Digesting food burns calories, protein most

Every time you eat, your body spends energy breaking the food down and storing it. This is the thermic effect of food, usually put at about 10 percent of the calories you eat, with small lab studies measuring roughly 4 to 9 percent of a meal. Protein costs the most to process, by a clear margin over carbs or fat.

Why it works

The body uses energy to digest food, absorb it and process the nutrients. Protein is thought to cost the most because its amino acids are broken down, the nitrogen is cleared as urea, and some of what is left can be turned into glucose, each step taking energy. The linked studies measured the totals, not these steps.

The evidence

• Calcagno, M., Kahleova, H., Alwarith, J. et al. (2019). The Thermic Effect of Food: A Review. Journal of the American College of Nutrition.

  This review describes the thermic effect of food (the rise in metabolic rate after a meal, from digesting and processing it) as a modifiable part of energy expenditure. It reports that the evidence suggests TEF is increased by larger meals (as opposed to frequent small meals), by carbohydrate and protein (as opposed to dietary fat), and by low-fat, plant-based diets. Age and physical activity may also play a role, while the effects of habitual diet and meal timing remain unclear.

• Ogata, H., Kobayashi, F., Hibi, M. et al. (2016). A Novel Approach to Calculating the Thermic Effect of Food in a Metabolic Chamber. Physiological Reports.

  In 7 healthy young men studied in a metabolic chamber for 33 hours with and without breakfast, the authors tested a new way to calculate the thermic effect of food that removes the noise from spontaneous movement. Their method gave a morning TEF of 4.1% (plus or minus 2.5%) of meal energy, with all values positive, while some older methods produced negative or highly variable results. The new method matched the gold-standard measure closely (mean difference of about 1.3%, no significant bias) and tracked physical activity far better (R-squared 0.45 versus 0.17 for a conventional method).

• Crovetti, R., Porrini, M., Santangelo, A. et al. (1998). The Influence of Thermic Effect of Food on Satiety. European Journal of Clinical Nutrition.

  Ten healthy, normal-weight women each ate three meals of about 557 kcal that were matched for calories but differed in makeup: high protein (68% of energy from protein), high carbohydrate (69%), or high fat (70%). The high-protein meal more than doubled the thermic effect of food compared with the high-fat and high-carbohydrate meals, and it also produced the strongest feeling of fullness. The authors conclude that the thermic effect of food contributes to how filling a meal feels.

  n=10

• Guarneiri, L.L., Adams, C.G., Garcia-Jackson, B. et al. (2024). Effects of Varying Protein Amounts and Types on Diet-Induced Thermogenesis: A Systematic Review and Meta-Analysis. Advances in Nutrition.

  In acute studies, higher-protein meals produced greater diet-induced thermogenesis (SMD 0.45) and total daily energy expenditure (SMD 0.52) than lower-protein meals; the DIT effect was significant in normal-weight but not overweight/obese participants. In chronic studies, higher-protein diets raised TDEE (SMD 0.29) and resting energy expenditure (SMD 0.18) but not DIT. Protein type made no difference.

• Kinabo, J.L., Durnin, J.V. (1990). Thermic Effect of Food in Man: Effect of Meal Composition and Energy Content. British Journal of Nutrition.

  Sixteen non-obese women each ate four test meals that varied in makeup (high-carbohydrate-low-fat vs low-carbohydrate-high-fat) and in size (600 kcal vs 1,200 kcal). Meal composition made no significant difference to the thermic effect of food, but meal size did: the 600 kcal meals produced about 54 kcal of TEF (roughly 9% of the meal) and the 1,200 kcal meals about 81 to 85 kcal (roughly 7%). The authors note metabolic rate had not fully returned to baseline by 5 hours, so these figures likely underestimate the total thermic effect.

  n=16

• Quatela, A., Callister, R., Patterson, A. et al. (2016). The Energy Content and Composition of Meals Consumed after an Overnight Fast and Their Effects on Diet Induced Thermogenesis: A Systematic Review, Meta-Analyses and Meta-Regressions. Nutrients.

  Higher energy intake increased DIT (about 1.1 kJ/h for every 100 kJ more). Meals high in protein or carbohydrate produced higher DIT than high-fat meals, although the difference was not always significant. Medium-chain triglycerides produced more DIT than long-chain, and one bolus meal more than the same food spread over several small meals or snacks.

Why we call confidence medium

Medium: the protein ranking is well supported, but the 10 percent figure is a convention from narrative reviews rather than a pooled finding. Guarneiri's 2024 meta-analysis found higher-protein meals raise diet-induced thermogenesis and daily energy expenditure in acute studies, and Quatela's 2016 meta-analysis found high-protein or high-carbohydrate meals raised it more than high-fat meals, though not always significantly. Small chamber studies agree on protein (Crovetti 1998, 10 women) but found carbohydrate and fat meals similar (Crovetti 1998, Kinabo 1990). Measured values in the linked studies were about 7 to 9 percent of a meal's calories (Kinabo, likely an underestimate) and 4.1 percent for breakfast (Ogata 2016).

Where it applies

Healthy adults