Longer fasting regimens – 24 hours or more

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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.

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.

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, especially for overweight or obese individuals. 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.

Another name for a similar fast is OMAD, short for One Meal A Day.

What you need to know about OMAD


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. In response, hunger is suppressed and total energy expenditure is maintained. So – appetite goes down and TEE stays same or goes up. Remember that chronic everyday caloric restriction strategies produce the opposite. Appetite goes up and TEE goes down, likely leading to inferior results.

You can extend fasts much longer. The world record was 382 days (not recommended!), 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.

Earlier

Short fasting regimens – less than 24 hours

More

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

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116 comments

  1. Cherrie
    I have been eating once a day for weeks and lost a nice amount of weight. I still have about 40 lbs to go. Is it safe to be fasting for 24 hours every day?? I am not hungry till dinner time every day so I’m not starving myself.
    Reply: #102
  2. Kerry Merritt Team Diet Doctor

    I have been eating once a day for weeks and lost a nice amount of weight. I still have about 40 lbs to go. Is it safe to be fasting for 24 hours every day?? I am not hungry till dinner time every day so I’m not starving myself.

    We actually don't recommend OMAD (one meal a day) as a daily thing. Check out this article for more info! https://www.dietdoctor.com/intermittent-fasting/omad

  3. Rebecca Bentley
    I am currently doing 36 hour fasting. I will eat a meal then start again. It’s fairly easy with my work schedule. I am taking vitamin supplements per my MD. I’m fasting for my brain/memory health. My sleep has improved and I feel my thinking is sharper.
  4. Nathan
    How to tackle loose stools on long fasts
    Reply: #105
  5. Kerry Merritt Team Diet Doctor

    How to tackle loose stools on long fasts

    This article should help you out! https://www.dietdoctor.com/intermittent-fasting/side-effects#diarrhea

  6. Joescalon
    For anyone with a new Fitbit, there is now a free fasting timer app called Autophagy in the Fitbit app store...enjoy.
  7. salsuradi
    I've done 7-day fasts on two occasions and had great results. On keto I've lost 40 lbs. I want to try intermittent fasting, 24hr. If I have only one meal a day, am I supposed to get all my macros for the day out of that one meal? That seems like too much. Any advice?
    Reply: #108
  8. Crystal Pullen Team Diet Doctor

    I've done 7-day fasts on two occasions and had great results. On keto I've lost 40 lbs. I want to try intermittent fasting, 24hr. If I have only one meal a day, am I supposed to get all my macros for the day out of that one meal? That seems like too much. Any advice?

    Check out our guide to One Meal a Day. https://www.dietdoctor.com/intermittent-fasting/omad

  9. Annie Tamlag
    Hello, I’m totally confused. Doing a 7 days fasting with bone broth, water, tea, coffee and added some vitamins, minerals, fish oil, MCT oil, cream, coconut milk and I’m experiencing ketogenic effect. I’ve also exercised without being unwell. But some people say that I’m not fasting and that I will not experience autophagy since I’m drinking bone broth. And they say that water fasting is the only way to obtain autophagy. In Dr. Fungs book there’s a list of what is allowed to drink during fasting which includes bone broth and MCT oil. So what’s correct to obtain autophagy?
    With kind regards Annie Tamlag, Norway
    Replies: #110, #114
  10. Kristin Parker Team Diet Doctor

    Hello, I’m totally confused. Doing a 7 days fasting with bone broth, water, tea, coffee and added some vitamins, minerals, fish oil, MCT oil, cream, coconut milk and I’m experiencing ketogenic effect. I’ve also exercised without being unwell. But some people say that I’m not fasting and that I will not experience autophagy since I’m drinking bone broth. And they say that water fasting is the only way to obtain autophagy. In Dr. Fungs book there’s a list of what is allowed to drink during fasting which includes bone broth and MCT oil. So what’s correct to obtain autophagy?
    With kind regards Annie Tamlag, Norway

    You may find this helpful.
    https://www.dietdoctor.com/renew-body-fasting-autophagy
    https://www.dietdoctor.com/video-post-what-breaks-a-fast

  11. 1 comment removed
  12. Archangel
    I love fasting and Dr.Fung was the one who opened my eyes to truth about what our bodies can do. I have no problem fasting couple of days. I did multiple times 5 days and 7 days fasting. I do water fasting. I know my body has enough reserve for short terms fasting like those. I don't rush and dump vitamins, bone broth, fish oil or anything. I believe those people who takes so much supplements even for 7 days fasting are the one still addicted to eating. I once did 21 days water fasting and the more days go by the better I felt. I stopped just because I had no doctor observation or anything. My question is I'm determined to go 21 days again maybe beyond that this time. Is there any blood test for kidney or any organ or hormone to take just to confirm nothing is happening inside me that will result badly later? Or feeling good is enough to continue fasting? I have enough fat reserve 😁
    Reply: #113
  13. Kristin Parker Team Diet Doctor

