Why protein 'burns' calories just by being digested

Different foods cost your body very different amounts of energy just to process

Your body spends part of any food's own calorie value just digesting, absorbing, and processing it — this is called the thermic effect of food. But the effect varies a lot by macronutrient: roughly 5–10% for carbs, just 0–3% for fat, and a lot more for protein — 20–30%.

That means out of 100 calories from pure protein, your body actually absorbs and uses only about 70–80, with the rest going toward the digestion process itself — breaking complex protein chains down into amino acids takes noticeably more enzymatic work than breaking down carbs or fat.

The practical effect isn't huge, but it's real: a higher-protein diet at equal total calories tends to burn slightly more energy just through digestion, on top of protein's other well-known benefits — preserving muscle in a calorie deficit and better satiety.