Daily metabolic rate in healthy infants.

Background: Previous estimates of daily metabolic rate in infants were based on short-term unstandardized measurements of energy expenditure (EE).

Objective: Determine 24-hour metabolic profiles in infants.

Methods: Energy expenditure (kcal/min by indirect calorimetry) and physical activity (oscillations in weight/min/kg body weight) were measured in 10 healthy infants (5.0+/-0.8 months, 68+/-3 cm, 7.3+/-0.8 kg) for 24 hours in the Enhanced Metabolic Testing Activity Chamber while allowing parental interaction. Energy intake, 24-hour EE, resting metabolic rate (RMR), and sleeping metabolic rate (SMR) (kcal/kg/day) were determined. In addition, extrapolated 24-hour EE, RMR, and SMR from the first 4 and 6 hours of data were compared with 24-hour measurements.

Results: Twenty-four-hour energy intake, EE, RMR, and SMR (mean+/-SD) were 78.2+/-17.6, 74.7+/-3.8, 65.1+/-3.5, and 60.3+/-3.9, respectively. EE and physical activity showed a decrease at 11:30 pm and a return to daytime levels by 5:30 am, suggesting a metabolic circadian rhythm. Extrapolated 24-hour EE, RMR, and SMR from the first 4 hours (72.2+/-6.6, 65.9+/-8.7, and 64.9+/-6.4) and 6 hours (74.8+/-6.7, 65.8+/-6.6, and 64.8+/-5.6) were similar to 24-hour measurements.

Conclusions: An apparent circadian rhythm in metabolic rate and physical activity was detected by 24-hour measurements. Furthermore, shorter-term measurements of the variables were comparable with 24-hour values.