Objective: Because of the difficulty in obtaining accurate dietary intake information on children and adolescents, the World Health Organization (WHO) has recommended that energy requirements in children and adolescents are best established by estimate of energy expenditure in free-living individuals. To define the energy requirements of healthy female adolescents, we used the doubly labeled water method to estimate the total energy expenditure (TEE) of nine female Caucasian adolescents (mean age +/- SD 13.2 +/- 1.8 years; range 10.3-16 years).
Methods: We collected a baseline plasma and saliva sample from each subject after she received by mouth 125 mg of 18O and 100 mg of 2H2O/kg body weight. A 3-hour postdose plasma sample was also collected. Subsequently at home, each subject collected one postdose saliva sample daily for the next 10 days. The 24-hour respiratory quotient (RQ) of each subject was measured by whole-room calorimetry. The 18O and 2H abundances of the plasma and saliva samples were measured by gas-isotope-ratio mass spectrometry. Isotope dilution spaces of 18O (No) and 2H (NH) were calculated from the enrichment levels of these isotopes in the 3-hour postdose plasma sample. The fractional turnover rates of 18O (ko) and 2H (kH) were calculated from the monoexponential decay curves of these isotopes in the saliva samples. The average CO2 production rate (rCO2) over the 10-day period was calculated as rCO2 (mol/day) = 0.4584 x (ko x No-kH x NH). The average TEE was calculated from the rCO2 using the Weir equation.
Results: TEE calculated by the isotope method was 2322 +/- 281 kcal/day (range 1850-2656 kcal/day), which is higher than the levels of energy expenditure (1910-2140 kcal/day) reported by the WHO for healthy female teenagers with a desirable body composition and appropriate level of physical activity.
Conclusion: Our results suggest that the activity factors used by the WHO in the calculation of energy expenditure of female adolescents between 10 and 16 years of age might have been underestimated.