Q&A: who can use intermittent fasting?

Ask Dr. Jason Fung

Do you want to lose weight or improve your diabetes using intermittent fasting? Are you unsure if fasting is appropriate for you?

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

Who Can Use Intermittent Fasting?

Is fasting an option for children who need to lose weight?

Fasting is not an option for children. My advice is to severely restrict added sugars and snacking. Reducing down to 2 meals per day is also possible, but not longer duration fasting.

My daughter who is 31 and a healthy weight exercises (rowing) four times a week. She wants to know if she can fast or is this not recommended for people who exercise?

Not only is it safe, but training in the fasted state has several theoretical benefits that many elite level athletes are using. So, yes, it is highly recommended.

Can women fast during pregnancy and after birth during breast feeding?

I don’t advise fasting during pregnancy or breast feeding. Short term (<24 hrs) fasts are OK, but definitely not longer term fasts. There is a concern of nutrient deficiency which I think far outweighs any potential benefit.

How should intermittent fasting be used in conjunction with resistance training to maximize muscle growth and fat burning?

Should there be any differences between intermittent fasting on training days and non-training days? And during the fasting periods – or days – is it advisable to take supplements such as BCAA’s to prevent muscle loss?

There are many different schedules. Most people fast for 24 hrs and then do their training – this is called ‘training in the fasted state’. Since growth hormone is high, you theoretically will recover and build muscle faster in this state.

There is minimal muscle loss during fasting, so BCAA is not needed, but often used by bodybuilders. It’s efficacy is unknown, with mostly anecdotal evidence. Many athletes follow a schedule of 24 hr fast, then exercise, then breaking the fast with a high protein meal.

How appropriate is intermittent fasting for teenagers?

Not appropriate. Certainly occasional short fasts, (less than 24 hours) are fine but not longer ones. Even most religions do not make children fast because their bodies require more nutrients to grow.

Is fasting still a good idea if you are trying to conceive? Should Intermittent fasting be used instead, or none at all?

You can certainly try. Fasting should not be used during pregnancy however.

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 or ask him directly on our membership site (free trial).

Jason FungAsk Dr. Jason Fung

We also have a 45-minute presentation by Dr. Fung on “the key to obesity” – insulin resistance – and how to reverse it, as well as one on type 2 diabetes. These presentations are on the membership pages (free trial).

Dr. Jason Fung

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


  1. Susan
    I have been on lchf for about 3 1/2 months, lost 17 pounds, reduced insulin from 50U to 0. Reduced 10mg Er Glipizide to 5 mg. My a1a was 6.1 last week I have been Diabetic 2 for over 20 years, I am a 77 year old female. My question is my Keto stick says my ketone level is small, is this OK or do I need to eat more fat? I also do IF for over 18 hours daily. Thanks
  2. Ranny
    Dear Dr. Fung,

    Your videos and blogs explain the significance of LC and HF very clearly. What is the role of proteins in this program? I may have missed it, but I haven't seen much mention of proteins. Your website provides information on how much carbohydrate is appropriate; similarly, do you have any recommendations on how much of proteins per day are appropriate and what are the best sources for a vegetarian? Thank you.

    Reply: #3
  3. Peter Biörck Team Diet Doctor
    Hi Ranny!

    If you are a member then you can direct a question to Dr Fung here:

    I hope that's OK?!

    /Peter Team DietDoctor

    Dear Dr. Fung,
    Your videos and blogs explain the significance of LC and HF very clearly. What is the role of proteins in this program? I may have missed it, but I haven't seen much mention of proteins. Your website provides information on how much carbohydrate is appropriate; similarly, do you have any recommendations on how much of proteins per day are appropriate and what are the best sources for a vegetarian? Thank you.

  4. Eunice
    Dear Dr. Fung,
    I want to thank you for your informative videos. I am 70 years young, but your videos are presented in a way that even I can comprehend most of the information. About 6 months ago my Dr. put me on metformin as I had been pre diabetic for some time, and when my fasting sugars were between 140 and 150 I started taking the medication. I gained weight! hungry all the time. about 2 weeks ago a friend told me about the Diet Doctor website. I was truly impressed. I eased my way into the program and started LCHF in earnest 1 week ago. and I am no longer taking metformin. I check my sugar at home and my fasting still a bit high 135, but after I eat (2hr) from 99 to 114. I have lost 10 lbs I now weigh 257 lbs , I have lost 2 in from waist. I am not hungry at all, keep carbs below 20, and short term fast from 6pm to 12 noon for two days now , think I will continue this. Does that sound reasonable (the fasting times)? I am barely mobile due to congestive heart failure, and other things. I am going to try to get into pool this summer and swim. thank you again for your wonderful life changing information.
  5. Greg Gilligan
    Dr. Fung,

