Since our bodies do not have the proper enzymes to break down cellulose, and I eat a serving of corn or nuts listed as a 100 calories, did I really consume 100 calories?
The amount of food energy associated with a particular food could be measured by completely burning the dried food in a bomb calorimeter, a method known as direct calorimetry. However, the values given on food labels are not determined this way, because it overestimates the amount of fuel that actually enters the blood through digestion because it also burns the indigestible dietary fiber so that not all food eaten is actually absorbed by the body (fecal losses). Instead, standardized chemical tests or an analysis of the recipe using reference tables for common ingredients are used to estimate the product's digestible constituents (protein, carbohydrate, fat, etc.). These results are then converted into an equivalent energy value based on a standardized table of energy densities.