Protein Loses a Fifth to a Third of Its Calories in Digestion
Fat costs almost nothing to digest and carbs cost a little, while protein takes far more work to turn into energy.
Andrew Huberman · Essentials: The Science of Eating for Health, Fat Loss & Lean Muscle | Dr. Layne NortonEat 100 calories of protein and your body keeps only 70 to 80 of them. The rest is spent during digestion.
A calorie is just a unit of energy, specifically heat, says Layne Norton, a PhD scientist who studies fitness and nutrition. All calories are the same size. What differs is how much energy it takes your body to break food down and turn it into fuel it can use.
Scientists call that cost the thermic effect of food. Every meal requires work, breaking chemical bonds in food and converting the pieces into energy. That work itself burns calories, so the net gain is always a little less than what was eaten.
Fat is cheapest to process. Eat 100 calories of fat, and you keep about 97 to 100, because fat is the easiest thing for the body to convert into energy.
Carbohydrates cost a bit more. Eat 100 calories of carbohydrate, and you keep about 90 to 95. How much fiber the food contains matters a lot here, since some fiber and plant-bound nutrients cannot be fully digested.
Protein is the most expensive to process. For every 100 calories eaten, 20 to 30 are spent on digestion.
That discount does not make protein calorie free. Its carbons are unlikely to wind up directly in fat tissue, Norton says, but in a high calorie diet they still add energy that, in his words, "can provide a calorie cushion for other things to be stored in fat." Protein also tends to be more satiating. More broadly, Norton says "all sources of calories may have differential effects on energy expenditure and appetite."
Questions people ask
Can I eat unlimited protein and not gain weight?
No. You still keep 70 to 80 of every 100 protein calories, so a large surplus can still maintain or add weight. Protein itself is unlikely to be stored directly as fat, but its calories still count and can leave room for other calories to be stored.
Does fiber change the math for carbohydrates?
Yes. Insoluble fiber is generally not digestible, and carbohydrates locked inside plant structures can be inaccessible to digestive enzymes. That lowers the usable energy from some high fiber foods, and differences in gut microbes may affect how much energy people extract.
How big is digestion compared with the rest of my daily calorie burn?
Digestion is a relatively small share, about 5 to 10% of total daily energy expenditure. By comparison, resting metabolic rate is about 50 to 70%. Digestion is very difficult to measure, so researchers often make an assumption about it and use a constant.