Why does blood sugar increase during a fast?

Ask Dr. Jason Fung

Why is my blood sugar increasing during a fast? Will reversing diabetes prevent arterial plaque production in the future? And can keto and intermittent fasting help reverse thyroid disease (Hashimoto’s)?

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

Increasing blood sugar when fasting

Why does my blood sugar level increase from 86 to 112 mg/dl (4.8 – 6.2 mmol/L) when I do a 20-hour fast? Is that normal? I’m on an LCHF diet.

Thank you doctor,

Answer:This is a normal process. When you fast, insulin levels start to drop and this triggers a surge of counter-regulatory hormones, including noradrenalin and growth hormone. This is normal, and meant to pull some of the stored sugar from the liver into the blood. If your liver is full of sugar, it may release lots of sugar into the blood, causing the blood sugar to rise.

So, yes, blood sugar may rise during fasting. The most important question to consider, though, is where this sugar came from. If you are not eating, the rise in blood sugar may only come from your own body. You are simply moving sugar from the liver to the blood. It means that there is too much sugar stored inside your body and you need to empty it out, either with LCHF diet or intermittent fasting

Dr. Jason Fung

Arterial plaque

Reversing diabetes and therefore inflammation should stop future arterial plaque production, right? Does reversing diabetes cause plaque to recede from arteries already affected? Are there any studies on this?


Answer: Diabetes is a very strong risk factor for heart disease and arterial plaque. Reversing diabetes should theoretically lower the risk of plaque disease, but there are no studies that conclusively demonstrate this. That is probably because most people consider type 2 diabetes to be a chronic and progressive disease, like aging. So, yes, I think it will reduce risk of heart disease, but no, there are no studies to prove this.

Dr. Jason Fung

Hashimoto’s, fasting and keto

I believe there is a way to reverse Hash! I feel so much better on keto and IF than any treatment over the past three years. Have you had any patients reversing Hashi with this program?

Thank you so much for all your work for all of us,

Answer: I don’t see Hashimoto’s very frequently so don’t have much experience. Fasting and low carb diets usually drop insulin so are effective for diseases of excessive insulin such as obesity, type 2 diabetes and PCOS. However, it may also have a smaller effect on inflammation so it may certainly benefit Hashimoto’s as well, but there are no studies to prove this, and I have only limited clinical experience with Hashimoto’s.

Dr. Jason Fung



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

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

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