> 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.