Fat-Free Mass Normalization Impacts Cardiorespiratory Fitness in Overweight Adolescents

Accurate assessment of cardiorespiratory fitness (CRF) in adolescents is critical. However, normalizing oxygen consumption (VO2) to body mass (BM) may underestimate CRF in overweight (OW) youth by including metabolically inactive fat mass. This study examined differences in VO2 normalized by BM and fat-free mass (FFM) between normal weight (NW) and OW adolescents. Thirty-eight participants (19 NW, 19 OW; 12–17 years) underwent anthropometric, body composition, and cardiorespiratory fitness assessments. VO2 at the aerobic threshold (VO2AerT), anaerobic threshold (VO2AnT), and peak exercise (VO2peak) were measured and expressed in absolute terms and relative to BM and FFM. Group differences in the main outcomes were analyzed using a one-way ANOVA, and Pearson correlation was used to examine associations between VO2, BM and FFM. When normalized by BM, NW adolescents showed significantly higher VO2AerT (18.7 ± 3.6 vs. 14.5 ± 2.3), VO2AnT (28.8 ± 6.3 vs. 23.6 ± 4.7), and VO2peak (37.7 ± 6.7 vs. 29.1 ± 7.0) compared to OW peers (p < 0.05). No significant differences were found when VO2 values were normalized by FFM. A group difference was observed in the VO2peak vs. BM slope (p = 0.03) but not in the VO2peak vs. FFM slope. FFM normalization provides a more accurate assessment of CRF by accounting for differences in body composition, underscoring the importance of evaluating the aerobic capacity of metabolically active tissue rather than total body weight in youth populations.