Testing Compensatory Mechanisms: Higher Resting Metabolic Rates Than Predicted In Highly Active Children And Adolescents

High physical activity (PA) levels may compromise the energy available for vital functions (growth, maintenance) in children/adolescents. A new hypothesis suggests that physiological and behavioral compensatory mechanisms act to buffer daily energy expenditure due to high PA levels. One mechanism could be a reduction of the resting metabolic rate (RMR), but little information in children/adolescents is available. PURPOSE: To test the hypothesis that higher PA levels are associated with lower RMR in children/adolescents. METHODS: Twenty-two individuals (9 girls, 13 boys; 12.2 ± 2.74 years of age) with high PA levels (153 average moderate-vigorous physical activity minutes/day) were recruited. RMR was measured twice using indirect calorimetry (7 days between each RMR). PA levels were assessed over 7 consecutive days using accelerometry, and sedentary, moderate, vigorous, and very vigorous physical activity (minutes/day) were calculated. Body composition was measured using dual-energy X-ray absorptiometry. Measured RMR was compared with 7 different equations. A one-way repeated-measures analysis of variance (ANOVA) with Bonferroni correction was used to compare measured vs predicted RMR. Forward stepwise multiple regressions were applied to determine whether percent fat-free mass (%FFM) and PA levels influenced the bias. RESULTS: With the exception of the Institute of Medicine (IOM) equation, all other equations underestimated the measured RMR. - Mean (kcal/day) Standard Deviation Bias Measured RMR (45 measurements) 1439.4 226.1 - Aca-Tek et al., 2023 1302.8 298.0 136.7 Harris-Benedict, 1918 1379.6 268.4 59.8 IOM, 2004 1464.3 336.3 -24.9 Müller (a), 2004 1425.7 237.1 13.7 Müller (b), 2004 1359.5 243.4 79.9 Pontzer et al., 2021 1133.0 303.3 306.4 Reale et al., 2020 1394.6 333.3 44.8 Parameters including %FFM and/or moderate PA were correlated with the equation’s bias. CONCLUSION: The presence of physiological compensation is not supported in our sample, given that the measured RMR values were generally higher than the RMR prediction equations. The prediction bias is mainly explained by the high PA levels and the low %FFM of our sample. GZR: NextGeneration EU funds Margarita Salas Postdoctoral Fellowship; OPN: VT Presidential Postdoctoral Fellowship; KRH & EL: VT Translational Obesity Research Interdisciplinary Graduate Education Predoctoral Fellowship.