GLOSSARY

Physiological Fuel Value describes energy made available from nutrients under a specified physiological accounting system. Equal nutrient masses need not supply equal energy, and usable yield differs from complete combustion.

Food energy can be expressed at several stages. Gross energy is released by complete combustion, digestible energy subtracts fecal losses, and metabolizable energy also accounts for urinary and gaseous losses. Net energy additionally considers costs associated with using nutrients, with details depending on the accounting system. These quantities answer different questions. A food's laboratory combustion value therefore cannot simply be substituted for the energy made available to a person.

A nutritional Calorie commonly means a kilocalorie. One kilocalorie equals 4.184 kilojoules, so a unit conversion changes the number without changing the underlying energy. Check whether a label refers to a serving or a fixed food mass before comparing foods. A calculation can be numerically correct yet misleading if portions or units differ. Energy is a property of intake and expenditure, rather than a direct rating of a food's overall nutritional quality.

General Atwater factors approximate metabolizable energy from the amounts of protein, carbohydrate and fat. Food-specific factors refine this estimate for composition and digestibility. Fiber and sugar alcohols require additional attention because their usable energy differs between substances and labeling systems. Label energy is therefore an estimate rather than a measurement of every eater's absorption. For athletes, the practical value comes from consistent portion estimates and adequate overall supply rather than false precision in individual foods.