It's also been hard to show that fasting works in humans. Take caloric restriction, which is a dietary intervention related to fasting, though it's not the same since it allows for some eating, just far less than normal. We have extensive evidence that caloric restriction can dramatically extend the lives of certain lab mice and even keep them physically "younger" when compared to mice allowed to eat whatever they want. But we don't know that drastically reducing calories in people will do the same. Not all mice respond in the same way, says Longo, and in people we think that some of the negative side effects of a calorie-restricted diet (30% below normal) would outweigh the health benefits.


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