This is what I’ve heard as well. I’m sure there is some factual evidence to support this. I also believe that there is a cost for getting to that point. Personally, the longest I’ve gone is 48 hours. I figure I can achieve autophagy in that time. I also exercise, which also increases autophagy. It seems to me that the people who are fasting longer are trying to lose a lot of weight or they are fasting for religious purposes.
Autophagy explodes around day 10 from what I’ve been told by some very smart people/read. Your body will be deep in ketosis and to provide your brain necessary glucose it will utilize damaged cells or any unnecessary tissue.