    Many thanks for your dedication to science and the truth, and presenting it in such an accessible way. I was drawn to fasting originally without a clear motivator. Several years ago I did a couple of longer fasts (7/14 days) just wanting to "cleanse" my body. In the past few years, seeking to address a weight problem, and sometimes being good about LCHF, I started to mix in intermittent fasting of 24 - 36 hours on Day 7 to counteract the one Day 6 "cheat day" that followed 5 days of LCHF. Then I saw your online videos, and bought and read both your books. Wow. Again, thank you.

    So toward my question: after absorbing your research and advice, I started more frequent water and tea only intermittent fasting with a fairly flexible approach, just fitting in 16 hour or 24 or 36 hour fasts when I could, but with fairly high frequency. Results started to happen and I was pleased. Then my health club offered a 21 day body composition challenge with a couple cool prizes. With the foundation of a few weeks already of frequent intermittent fasting, that gave me the motivation to go for the 21 day (also water and tea only) fast and really shed the weight and address decades of nutritional abuse (I'm 53, 5'9" and peaked at 198 pounds, just shy of a 30 BMI).

    So I am on Day 8 and dropping weight quite successfully - 15.7 lbs as of this morning. What I am struggling with though (besides fairly persistent hunger!) is heartburn. In the reading I've done that seems to be something that should fade away fairly quickly, but it has been nagging me for several days now. In fact, it is more persistent than all but my worst periods of nutritional excess. I am drinking tons of water and include a few teaspoons of apple cider vinegar once or twice a day but the heartburn persists. Any suggestions? Everything else is fine: energy levels high, no nausea, dizziness or weakness, etc.

    I'll appreciate any suggestion you might offer.


  6. JennaJ

    Dr Fung,

    Can you please update your video links. I can only view the metabolism video and the 'What is fasting video' All other links lead to the "What is fasting video."

    My husband and I have been going 3 weeks now with the 20:4 plan, and I have gone from 221lbs to 213lbs! I am a firm supporter of your work, and I'm looking forward to reading the book you wrote when it arrives this Thursday to get more in depth about the research you have done. Thanks for a free alternative to dieting! I've found it's far easier to count minutes than calories!


    Reply: #7
  7. Inger Swanberg Team Diet Doctor

    The links are fine. If you are not logged in to the membership, you can only view the What Is Fasting video.

  8. Suity
    Please help me I want to cure my type 2 diabities
    Reply: #9
  9. Peter Biörck Team Diet Doctor
    Hi Suity!

    You can find more info here:

    Please help me I want to cure my type 2 diabities

  10. Kisha watson
    I plan on doing intermittent fasting: m-f with my eating window from 4p-12a (I often get home late). Is it possible to switch it up on the wknd, like a eating window of 12p-8p (or something like that) without any issues?
  11. Bobbie
    Question. HGH injections in the absence of fasting induce most of it's anti-aging effects by increasing IGF-1 levels, which is the real anti-aging hormone that speeds cellular regeneration throughout the system. But increasing HGH through fasting drastically decreases IGF-1 levels, which in turn would limit cellular regeneration beyond the direct effects of HGH (skeletal and muscle growth). This line of reasoning suggests that fasting is counter productive for anti aging purposes.
  12. Patricia Ormsby
    I wonder if there are any doctors in Japan who can provide counseling on use of intermittent fasting with diabetes (taking insulin). Husband has 30+year history of diabetes, the last 15 on insulin, with good blood sugar control (a1c 5.8). He has had positive experiences with fasting in the past, but never sustained. He is pretty satisfied with the insulin approach, but notes problems with fatty liver and weight gain now. Also a recent sudden bout of low blood sugar scared both of us. I think he would be ready to give fasting a try, and I would be willing to observe an identical protocol to be supportive. My fear is coming off insulin without professional support could be dangerous.
  13. Thomas
    Hello all,

    I am on folic acid and Ramipril - will these affect insulin?
    if so, am I best to take these during my eating window?

