Are there any dangers fasting while breastfeeding?

Ask Dr. Jason Fung

Can you take supplements when fasting? What kind of fasting can you do while breastfeeding? What is my recommendation on fasting for athletes? And, is a keto diet beneficial for migraines?

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

Supplements while fasting

Are there any known issues with taking supplements, like vitamins or minerals, while fasting? And if you take supplements, could you take them with anything else than water, like for example broth? Would any of this negate the effects of fasting, or have other negative effects?


There are no major issues with supplements during fasting. Iron can cause some constipation, which can be an issue during fasting anyways. You could take it with bone broth, and there should be no major negative effect.

Dr. Jason Fung


Is there any danger to fasting while breastfeeding? What kind of fasting – like what length – could be recommended if it’s OK?


I don’t recommend fasting for more than 16 hours during breastfeeding. Nutrition is very important during that post partum period, so there is too much risk and it is a temporary situation. There will always be time to fast longer after the baby is weaned. The body needs to heal from the baby, which takes a lot of nutrients out of the mother. Breast milk needs to be produced, which also takes nutrients out of the mother.

Dr. Jason Fung

Fasting for athletes and youth

What do you recommend for athletes training and fasting? Training for mountain biking, basketball etc. And what do you recommend for fasting for youth (12 and up)?


I recommend athlete use training in the fasted state. 16-24 hours of fasting followed by workout and then eating – with some higher emphasis on protein. 24 hours is ideal, but elite athletes often have trouble eating enough calories during the day. Similarly, we suggest higher than usual protein for muscle growth, which is a totally different situation from weight loss, where we only recommend moderate protein. For youth, I don’t advise more than 24 hours of fasting routinely, so would advise more of a no-snacking or time restricted eating protocol. Fasting is not optimal for growth, which is important for teenagers.

Dr. Jason Fung

IF and keto for migraine?

Hi – just wondering if you recommend IF with keto for migraine with aura – or keto only. I’ve seen some studies RE keto, but nothing with IF. I used to do IF, but moved to keto. I now have a MCT oil coffee with cream at 7am and first meal at 1pm.


Fasting is predominantly about lowering insulin and raising the counter-regulatory hormones. This would not be expected to improved migraines much, if at all. During initial fasting, you can get headaches, and some people find it triggers migraines, too. Keto would be more preferable in that case.

Dr. Jason Fung



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