Sex Differences in Substrate Oxidation During Incremental Exercise: An Exploratory Study on the Role of Body Composition

Background: Men display higher absolute fat oxidation rates during exercise than women, but whether fat-free mass (FFM) moderates the substrate–intensity relationship in a sex-specific manner remains untested. Methods: In this cross-sectional laboratory study, we evaluated 37 competitive athletes (20 women, 17 men; age 22 ± 5 years) using dual-energy X-ray absorptiometry (DXA) and indirect calorimetry during incremental treadmill exercise (538 min-by-min observations) to evaluate a sex × FFM × intensity interaction using linear mixed-effects models. Results: The primary sex × FFM × intensity interaction was non-significant (β=+0.014, 95 % CI −0.021 to +0.049; p=0.443). Absolute maximal fat oxidation (MFO) was higher in men (0.826±0.234 vs. 0.655±0.175 g⋅min−1; p=0.038), but it disappeared when normalised to FFM (15.45±4.19 vs. 15.74±4.85 mg⋅kg FFM−1⋅min−1; p=0.847). Interestingly, a strong sex × adiposity (DXA body-fat percentage, %) interaction was identified (β=−0.440, q<0.001), with higher adiposity linked to higher fat oxidation in men (β=+0.339, p<0.001) but lower in women (β=−0.115, p=0.010). Conclusions: Adiposity was identified as the most significant sex-specific moderator, with evidence showing that it is associated with greater fat oxidation in men and lower fat oxidation in women. The male advantage in absolute fat oxidation reflected greater FFM, because MFO per kilogram of FFM did not differ between sexes; muscle oxidative capacity was not measured directly. However, the exploratory nature of the study requires confirmation in future research.