Could you fast for health without weight loss?

Ask Dr. Jason Fung

What is the beneficial difference between early time-restricted feeding (eTRF) and TRF? Can you do intermittent fasting for health benefits but not lose weight? How does fasting differ with aging? And, could it be dangerous to fast when on metformin?

It’s time for this week’s Q&A about intermittent fasting and low carb with Dr. Jason Fung:

Benefits of eTRF compared to ‘normal’ TRF?

I’ve read your post about the eTRF study with feeding window from 8 am to 2 pm. The study compared eTRF to a normal 12-12 hour feeding pattern. I get the benefits of TRF (as I’m doing 16-8 TRF for a couple of months now). What I didn’t understand from the study were the specific benefits of EARLY time restricted feeding. Is there any additional advantage to have the eating window early in the day? Could you elaborate?


Yes, there is theoretically some advantage to eating earlier in the day. I outlined it in some of my posts on the circadian rhythm.1 Briefly, insulin effect is higher in the evening than the morning. You can eat the same food, but the insulin effect will be higher in the evening. This means there is more of a fattening effect for foods eaten late at night. The other issue is that hunger tends to peak at about 8:00 pm. So you are hungrier, and therefore will eat more, and for the amount you eat, you’ll have more insulin effect. A double whammy.

Dr. Jason Fung

Intermittent fasting for health benefits but NOT weight loss?

Hi there, I have been following a ketogenic diet for 14 weeks and really enjoy it. I’ve lost about 15 lbs (7 kg) in that time and my BMI is now around 20. My main reason for starting the keto diet was for the health benefits (I have moderate ME/CFS) and I’ve seen an amazing reduction in brain fog and slightly less disturbed sleep. I’ve read about Autophagy and I think it could really improve my health even further. Having watched your (and other) videos on fasting, the emphasis tends to be on weight loss and my concern is that I don’t particularly have very much weight left to lose…. but yet I really want to give intermittent fasting a try! Is it possible to fast without invoking weight loss? Are there any particular guidelines available on fasting for autophagy and not weight loss?


Fasting does not necessarily involve weight loss. My best guess is that autophagy starts at around 18-20 hours so a 24-hour fast once in a while will give you the benefits without necessarily any weight loss.

Dr. Jason Fung

Effect of age on diet/fasting

Today I watched an interview with Dr, Joseph Antoun, he briefly touched on the subject of age and how it affects levels of protein’s effect on MTOR… I am very interested in the effects of age on diet, IF, 5-day fasts, etc. It has occurred to me that age from 1 to 20 would indicate a different diet than someone 20 to 45, and change again 45 to 65, and change again 65 to (with a little luck) 100 years old… Please go into this subject with your thoughts on all sorts of people, say type 2 diabetics at a young age, middle age, old age. How could they be treated? and can you give your thoughts on obesity and the effects of same with age? Along with any other thoughts you might have. I have seen little to nothing that would indicate a differing treatment for some different ages.


In childhood, the emphasis is on growth, so mTOR needs to be higher than in adulthood, where people shouldn’t be growing. So for adulthood, I don’t treat an obese type 2 diabetes of age 40 and one of age 65 and differently. Otherwise, you need to ask Dr. Antoun.

Dr. Jason Fung

Is it dangerous to fast while taking metformin? Valter Longo asserts this in the movie “Fasting”. I will put his quote down below.

“Metformin is a gluconeogenesis inhibitor. Fasting requires gluconeogenesis. You take a blocker of gluconeogenesis and something that requires gluconeogenesis to survive; if you don’t have gluconeogenesis, if you block it on one side and you require it on the other side, you can see how you have a very problematic situation. The combination of fasting or the improvisation with fasting could be extremely dangerous .” Do you agree with his statement?


No, if blood sugar is high, then I don’t see a problem blocking gluconeogenesis. We’ve had thousands of people fast with metformin without problems. If blood sugar is not high, then you don’t need and should not be taking metformin during fasting.

Dr. Jason Fung



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