I think how to fast is a good question. Longo might have gone with his fasting mimicking diet because he didn't think people would comply with total fasting, and their doctors wouldn't support them in it, which increases the risks. But the question of where the therapeutic effects of fasting start is a genuine one for science. I often see a day of fasting on water given as preferable to one with beef broth, or one where somebody's taken in 500 calories. Is it? It might seem prudent to assume that that's so, but for all we know, some small calorie intake might not only not ruin the therapeutic effect, it might make things better.

I just feel like there's a risk of jaundicing people against effective interventions that they're actually willing to do, and have easier access to medical support for, in defense of fasting purity. Some of the arguments for making a fast pure water don't really hold up to scrutiny. I've seen Dr. Fung talk about the micrograms of protein in a cup of coffee as preventing autophagy, a claim that astonishes me with how poorly founded it is. Maybe in cells isolated in a petri dish, but you'll have way more of your own proteins in your digestive tract being autodigested in the normal course of events than you'll ever get from that cup of coffee.