    I love fasting and Dr.Fung was the one who opened my eyes to truth about what our bodies can do. I have no problem fasting couple of days. I did multiple times 5 days and 7 days fasting. I do water fasting. I know my body has enough reserve for short terms fasting like those. I don't rush and dump vitamins, bone broth, fish oil or anything. I believe those people who takes so much supplements even for 7 days fasting are the one still addicted to eating. I once did 21 days water fasting and the more days go by the better I felt. I stopped just because I had no doctor observation or anything. My question is I'm determined to go 21 days again maybe beyond that this time. Is there any blood test for kidney or any organ or hormone to take just to confirm nothing is happening inside me that will result badly later? Or feeling good is enough to continue fasting? I have enough fat reserve 😁

    Your doctor will be able to let you know if there are specific tests and functions they would want to monitor over a longer fast.

  14. Michele
    Too much protein, for example from bone broth, discourages autophagy. According to Dr. Valter Longo of USC, a fasting mimicking diet permits autophagy. In his cancer treatment protocols explained in his book, The Longevity Diet, he suggests up to 400 calories of vegetables plus 400 calories of fat from nuts, seeds, olives will have the same benefits as a water-inly fast. His research is solid. He sells a product (but takes no profits) called Prolon which is a 5-day fasting mimicking food program originally designed gor cancer patients but beneficial to most people. I’ve done it 2x.
    Reply: #115
  15. Angie
    Hi Michele,
    What were your results on Prolon?
    Thanks!
    -Angie
  16. John Braddick
    I eat between the hours of 10am and 6pm every second day which gives me a total of 40hrs fasting over 2 days. I've been doing it for a week and will try for a month. I'm a 73yr old man in reasonable health and want to lose 10kg do you see any problems, I drink only water or unsweetened black coffee while fasting, I don't count calories during the 8 hour fasting period but I try to cut out carbs as best I can.
    Reply: #117
  17. Kerry Merritt Team Diet Doctor

    I eat between the hours of 10am and 6pm every second day which gives me a total of 40hrs fasting over 2 days. I've been doing it for a week and will try for a month. I'm a 73yr old man in reasonable health and want to lose 10kg do you see any problems, I drink only water or unsweetened black coffee while fasting, I don't count calories during the 8 hour fasting period but I try to cut out carbs as best I can.

    If you are feeling well, there shouldn't be any issues with intermittent fasting. Of course, you should consult your doctor if you have any concerns regarding your fasting regimen.

  18. Nids
    If I follow a pattern of 11am.7.00pm... can I drink my normal tea once a day with may be 4 spoons of milk n one spoon sugar
    Also can I drink my green tea n water before 11.ooam and after 7.00pm
    Reply: #119
  19. Kerry Merritt Team Diet Doctor

    If I follow a pattern of 11am.7.00pm... can I drink my normal tea once a day with may be 4 spoons of milk n one spoon sugar
    Also can I drink my green tea n water before 11.ooam and after 7.00pm

    This video about what breaks a fast should be helpful for you! https://www.dietdoctor.com/video-post-what-breaks-a-fast
    We also recommend avoiding milk due to the sugar content. Heavy cream is a better option, as it's less sugar and higher fat. The spoon of sugar should also be avoided. You may want to try a natural sweetener instead, such as stevia, erythritol or monk fruit. Here's our sweetener guide for more info! https://www.dietdoctor.com/low-carb/sweeteners

  20. Mike
    I am about to break my fast (after 68 hours fasting ) today Wednesday 24.6.20. I weighed this morning after 59 hours of fasting and have lost 16 lbs since weighing on Monday. ( 7 lbs lost on Tuesday weigh in and 9 lbs today = total loss of 16 lbs ). I feel really good and have drunk a lot of water and some coffee plus taken vitamin supplements during my fast.
    I appreciate that I have only really lost water over this time and will gain weight after I start eating.
    I am 5 ‘ 10” and very overweight weighing at 20 stones 9 lbs on Monday but I am big boned and have reasonable muscle mass for my age of 69 years.I am in reasonable health only taking tablets for blood pressure.
    My weight this morning was down to 19 stones 7 lbs.
    I intend going 16:8 lifestyle and reducing carbs in trying to get down over several months to 14 stones plus the occasional longer fasts of 24/48/60 hours from time to time.I also will seek to reduce my heavy daily alcohol intake ( either lagers or G&T in late evening followed with bottle of red wine with large late dinner ) and especially reduce beer/lager consumption.I also need to reduce heavy salt intake. I intend exercising more but difficult as my wife has to shield from coronavirus and I do not want to put her at risk by venturing away from my house.
    Is there anything unusual in losing so much water weight in such a short time ?
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