  14. Wendy Cirigliano
    Hello. We came across your articles and videos shortly aftert my husband was diagnosed in a walk in clinic with diabetes. He had been having the classic symptoms for almost a year and after many months of my urging, had a friend test his blood with a prick in the finger. His bs was 457! He immediately started a fast so that he could do a full blood panel the next day at the clinic. At this time his fasting bs was 279 and was prescribed metformin. He immediately began a low carb diet and throughout the week his ranges were 270-320. His panel came back and his AC1 is 14.4. I found your videos and immediately asked him to watch because it seemed so accurate in my understanding of how the body works. He watched and immediately started a fast- drinking water and tea and on the fourth day drinking a cup of bone broth. His levels have been dropping having slight increases in the mornings. However, today, the fourth day, he hasn’t been above 200. Tonight’s bs is 151!! This is less than 2 weeks after diagnosis! Thank you so much for your dedication in understanding diabetes and educating us!! Our family is excited for the victory that your knowledge will provide! Thank You!!
  15. Jay

    I have been watching the iThrive series, and have been concerned by the comments that the high fat diet ( ketogenic) actually contributes to fatty liver and fat in the cells and over time would make the problem of sugar metabolism worse. Please could you comment and explain how the excess fat is metabolised, does it stay in cells? or is it used by the body and excreted? does the Keto diet exaberate fatty liver or fat in cells?

    Are there any tests that could be done before/ after ketogenic diet to allay fears of this problem?

    Many Thanks

  16. Elizabeth Caldwell
    Dr. Fung,
    I read your book, "The Obesity Code," and was fascinated by this new information (new to me). All of the research seemed to make sense. However, I was still left with a few questions. One in particular is regarding people who lean towards hypoglycemia. I desperately want my mom to try more of a LCHF diet along with some intermittent fasting, but she seems to get sick if she goes more than 6 hours without eating something. Could this be caused by a micro-nutrient deficiency? I would love to hear your advice for situations like this.
  17. Jeanette
    Dr. Fung,

    I just wanted to say Thank you so much for the valuable information you have shared on your videos, I credit my success in being on my way to better health on your videos and books. I am a Diabetic, and was getting nowhere, my A1c kept increasing and in April it was at an all time high of 8.9, but decided to bite the bullet and change my diet and lifestyle, watched your videos, read all your books and went on Low carb/High Fat diet with intermittent fasting, I have been able to lose 25 lbs. and I just got my blood work done and have decreased my A1c by 2.5 points and it is now 6.4, my doctor is so pleased she says I am now in the pre-diabetic range and has permanently deleted my Diabetes medications from my patient profile. I know I have a lot more work to do, since my fasting glucoses are still on the higher side, running between 130-150, so thank you so much!

    I also want to thank the team on Dietdoctor.com, who has helped me learn about the ketogenic diet, and reading/watching the information you have on your website has also been a major key to my success.

    So very grateful to all of you,


  18. Marilyn
    I would appreciate information on how Keto and intermittent fasting affect osteoporosis please and thankyou. Marilyn
  19. Tash Lawlor
    I have a simple question. It's the only thing stopping me from following you at this point. It's my understanding meat increases IGF1 which would stimulate growth (tumor) how do you get around this eating meat in Ketogenic?
  20. Alicia Wassmer
    Is there a way to find a family doctor in Utah who understands this health model? I’m running into a lot of ignorance and would like a practioner for my daughter and I who understands these concepts.
  21. Abdi
    Can you Dr. Jason or Dr. Andreas address this article from the Lancet....i have been on LCHF diet ever since i joined your programs but this article scared the crap out of me....may be when you read it you can put in a layman;s terms as usuual. Thanks, Abdi :-)

    Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis


  22. Tina Crawford
    I take meds (synthroid and a water pill) in the morning which is during my fast. Will this interfere with my results of fasting? I also chew gum during my fasting/sugar free-will this interfere with the digestion process during my fasting time?
    Reply: #23
  23. Kristin Parker Team Diet Doctor
    For medicine, taking what is prescribed is more important than if it will affect fasting. Gum will affect each person differently.
  24. 1 comment removed
  25. Darrian Martin
    I've been researching alot of diabetes and the effects fasting has on the body. I in fact before even finding you or having a reason to started the keto diet four weeks ago. i cut 20 lbs. I know the power that both keto and fasting have but my primary concern is the effects they have on type one diabetes. Does it work the same? Do you or your team have any insight on this?
    Reply: #26
  26. Kristin Parker Team Diet Doctor
    With Type 1, it is extremely important to pay attention to your blood sugar. If your blood sugar drops too low, it will be important to eat even if it means breaking the fast. Different people will respond differently to fasting.

