Why does the human body get less energy from food than a calorimeter measures?

A bomb calorimeter measures the total potential energy in food by burning it completely through combustion, a process that releases all available chemical energy as heat.12

The human body, however, is not as efficient as a calorimeter. It derives less energy from food because it cannot achieve the same level of complete oxidation, and some potential energy is inevitably lost during the processes of digestion and metabolism. While a calorimeter provides a measure of the total heat of combustion, the body’s actual energy yield is lower due to these physiological limitations. Consequently, nutritionists use corrected physiological values—often referred to as Atwater factors—to estimate the usable energy from carbohydrates, proteins, and fats rather than relying solely on the raw data produced by a calorimeter.2