Determination of energy expenditure during heavy exercise, normal daily activity, and sleep using the doubly-labelled-water (²H₂¹⁸O) method

Energy expenditure of four subjects was measured by the doubly-labelled-water (²H₂¹⁸O) method to determine if energy expenditure could be determined over short periods. Three subjects were studied while they performed 8 h of heavy exercise. Estimated energy expended during 8 h of high-intensity exercise (55% VO₂ max) for three subjects was 757 ± 118 kcal/h by the doubly-labelled-water method using urine and a two-point calculation, which compared favorably with 735 ± 82 kcal/h obtained by respiratory gas exchange. For the fourth subject, daytime, nighttime, and daily energy expenditure was calculated by both the two-pair method and decay-curve analysis of urine and saliva samples collected in the morning and at night. Daytime and nighttime energy expenditures differed significantly (p<0.05).