed tea. It's amazing! Never felt better. I'm concerned how to break my fast. Dr. Fung says a 1/4 cup of Macadamia nuts, followed 30-60 minutes later with a small salad using cottage cheese as a dressing. I have seen other that have said stay away from greens as it's too hard to digest and you will get cramps. My biggest fear is that the fast went so well that my breaking the fast and the pain associated with it will prevent me from doing this again. Another fear I have is a couple years ago (not on a fast) I consumed raw almonds. I had a stomach ache so bad, I thought I was going to die. Such pain. I don't want to relive that. Can anyone provide what they did to break a 14 day fast painlessly?
    Replies: #41, #48
  40. Carol
    Keep up the good work and you may not need the knee replacements. Contrary to popular belief joints can regenerate. Turns out Keto n IF just might be as good as stem cell replacement.....upregulating the good stuff n downregulating the bad.
  41. Kristin Parker Team Diet Doctor
    You may wish to follow Dr. Fung's advice, paying close attention to smaller serving sizes to ease out of the fast.
    Reply: #42
  42. Gregory
    Thank you, Kristin. It was much easier than expected. Took Dr. Fungs advice. I went with 1/4 cup of Macadamia nuts, waited and hour and had some bacon and avocado. Worked for me! Thanks again
  43. Leah
    Have any of you found a way to not gain the weight back after a fast? ie: 5/2 fast or shorter/longer?
  44. Brandon
    You have to be extremely lean in the first place from what I’ve heard
  45. Paul
    Water contains no calories.
  46. Nate K
    I fasted for 25 days(only water) about 1.5 years ago. During that time I lost a little over 60lbs. I have since gained back most of that weight and am now on day 3 of fasting, but have decided to do green tea and black coffee in addition to water. My goal is to fast until I lose approx 60lbs again and then want to start intermittent fasting regularly. Is there any reason why my stomach would be much more upset with drinking green tea and black coffee this time around than when I did water only?
    Reply: #47
  47. Kristin Parker Team Diet Doctor
    It's most likely the coffee, which can be rough on the stomach.
  48. John
    How does he figure that Coffee is fine but Black Tea or White Tea or Green Tea is ok? Where is the documentation for this. Caffeine is in all of those by the way. Many Millions of people do ketogenic diet around the world, carnivore diet, water fast etc??? On top of that all kinds of athletes and diabetics drink tea. You would think if there was a problem there would be all kinds of research. Caffeine is what spikes insulin and epinephrine yata yata yata not to mention EGCG is mostly derived from tea. I have met plenty of people that lost 150lbs.-300lbs. most of them got stuck early on with various diets and it was not until they gave up caffeine completely that they started to lose fat again. I am dreading the thought of giving up tea and coffee but I have to admit I have noticed better results when I have given up all sources of caffeine in the past. That said I would never present that as a fact or as statistically significant to anyone but me since the data set is so small! Also I advise someone to make it 3 days to 7 days into a fast before giving up an addictive but otherwise unneeded substance. Since the first 3-7 days is the hardest might as well get past that point before you add more misery and suffering on top of what is already a difficult situation for 99% of us. I do not drink coffee or tea for energy or to wake up I really enjoy the taste of them. That said there is a time for everything and at some point one has to put away childish things and make sacrifices for what is more important and thankfully water fasting has taught me that tea and coffee are not must haves rather I should maybe make them a treat!
  49. Brenda
    Dr Fyng also says that natural fats such as butter and cream have minimal impact on insulin. So have your bulletproof coffee & enjoy the fasting!
  50. ip
    Hi Larisa,
    Drinking bullet coffee in a morning is still fasting as per the Dr Fung. When I fast I drink it in the mornings only.
  51. Julie
    Hello, I am trying to figure out while doing a 24hrs fast OMAD at the 23rd hour I would break it with my 1 meal. In this 1 meal I am confused about the food consumption. If my routine normally is IF 18:6 with 2 meals a day at 1285cals 83g Pro 93g Fat 20g Carb Do you consume this all at once or is it ok to cut in half? Less than 1000cals does not sound healthy to do and with reading about going 35g over in protein that can spike your insulin and kick you out of ketosis. How would you handle this situation? I am 42yrs female, 135lbs at 5'3 height, my goal is to be 125-128lbs. I really want to nail this Keto plan and have myself under control properly but there is so many dif opinions out there that has me always 2nd guessing and changing what to do with myself. I finally got my macros to what works with my body I am always reaching my protein goal, and my carbs are always under 20g, my fat I play around with usually lower than my Protein as I was told 70-93 fat to stick with. Any help would be much appreciated. Thank you!
  52. Kristin Parker Team Diet Doctor
    There is no one right answer for everyone. Our number 1 tip is to eat to hunger. For whatever meal you are eating, eat until your hunger is satisfied; that amount is going to be different for different people.
  53. Cristine Gutierrez
    I’ve been on the16/8 since Mid January and I’ve lost 29lbs. I just finished my first 24 hour fast and it really wasn’t as hard as
    I thought it would be. I’ve hit a plateau and I just have 15 lbs to reach my goal so I’m hoping fasting longer will help me break the stall. This is definitely a lifetime process for me, I’ve struggled for the last 14 years to get this weight off. Finally I’ve found something that works and is good for me.

Leave a reply

Reply to comment #0 by

Older posts