I think it is an interesting and underappreciated aspect of calorie counting as well. I think calories are a decent first order approximation for foods that humans (and animals) evolved eating, because we are efficient extracting chemical energy.

The alternative would be empirical animal studies that look directly at weight as a function of feed. You will note that agribusiness doesn't mess around with calories when money is on the line. Instead relies on empirical data for mass as a function of feed type.