“Can you get type 2 diabetes again once you’ve reversed it?”

Ask Dr. Jason Fung

Can high levels of B12 be a sign of kidney failure? What effect can fasting have on autoimmune disease? What is the best way to break a fast? And, is it possible to ever really cure type 2 diabetes?

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

Could you talk about high vitamin B12?

Can abnormally high B12 levels be a sign of kidney failure if diabetic and doing intermittent fasting?


B12 levels are not a sign of kidney failure. Mostly we worry about low vitamin B12, especially in vegans.

Dr. Jason Fung

Impact of fasting on autoimmune disease

I read your response about autoimmune disease and fasting and you stated autoimmune disease has little to do with diet.

Are you familiar with Dr. V. Longo’s studies regarding fasting? He is doing research that is starting to show that fasting and autophagy can be very effective in improving various autoimmune disease.


We don’t know what causes most autoimmune diseases. However, fasting may potentially improve them by regeneration of the immune system. Dr. Longo’s studies suggest this is possible, but there are several caveats. This research is done in animals, and it may or may not apply to humans. Second, autoimmune diseases are all different and we don’t know if some of them are more responsive or not. Third, you generally need longer fasts going out to about 7 days. Fourth, we don’t know how often you would need to repeat these fasts.

In medicine, it is all about risk versus reward. The reward is theoretical but potentially important. The risk is minimal, although 7 days of fasting is generally not fun. My own advice is that everybody should give it a try, under proper supervision, since there is little risk. I would repeat a 7 day fast in about a month or 6 weeks as it may not respond to immediately. If you notice a significant improvement, then I would consider using it on a regular basis. If you notice no change, then I would not continue.

Dr. Jason Fung

Breaking a fast

Good day Dr. Fung,

Firstly I want to thank you for writing your book, The Obesity Code. I received it for a Christmas present and I will be reading it in 2019.

I have two questions:

  1. What is the correct way to break a 16-8 fast while on a ketogenic diet? A normal LCHF meal or drink a bullet proof coffee?
  2. Can I IF on a 16-8 window every day or must is it better to fast one day and eat normally the next day?

Look forward to hearing from you.

Ben de Swardt

  1. There is no ‘correct’ way. General principles would be that longer fasts need to be broken more gently. 16:8 is fairly short so it likely doesn’t matter much. Also, a second general principle is to eat natural foods. I would prefer an LCHF meal rather than a bulletproof coffee for that reason.
  2. Yes, many people (myself included) use a 16:8 schedule daily. Skipping breakfast becomes quite easy after a while, so I fall into a 16:8 rhythm quite naturally. For other reasons, I think that eating earlier in the day is better than late, so skipping dinner is probably better than skipping breakfast, but for social/work reasons, skipping dinner regularly is difficult for many people.

Dr. Jason Fung

What does reversing type 2 diabetes mean to you?

When doing LCHF and IF with awesome results (A1C around 5.6, down from a high of 9.0) it looks like reversion of diabetes. If symptoms (ie high glucose) are all normal, does it mean one has to maintain this lifestyle forever? People without diabetes can have all the pizza and beer they want with little effect on blood sugars. While one who has reversed diabetes can never be truly cured. Is this correct?


If you had type 2 diabetes (T2D) and then you get your A1C under 6.0 without medications, then you are defined (in Canada) as non-diabetic. In other words, your T2D reversed. But if you return to the diet that gave you T2D, you will get it again. Think of your body like a sugar bowl. If you eat lots of sugar and glucose, the bowl fills up. If you eat more glucose, then it overflows and spills into the blood. That’s T2D. If you empty the sugar bowl, then you won’t have T2D, but it doesn’t mean that you will never get it again.

Dr. Jason Fung


Q&A videos

Top Dr. Fung videos


More with Dr. Fung

All posts by Dr. Fung

Dr. Fung has his own blog at idmprogram.com. 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.

Older posts