    Here's the bulk of our information about lchf for Type 1 diabetics - https://www.dietdoctor.com/diabetes/type-1

  27. Lelia Cahill
    What are the effects of water fasting if you have cancer in the lymph nodes of your pelvis as a man?
  28. LeliaCahill
    Do you use vitamins while fasting
  29. Christine
    Dear Dr.Fung,
    I am a 63yr old woman using intermittent fasting as an alternative to caloric restriction. I realised, after decades of half starving my body, to keep weight down, after 5 children, that this was the reason for my low energy levels. So thank you for that. I am 5ft 3.5 inches, 114lb, very small frame, BMI 20 and 33% body fat ( after a recent DEXA scan ) and have lean muscle but lack sufficient strength. I use fasting to help acute gastritis and wish to go on a 5 day fast for its health benifits ( autophagy). My questions are; is this okay at my fat percentage and will I risk loosing too much weight ? I have just about overcome my 1970's inherited fear of dietry fat !

    Christine from Wales UK

  30. Daniel H Kapika
    Aloha Dr. Jason Fung your information is very good. Thank you for sharing. I have type II diabetes but now I have a better understanding on
    Diabetes. I agree that I put my self in the his position and I also it starts with obesity and understanding it. Thank you so very much for helping me understand what is happening. I’m going to try the intermittent fasting. I went off my insulin for three weeks I lost 15
    Pounds went back on it gained 22 pounds in a week and a half. I will go and talk to my doctors and let them know about my test.

    Thank you and much mahalo from Hawaii Daniel H Kapika

  31. Joshua
    Hi Dr. Fung,
    I know that the advice is to completely eliminate sugar from my diet. However there is coffee and tea I drink with less than one teaspoon (closer to 1/2) I drink a cup. I don't see myself getting rid of that. So it would be nice to know what the effect of small doses has on insulin levels (insulin index). Any idea?

    PS: Bought the obesity code it was very interesting.

  32. Karen Towle
    Hello Dr Fung
    Thank you for your intermitten fasting book. It really helped me. I am hoping you can help me with my kidney? Doctors found aneurysms off of my celiac artery and iliac artery. They are small and not growing so far. I am having a problem with Extremely high blood pressure and found out my kidney is dying on me. The docs told me I can live with one kidney because they don't want to operate. They want to lower my BP with Meds instead. I am taking 40mg of Lisinipril and now two 1 mg tablets of Doxozine at night. Is this effecting my kidneys more? Do you have any suggestions? I want to save my kidney if possible.


  33. Joe Gamal
    Hello Dr Fung - big fan of your work. Diagnosed T2 in 2010. I was on Glucophage 500MG controlled for about eight years and you helped me get off it! Now controlled HbA1C for three years around 5.5 on average. But recently my C-Peptide number as an indicator of pancreatic health has continued to deteriorate, so 1.1, 1.0, 0.9 and now 0.7 over the course of the last four years. I lead a very active lifestyle strictly with a LCHF diet, but it's taken more and more effort to keep my blood sugar stable. I'm six foot six and about 180 pounds, Anti GAD is 5 u/ML, Anti Pancreatic Islet Antibodies negative and fasting insulin at 1.57 uU/ml. Is a move to insulin injections inevitable for me despite my gargantuan efforts? Truly grateful for any pointers.
  34. Clarence T. Ang
    Hello Dr. Fung , what is the right amount of water to drink when we do our IF , is there a regulated amount ? Thank You.
    Reply: #35
  35. Kristin Parker Team Diet Doctor

    Hello Dr. Fung , what is the right amount of water to drink when we do our IF , is there a regulated amount ? Thank You.

    There is no specific recommended amount of water to drink as it can vary based on many factors.

  36. Sarah Kelsch
    Hi Dr. Fung,

    I was recently diagnosed with insulin resistance at 22, which was a surprise to me. I am overweight, but not terrible unhealthy, and I don't have a family history, or PCOS. Doing intermittent fasting lately, I have lost 15 pounds and am starting to feel good! But I have been monitoring my blood sugar, and am a little discouraged. Yesterday, it was 82 20 hours into an 24 fast, then last night about 1.5 hours after eating, it was 121. But, I took it again 8 hours after eating, and it was 101, and then again 12-13 hours after, and it was 124-129. This fasting blood glucose is actually in the diabetes range, so I was understandably startled. But since my fasting blood glucose is actually as high as my postprandial, does that mean my glycogen stores have been activated and that is what is upping my blood glucose? Could glycogen get them that high? It seems to me that my insulin is working since my postprandial blood glucose is fine, as well as my long term fasting glucose. Do I need to be worried?

    Thank you so much for your advice! Your book is genuinely changing my life. You believe in people and help them remove blame from their shoulders, and that is so important. Thank you.

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