Q&A: what kind of fasting should I do?

Ask Dr. Jason Fung

Do you want to lose weight or improve your diabetes using intermittent fasting? Are you unsure what kind of fasting to try?

My simplified suggestion would be to try “16:8” fasting first, but there are many options and different variants may suit different people best. So let’s see what a real expert suggests.

Dr. Jason Fung, the Canadian nephrologist, is a world-leading expert on intermittent fasting and LCHF, especially for treating people with type 2 diabetes.

The most interesting questions and answers so far are now available to everyone. Here’s a few selected questions and answers about what kind of fasting you may want to try.

Different Kinds of Fasting

Are there significant differences in benefits from 24-hour fasting vs. multi-day fasting vs. 16:8 fasting?

Dr. Jason Fung: The main difference, as you may suspect, is that shorter fasting periods are less effective and are usually done more frequently. So a 16:8 fast is often done daily, whereas a 24 hr fasting period is done 2-3 times per week. For more severe insulin resistance, I tend to prescribe longer fasting periods, whereas for maintenance I tend to prescribe shorter ones.


The fasting protocol that fits my life style best is a fasting all day with a 4 to 5 hour eating window in the evening. I feel that I could do this daily during the work week. Is this recommended? How many days a week of IF is healthy?

Dr. Jason Fung: Periods of fasting less than 24 hours (20 hours fasting, 4 hr eating) or ‘Warrior’ style fasting can be done daily. The term ‘healthy’ always depends upon what your goals are. If you are simply trying to lose weight, then fasting can be done as needed for that. There are no negative health consequences to eating only during 4 hours of the day.


I have done 18hr, 24hr and 3 days without any real difficulties and have switched it up during the week, depending how I feel and whether I have social plans. Is it a good idea to change the fasting routine regularly, or would I be better off sticking to a 24hr regime for consistency?

Dr. Jason Fung: Personally, I believe that it is much better to switch things up so that the body does not have a chance to adapt. However, sometimes this inconsistency leads to people not fasting at all, which is also bad.

So it all depends upon your ‘style’. If a regular routine works better for you for compliance reasons, then do so. However, physiologically, I think changing things up all the time works better.


I’m very afraid of fasting because every time I tried, I get the common cold. How should I begin the fasting process? Should I start with a shorter fast and then progressing with more and more hours?

Dr. Jason Fung: I don’t think there is any link. You can certainly try skipping breakfast 2-3 times per week and working upwards from there. Some people prefer to work themselves in slowly, and others to jump in with both feet. Kind of like a swimming pool. Some wade in, others cannonball right in. Your choice.

More Questions and Answers

Go to the page with all top questions and answers or chose a topic below:

  1. Who can use intermittent fasting (IF)?
  2. Different variants of IF
  3. Things to consider during fasting
  4. Blood sugar and other tests during fasting
  5. Dietary advice between fasting periods
  6. Type 2 diabetes (and IF)
  7. Positive effects of IF beyond weight loss and type 2 diabetes
  8. Potential negative effects of IF and LCHF
  9. IF and ketosis
  10. Recommendations on blood tests


Do you have other questions about fasting for Dr. Jason Fung? Watch our in-depth interview with him.

Jason FungAsk Dr. Jason Fung

Dr. Jason Fung

You can also visit Dr. Fung’s website intensivedietarymanagement.com.


  1. Steven Richards
    I been experimenting lately with IF lately, and enjoying it more than ever. Decided to only eat when I'm actually hungry. Not a hunger pang that goes away for hours, but one that keeps coming back or won't go away. Feels awesome. Been treating water the same way. Think I used to eat and drink too much, or at least too often.
  2. Norman Roux
    Dear Doctor
    I am a non insulin dependent T2 diabetic and understand IF. It does drop my bloodsugar levels but I lose weight and I need to gain. I also gym 6 days a week. What would be my best IF options?
  3. Galina L.
    The best place on internet to learn how to fast http://gettingstronger.org/2010/11/learning-to-fast/
  4. Marguerite Junod
    I have been using the diet for a few weeks. I lost 7 pounds in a little over a week but have't lost any more since. Before I started this diet I was already not eating carbs, no processed food, almost no sugar but I have increased my fat intake which has led to not being hungry and only eating at 10am and again at 6pm. I don't feel hungry all day and fill up quickly. I also go to the gym a few times weekly and do floor exercises for specific body areas daily. I also grow most of what I eat and therefore am doing a lot of gardening in my 50 by 75 foot garden. I am hypothyroid and have high cortisol levels. I want to lose around 50 pounds.
  5. ROD
    I take supplements (lots of them) in the a.m., and some in the evening. If I do intermittent fasting, how can I fit these supplements into the schedule? Which I.F. protocol would best allow supplementation, and still be effective for the benefits of fasting, even beyond weight loss?
    Reply: #6
  6. Jennifer
    Hi ROD, I assume your supplements must be taken with food? If so, do you have to take some in the morning and some in the evening or can you take them all at once? I would suggest one 36h fast a week so you have dinner (and supplements) as your last meal on day 1, then you drink water only (and no supplements or the ones you can take without food) on day 2 and then you have breakfast (with supplements) on day 3 (and then lunch and dinner as normal). If you HAVE to take supplements everyday a very small snack on your fasting day does not completely stop the benefits of fasting. Either bone broth, or a boiled egg, or a small salad, or coffee with cream/butter/coconut oil.

