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Background: We previously reported that differences in fat-free mass (FFM) estimated by isotope dilution of 18O-labeled water could not fully account for lower resting metabolic rates (RMR) in old men and women compared to RMR in young men., Methods: Since age-related changes in the distribution of water between extracellular and intracellular spaces could lead to overestimation of FFM in the old, we reanalyzed our data using estimates for total body and intracellular water (TBW and ICW, respectively) derived from published equations and included data from adolescent boys and girls studied under similar conditions., Results: In both sexes, the age-related reduction in RMR remained significant after adjustment for estimated body water compartments (p < .05). While adjusted RMR differed in boys and girls (p < .0001), it did not in old men and women (p = .15)., Conclusion: We conclude that aging per se reduces RMR in lean tissue, a difference which cannot be fully explained by changes in body water or its distribution. Investigators should be cautious when selecting models and equations to estimate body water compartments.

Several different equations based on height, weight, sex, and age are available to predict metabolic rate in children and adolescents. The purpose of this study was to determine which of the published standardized equations most accurately predicts metabolic rate in premenarcheal girls. Resting metabolic rate (RMR) was measured in 186 premenarcheal girls by using open-circuit indirect calorimetry with a ventilated hood. Measured RMR was compared with equations provided by the FAO/WHO/UNU, the Mayo Clinic, and Robertson and Reid for girls of the same age. The accuracy of each equation differed for each Tanner stage. The Mayo Clinic standards significantly differed from measured RMR among girls in Tanner stages 1 and 2. The Robertson and Reid standards differed from measured RMR for girls in Tanner stage 1. Only the FAO/WHO/UNU equation predictions did not differ from measured RMR for all Tanner stages studied. These results indicate that the FAO/WHO/UNU equation is the best predictor of RMR in premenarcheal girls at different stages of development. Furthermore, because RMR measured under inpatient and outpatient conditions did not differ significantly, an overnight measurement is not necessary for a reliable determination of RMR.

To evaluate the validity of equations for the calculation of basal metabolic rate, we compared measured metabolic rates in a population that included obese and nonobese adolescents with metabolic rates calculated from five equations commonly used to estimate metabolic rate. Of the available options, neither the Mayo Clinic nomogram nor the Food and Agriculture Organization/World Health Organization/United Nations University (FAO/WHO/UNU) equations produced estimates that differed significantly from measured values. In a second cohort of severely obese adolescent girls, the FAO/WHO/UNU equation that included both height and weight provided the most accurate estimate of metabolic rate. Because of their simplicity, we recommend use of the FAO/WHO/UNU equations to estimate metabolic rate in adolescent populations (boys: BMR = 17.5 weight (kg) + 651; girls: BMR = 12.2 weight (kg) + 746). However, when obese cohorts are studied, the FAO/WHO/UNU equation that includes both weight and height predicts metabolic rate most accurately (boys: BMR = 16.6 weight (kg) + 77 height (m) + 572; girls: BMR = 7.4 weight (kg) + 482 height (m) + 217).