How do you repair low metabolism after a low-calorie diet?

Ask Dr. Jason Fung

How can you repair a low metabolism? Can you drink whey protein drinks when fasting? Is higher glucose levels normal on a strict carnivore diet? And, what is the difference between fasting and chronic undereating?

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

Repair metabolism

How do you repair a low metabolism after doing a very low-calorie diet? There must be a lot of people out there (like me) who have taken advice from the GP’s with their official dietary guidelines and lost weight using a low-calorie diet, and as a consequence, wrecked their metabolism and eventually regained the weight.

There is not a lot of information out there to help us. As this obesity epidemic continues and people turn to low-calorie diets in desperation, they too will be looking for help when it fails.


This is certainly possible but can take some time. The key is to keep insulin low, and don’t reduce calories, which generally means following a low carbohydrate high fat diet. If calories are relatively high, and insulin low, then the body tends to use those ingested calories for energy and increases metabolic rate.

Dr. Jason Fung

Are whey protein drinks ok?

I typically drink a protein drink that is a whey protein. Is whey protein in protein drinks or bars not ok? Also, should I have apple cider vinegar during a fast? There is conflicting information as to whether it stops autophagy. And lastly, wouldn’t I have to avoid bone broth if I am want to reap the benefits of autophagy?


For autophagy, you should do a water-only fast. I generally advise people to avoid protein drinks and bars. Get your protein through natural foods.

Dr. Jason Fung

Is it normal to have higher glucose while on strict carnivore?

Is it a common occurrence for people’s bodies to have high glucose when on strict carnivore and OMAD intermittent fasting (22:2)?


No, generally meat does not raise blood glucose. However, if insulin levels fall faster, then glucose may still remain high.

Dr. Jason Fung

Fasting vs. chronic undereating

Hi Dr. Fung,

I’ve recently finished reading The Complete Guide to Fasting (loved it BTW!) and at one point there was a seemingly throwaway line about a risk to those practicing regular fasting, and that is ‘chronic undereating’. Can you please unpack this a little bit? Because in my mum’s traditional view, only eating dinner (eg after a 24 hr fast) is chronic undereating, though you recommend (and indeed I enjoy) 3 x 36 hour fasts during a week and as far as caloric intake goes, those weeks are rather slim (as is much of the point of fasting).

My assumption is that not eating (i.e. fasting) is very different metabolically than eating, but not eating enough… am I right?


Regular fasting is different from a chronic caloric restriction, such as, for example, a daily reduction of 500 calories per day, but still eating 3-6 times per day. Those are metabolically different because fasting allows insulin to drop and the body can then access its stores of food energy (calories) such as glycogen and body fat.

Dr. Jason Fung

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Earlier Q&A

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Dr. Fung has his own blog at He is also active on Twitter.

The Obesity CodeThe Complete Guide to Fastingthe-diabetes-code (1)

Dr. Fung’s books The Obesity Code, The Complete Guide to Fasting and The Diabetes Code are available on Amazon.

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