Energy is an abstraction representing approximately the maximum amount of calories can be used in a day. The lower it gets, the less effective you are, because your body is resorting to more difficult-to-extract energy sources (fat or protein instead of blood sugar, depleted glycogen reserves in muscles, etc). Additionally, your metabolism can only actually absorb so much nutrients in a given day, it takes time to digest food and extract nutrients from it.
The exact biological functions aren't really relevant to gameplay, but they are physical processes to base our equations on.
Points of reference
- A small, sedentary person uses ~1500 kcal/day
- An average male soldier marching all day uses ~6000 kcal/day
- An Olympic athlete in an endurance event can use as high as ~12000 kcal/day
- Calories metabolized by humans per-gram
- Fat: 9
- Protein: 4
- Uncooked starch: 2
- Cooked starch: 4
- Sugars: 4
- Cellulose: 0.2
- Alcohol: 7