Do you recommend going in and out of ketosis?

Ask Dr. Jason Fung

What do I need to know about exercising while doing extended fasting? How much salt should I add to water when I’m fasting? And do you recommend going in and out of ketosis?

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

Exercising while fasting

Is there anything I should bear in mind regarding extended fasting and endurance training? I’m looking at starting some extended fasts of around 36 hours, but I will also be exercising quite heavily as I’m training for a marathon. Any advice would be really appreciated.


There is a period of adaptation, about 2-3 weeks to get your body used to exercising while fasted.

Dr. Jason Fung

Water fasts and staying hydrated

Hi Dr Fung,

Do you have a recipe for how much salt to add per one liter of water, for a person to drink while on a water fast to maintain hydration and electrolytes? Every person drinks a different amount of water each day so the question is if a person only drank this salted water and no other liquids.


There is no single right answer. Some people don’t use salt at all. Others get dizzy if they do not take supplemental salt. The kidneys have an extraordinary ability to retain salt in the the event of sodium depletion. For shorter fasts (24-36 hours), it is likely not necessary to supplement.

Dr. Jason Fung

Going in and out of ketosis regularly?

Is it necessary or recommended that you switch out and into ketosis again on a regular basis? Online I read that some experts advise that, in order to maintain insulin sensitivity. For women I even read about a high-carb “week” during the menstrual cycle, altered with a ketogenic diet in the other 3 weeks. Or 5 days keto, 1 day fast and 1 day feast, with higher carbs.

I watched many videos on the dietdoctor website now (love it!) But have not come across any such advise yet.


Personally, I think it is beneficial to switch things up every few weeks, too, although most evidence is anecdotal. There is one small study MATADOR, that suggests this is true. In this study, alternating periods of calorie restriction with normal diets seemed to be better than continuous dieting. The same may hold true for ketosis for some people. But there is no evidence to suggest one way or the other, and different people will respond differently. Some people struggle to get back into ketosis once they go out, some experience cravings they can’t control, and others move back and forth with ease. The key is knowing which group you are in. Hopefully we will have data about this in the near future.

Dr. Jason Fung



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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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