    Otherwise 16:8 (first meal at 12:00 and last meal at 20:00) allows you to take morning supplements with lunch and evening supplements with dinner.

  7. Chris the Barbarian
    Awesome Advice, thanks again!

    I am currently on the 16:8, sometimes even 20:4 fasting protocol, and once or twice a month I even fast for 24+ hours, just because I'm not feeling hungry.

    My Question regards strength training in the morning; I always do it in the morning, and I generally like training in the fasted state. Now, Mr Berkhan (leangains.com) recommends taking some BCAA's right before training, and even afterwards. No carbs or other proteins, just bcaa's in powderform. Will this lessen the effect of the fast? Or is the small amount of Insulin not enough to make any impact?

  8. Walter
    Hi, Im starting with the fasting, reading all comments and posts, come to me some questions...

    After how many hours of stop eating, the body start to use the stored fat of our body, for example in a 16:8 fasting period...

    *How may of the 16hs we are fat burning machines...?
    *Depend on what kind of food you eat before ...?
    *Depend on how many calories you eat before ...?
    *Depend on how many fat you eat before...?

    Obviously I understand everybody have his own metabolism.

    Also, I think for me, start my fasting time around 3-4pm could work better, because I only need to be fasting until 10-12 pm after that I go to sleep, and I will keep in fasting for another 8 - 10 hs of sleep.

    Reply: #11
  9. John
    Dr Fung's website is incorrect as shown, try


    best regards,

    Reply: #10
  10. Dr. Andreas Eenfeldt, MD Team Diet Doctor
    Quite right, I fixed the link. Thanks!
  11. Martin
    Walter: I think your first question depends on your second. You could be fat burning 24 hours if you restrict your carbs and proteins enough. As I understand near 10 and 20 % of your energy daily needs respectively.
    Doing so, your third question will affect you winning or loosing weight.
    If your do the 10 and 20% carbs and protein, how many fat you eat will be the leveling of your daily energy, loosing or winning but always burning fat.
    But as you said "everybody have his own metabolism".
    Reply: #12
  12. Martin
    I tried to edit my comment but... anyways
    What I was adding at the beginning is that...
    My understanding of fasting is that it allows for the glucose reserve in the liver to diminish low enough so that the blood glucose itself goes low in the range where the fat will start to get used for fuel for most tissues.
    If in your feeding hours you eat loads of carbs (that will get converted to glucose replenishing your reserves) or protein, your "fat burning window" will get smaller.
  13. Eric
    Doctor Fung, The 120 hour ( five day) fast seems a great option for me.
    I seem to do fine on broth.

    PhD valter Longo use to suggest water or 200 calories for five days. Now he has a L nutra product that has 1090 calorories the first day and 750 for the next four days. Humm I do not see the advantage for me. Maybe compliance is the issue?

    WhAt is of interest is the drop in wbc, glucose,and insulin observed over. Five day fAst that can be done monthly. Seems if everyone could do a monthly five day fast or mimic fast then most of the health problems along its weight could disappear
    Exhaled in the CO2

  14. Dirk Van Giel
    First of all I would like to pay my respects to Dr. Fung, Dr. Eenfeldt and the entire LCHF-team for supporting so many thousands of people in their efforts of changing their lives for the good.
    I am a huge LCHF-fan and already lost 21kg in 10 months, virtually reversed the diabetes and looking at life again in a very positive way.
    I still was wondering what would happen to the neuropathy in my feet and if there is a chance it can be reversed as well... and if not : how do I need to proceed with this neuropathy ? Is there a treatment in the making ? Am I stuck with it for the rest of my life ? Do I need to keep taking the medicine I am taking now (Neurontine/Gabapentine) ?

Leave a reply

Reply to comment #0 by

Older posts