Absolute Aerobic Capacity Predominates over Body Composition and Regional Anthropometry in Explaining Maximal Fat...

Background: Maximal fat oxidation (MFO) is the highest rate of fat oxidation attained during incremental exercise, with cardiorespiratory fitness being its primary correlate. How body composition and regional anthropometry explain MFO variability remains poorly understood. While fat-free mass is a known correlate, regional morphology, such as skinfold-corrected girth, has been proposed to capture local muscle mass contributions. However, MFO is an absolute rate (g·min^-1) whereas peak oxygen uptake (VO₂peak, typically mL·kg^-1·min^-1) is conventionally reported relative to body mass. The statistical consequences of this dimensional mismatch, and the question of whether regional girths independently account for MFO variance or merely reflect body size artefacts, remain unexamined. Methods: Forty-three national-team collegiate athletes (21 men, 22 women) underwent a maximal incremental treadmill test with breath-by-breath indirect calorimetry, dual-energy X-ray absorptiometry (DXA), bioelectrical impedance analysis and a full International Society for the Advancement of Kinanthropometry (ISAK) anthropometric profile including six skinfold-corrected girths. The primary model regressed absolute MFO on absolute peak oxygen uptake (VO₂peak, L·min^-1). Sensitivity models added corrected girths, fat-free mass, sex, stature and body mass. Internal validity was assessed by leave-one-out cross-validation (Q²) and 5000-sample bootstrapping, with Benjamini-Hochberg control of the false discovery rate. Results: Absolute VO₂peak explained 60.4% of the variance in MFO (Q² = 0.563; β = 0.191 g·min^-1 per L·min^-1, 95% CI 0.142 to 0.240; bootstrap CI 0.133 to 0.227). Adding corrected calf girth changed the explained variance by 0.006 (p = 0.424) and reduced Q² to 0.541. No corrected girth survived the false-discovery-rate control, and fat-free mass added nothing (p = 0.268). When aerobic capacity was instead expressed per kilogram of body mass, model fit was substantially worse (R² = 0.415) and corrected calf girth appeared to be strongly and independently associated with MFO (p < 0.001). Conclusions: In trained athletes, absolute aerobic capacity explains MFO variance, whereas regional anthropometry and body composition provide no incremental information. Normalising predictor variables by body mass creates statistical artefacts that falsely attribute explanatory power to structural girths. Future studies with larger sample sizes and diverse athletic populations are needed to confirm these findings across different training statuses and metabolic profiles.