So last week there was a thread about a scientist who wrote a book:
https://news.ycombinator.com/item?id=30381000
and I am reading a copy of the book from my local library and it actually covers this topic. The relevant part of theory (which has been shown to be true for every species they measured with doubly labelled water/urine) is that all mammals are calorie restrained - there's a maximum amount of calories that can be eaten, and there's a maximum amount of calories that can be burned, the range for humans is not that big, humans have a much higher calorie per weight budget than our ape/monkey cousins, the extant hunter gatherer societies exhibit the same behavior, and that being inactive like many
modern humans means we have excess calories to burn with our bodys involuntary system like the immune system (one of the hunter gatherer societies had high immune system activity because of endemic worm/disease infection to use up a lot of calories), so the hypothesis is being physically active (up to a point) tamps down the activity of involuntary systems that do not need to be so active.