How efficient is the human body at metabolizing food?
I suspect that what you are actually looking for is the following:

- 1 gram of fat = 9 kcal
- 1 gram of protein = 4 kcal
- 1 gram of sugar = 4 kcal
- 1 gram of alcohol = 7 kcal


Those are general and inexact values. They're just often used to give a rough idea of the amount of energy we get from different types of food.