> I think the opposite. People that fast for a period (> 16 hours) report improved energy levels. This may be because after a while the body enters a ketogenic state where it burns body fat, which is a great source of energy and perfect for when on the hunt.

Ketogenesis requires over a week of carb restriction IIRC. I think fasting yields energy because the body increases various stress hormones to push you to find food so you don't starve.

No, it has to do with insulin levels. Google "Jason Fung insulin".

Depending on food type it can easily take over 2 days to digest food. https://www.mayoclinic.org/digestive-system/expert-answers/f...