GLOSSARY

General Atwater Factors Table

General metabolizable-energy factors are approximate food-energy accounting factors, not measured ATP yield.

General Atwater Factors are conversion factors used to estimate metabolizable food energy from nutrient composition. General factors provide practical approximations, with exceptions for some substrates and labeling conventions.

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.

For example, two portions can supply similar labeled energy while differing in volume, protein and digestibility. Compare the actual portion and the intended use around training before deciding which better supports the athlete. An energy number helps quantify supply, while the food's wider composition determines what else that supply provides.

Over a defined interval, a difference between metabolizable energy intake and energy expenditure changes stored body energy. This principle does not imply that daily scale weight directly reveals energy balance. Water, glycogen and gut contents also contribute to body mass. Intake and expenditure influence each other through behavior and physiology, so projecting the same daily deficit indefinitely creates unrealistic predictions. Interpret the energy equation together with its timescale and the composition of the tissues changing.

Energy requirements depend on body size and composition, activity, life stage and physiological circumstances. A maintenance estimate is a starting point for a defined situation rather than a permanent personal calorie ceiling. Training load, recovery, appetite and changes in body mass help evaluate its usefulness. Growth, illness or a change in training schedule may require reassessment. Adequate energy also needs appropriate carbohydrate, protein, fat and micronutrient provision; matching a calorie total alone does not guarantee a balanced sports diet.

Total expenditure includes energy needed at rest, the thermic effect of food and physical activity. Planned exercise is only part of activity expenditure. Work, walking and spontaneous movement can also matter. When an estimate already includes a component, adding it again double counts energy. Establish whether an exercise value is gross or net of resting expenditure, and use the same convention when comparing days or combining wearable estimates with a daily energy model.

Table references: FAO · 1