A calorie is not a calorie

The Thermic Effect of Food

Different foods go through different metabolic pathways.

Some of these pathways are more "efficient" than others.

The more "efficient" a metabolic pathway is, the more of the food energy is used for work and less is dissipated as heat.

The metabolic pathways for protein are less efficient than the metabolic pathways for carbs and fat.

Protein contains 4 calories per gram, but a large part of the protein calories are lost as heat when it is metabolized by the body.

The thermic effect of food is a measure of how much different foods increase energy expenditure, due to the energy required to digest, absorb and metabolize the nutrients.

This is the thermic effect of different macronutrients (7):

• Fat: 2-3%.
• Carbs: 6-8%.
• Protein: 25-30%.

Sources vary on the exact numbers, but it is clear that protein requires much more energy to metabolize than fat and carbs ( 8).

If we go with a thermic effect of 25% for protein and 2% for fat, this would mean that a 100 calories of protein would end up as 75 calories, while a 100 calories of fat would end up as 98 calories.

Studies show that high protein diets boost metabolism by 80 to 100 calories per day, compared to lower protein diets (9, 10) Put simply, high protein diets have a "metabolic advantage." There is also one study that compared two sandwich meals that had the same number of calories and macronutrients.

However, one sandwich was made with whole grains and cheddar cheese, while the other was made with refined grains and processed cheese (11).

Those who ate the whole grain sandwich burned twice as many calories digesting the meal.

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