GLOSSARY

Indirect Calorimetry estimates energy expenditure from respiratory gas exchange under suitable assumptions. Oxygen consumption and carbon dioxide production inform the calculation, with preparation and steady measurement conditions affecting validity.

For example, a resting respiratory test and a wearable estimate from a training day answer different questions. Before comparing their numbers, check the period, conditions and components included. A lower resting value than a daily total is expected and does not, by itself, show that either instrument has failed.

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.

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.

Basal measurements require stricter conditions than many resting measurements, including a postabsorptive state, physical rest and a thermoneutral environment. Resting energy expenditure and basal metabolic rate are closely related but not automatically interchangeable. Recent training, food, stimulants, illness and measurement procedures can affect results. A prediction equation supplies an estimate for a population and its inputs, whereas properly performed indirect calorimetry measures respiratory exchange under the conditions of that particular test.