Variation in total energy expenditure in young healthy free-living men

Interindividual and intraindividual variation in total energy expenditure (TEE) were examined in 17 healthy, free-living men (weight, 56.4 to 82.4 kg; age, 18 to 30 years). TEE over 14 days, resting metabolic rate (RMR), and body composition were measured two or three times during 77 days of fixed caloric intake using doubly labeled water, respiratory gas analysis, and isotope dilution, respectively. When individual data were averaged, TEE was most significantly related to fat-free mass ([FFM] r = .73, P = .001), body mass (r = .70, P = .002), and RMR (r = .63, P = .006). After adjusting TEE for BM, a significant inverse relation with age was found (partial r = -.52, P = .032). Stepwise regression analysis showed that 69% of individual variation in TEE was explained by BM, age, and fasting respiratory exchange ratio (RER). TEE/RMR averaged 1.73 +/- 0.25 (range, 1.38 to 2.32), and was independent of age and body composition. In 10 subjects in whom triplicate observations of TEE were performed, the average experimental variation for TEE was +/- 11.9% (range, 6.1% to 19.6%) compared with a theoretical estimate of precision of +/- 5.9% based on the reported isotope dose and analytical uncertainty. The difference between theoretical estimates of precision and observed experimental variation suggests that inherent random variation in free-living TEE is +/- 10% (ie, square root of 12(2)-6(2)) in subjects maintained on fixed caloric intake. We conclude that in young free living men (1) BM, age, and RER are important determinants of TEE; and (2) intraindividual variation in TEE is approximately +/- 10% due to fluctuations in physical activity levels within individuals over time.