Are artificial sweeteners allowed when fasting?

Ask Dr. Jason Fung

There are tons of questions about intermittent fasting, like these:

  • Are artificial sweeteners allowed when fasting?
  • Does fat interfere with autophagy during fasting?
  • 500-calorie meal on fasting day?

Dr. Jason Fung is one of the world’s leading experts on fasting for weight loss or diabetes reversal. Here are a his answers to those questions and more:

Are artificial sweeteners allowed when fasting?

I have a sweet tooth and its difficult for me to have plain black coffee or tea. Stevia or xylitol would be risking the results of the fasting?


Artificial sweeteners may stimulate insulin release. You can try fasting with the sweeteners – if you get good results, then continue.

Dr. Jason Fung

Does fat interfere with autophagy during fasting?

Hello Jason,
Specifically, does coconut oil, being 100% fat only and little else, have an effect on autophagy when it keeps ketones high as well?


Likely not, although research is lacking. Autophagy mostly turns off with protein.

Dr. Jason Fung

500-calorie meal on fasting day?

A number of fasting regimes (Every Other Day Diet and 5:2 Diet) recommend having a 500 to 600 calorie meal on ‘fasting’ days. Why is this? To me, this would spike insulin and temporarily break the fast. Wouldn’t it be better to just not eat anything on the fasting day? Is there a specific physiological reason why these authors recommend this, or is it to encourage participants’ compliance?
Thank you,

The reason authors recommend it is because they believe a full fasting day (0 calories) is too hard for most people. I disagree. However, they do get good results with this modification and I have no problem with these sorts of regimens.

Dr. Jason Fung



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  1. Ian
    I used artificial sweeteners to get over the craving for sugar (in coffee). After two months or so the "fake" aftertastes put me off. It took much less will power to give these up. But this might not work for everyone.
  2. 1 comment removed
  3. Wm
    Like you got a sore toe so u pound your finger with a hammer. Now your not concerned about your toe. Hmmm.
  4. Mohammed
    Hi - Can you go on a Keto diet without fasting ?
    Reply: #5
  5. Kristin Parker Team Diet Doctor

    Hi - Can you go on a Keto diet without fasting ?

    Yes, definitely! You can eat to hunger.

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