Body & HealthFact
Protein costs you a fifth of its calories just to digest.
Digestion itself uses energy, the thermic effect of food. It differs by nutrient: with protein, 20 to 30 percent of the calories go into breaking it down and processing it, with carbohydrates 5 to 10 percent, with fat only 0 to 3 percent. That is why you feel warm after a big meal: you are literally heating with food.
Useful: 100 kilocalories from protein yield less net energy than 100 from fat.
Source: doi.org