depend on the specific person

I'm not really sure how to pinpoint individual differences. I'm going to stop here but I honestly think it would be nice to break this down further. A potentially harmful practice could be taking some sort of average ability to digest food, and then start deriving standard deviations from it. I'm saying 'harmful' because I (1) do not know how to do this and (2) I have no idea if this is the right thing to do.

Now apply this argument to the calories themselves. Is it possible that two people eat the same food, yet one of them extracts 1000 calories from the food, and the other extracts 1500 calories?

I'd imagine that people who had a less economical digestion would probably have less offspring, but that's just a guess.

Well, you have just returned my question. I was curious whether there are ways to spend calories that most people would forget to think about when thinking about "work".

It would be greatly helpful to have a list of energy spendings by the body, then. Can someone provide directions?