What are the good and bad effects of fasting?

Ask Dr. Jason Fung

Is your insulin resistance cured? How does apple cider vinegar help with blood-sugar issues? And what are the good and bad effects of intermittent fasting?

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

Is my insulin resistance cured?

I was wondering how you know when someone has become insulin sensitive again? I have been resistant for many years and borderline type 2 diabetic until embarking on keto with intermittent fasting.

I did a test today where I deliberately ate outside keto and got 9.5 mmol/L (171 mg/dl) 2 hours later – tested a further hour later and was back to 6.2 mmol/L (112 mg/dl).

Is that a sign I might finally be resenzitised to insulin?


It is definitely possible. Insulin resistance is a reversible condition, as is type 2 diabetes. However, it develops over many decades and can often take a long term to reverse as well. Lab tests for insulin resistance include Fasting insulin, HOMA (comparing fasting insulin and fasting glucose) and the less sensitive fasting glucose and A1C. However, one of the most sensitive tests may be the insulin response to an oral glucose challenge, also known as the Kraft assay.

Dr. Jason Fung

How does apple cider vinegar help with the Dawn Phenomenon and how do I rid stored sugar in the liver?

I’ve never been told I’m pre-diabetic or diabetic, fasting blood tests show about 90 mg/dl (5 mmol/L), but that’s usually around 7:30 or 8:00 a.m. I have a home meter that has been showing fasting BG at 101 mg/dl (5.6 mmol/L), so I’m learning about Dawn Phenomenon.

I took apple cider vinegar one night before bed and morning BG was 86 mg/dl (4.8 mmol/L). What does the ACV do to accomplish that number, and more importantly, is it just a bandaid? How do I get rid of the stored sugar in the liver (or correct what I suspect to insulin resistance)?


Vinegar in general may work to decrease blood glucose and insulin response. The mechanism is not entirely known, but many people have found that adding vinegar to foods decreases the rise in blood glucose. This has been shown for example, in sushi rice (vinegared rice) and eating bread with olive oil and vinegar. It is a useful adjunctive treatment but limiting carbs and fasting are the most useful

Dr. Jason Fung

Good and bad effects of intermittent fasting?

Hello Dr. Fung!

Chris Wark recently posted an interview about a researcher that says fasting more than 12 hours a day on a regular basis is bad for you, skipping breakfast can increase your risk of disease/death and high-protein keto diets are dangerous. I wonder what you think about this?

My daughter and I have been following your books and doing a one meal a day protocol with 18-20 hour fast in between. It’s been so easy and with great results. I had encouraged my daughter to do the one meal a day and she has lost a lot of weight and feels great. I wanted to be sure that what we are doing is good.

Do you recommend limiting how many days in a row to do a limited fast, as we do? There is so many conflicting opinions out there, it’s hard to know what to think/follow. But I have followed you and trust your opinion.


It all depends upon your own personal situation whether something is good for you or bad for you. The main point of fasting is to lower insulin levels and raise levels of counter regulatory hormones (noradrenalin, growth hormone). In diseases of excess insulin, fasting is beneficial. Obesity, type 2 diabetes, fatty liver, PCOS are all examples of diseases of hyperinsulinemia. Therefore, fasting is good for you.

There are no rules to fasting duration – only what suits you specific conditions. Many people, for example eat one meal a day for years and feel well.
Dr. Jason Fung



Intermittent fasting for beginners

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Intermittent fasting Q&A

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More with Dr. Fung

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

The Obesity CodeThe Complete Guide to Fasting

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

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