GLOSSARY

Doubly Labeled Water (DLW) estimates free-living energy expenditure over time from elimination of stable isotopes of hydrogen and oxygen. It does not directly identify the timing or exact cost of each exercise session.

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.