Can the body really get as much energy AND building material as the nutrition content declaration says?

Suppose you eat a food that, according to its nutrient content declaration, provides X grams of protein, etc, etc, and Y calories of energy. When you eat that food, the energy in the X grams of protein is included in Y, right? But what if your body uses the protein for building material and not for energy? Then do you still get Y calories of energy? I mean, the same protein molecules can’t be used for both energy AND building material, can they? But, on the other hand, if Y _doesn’t_ include the energy that’s in the protein, Y must be false in another situation – namely when your body uses the protein for energy! Or am I missing something? (For now, let’s ignore the fact that any nutrient info is inexact in other ways anyway, and that each body will take up different amounts of the nutrients in any food depending on various circumstances.)