Question: how many calories does your body absorb?
Answer: Protein takes the most energy to digest (20-30% of total calories in protein eaten go to digesting it). Next is carbohydrates (5-10%) and then fats (0-3%). Thus, if you eat 100 calories from protein, your body uses 20-30 of those calories to digest and absorb the protein. You'd be left with a net 70-80 calories.