Is lean mass quantity or quality the determinant of maximal fat oxidation capacity? The potential mediating role of cardiorespiratory fitness.

Abstract

Background: Impaired fat oxidation is linked to cardiometabolic risk. Maximal fat oxidation rate (MFO) reflects metabolic flexibility and is influenced by lean mass, muscle strength, muscle quality - defined as the ratio of strength to mass - and cardiorespiratory fitness. The relationship between these factors and fat oxidation is not fully understood. The aim is to analyze the associations of lean-mass, muscle strength and quality with fat oxidation parameters in young adults, considering the mediating role of VO₂max.

Methods: A cross-sectional observational study. Eighty-one adults (50 males, 31 females; age 22.8 ± 4.4, BMI 25.70 ± 5.75, lean-mass 54.19 ± 8.78, fat-mass 18.66 ± 11.32) Body composition assessment by bioimpedance determine fat and lean-mass. Indirect calorimetry at rest and exercise was used for the calculation of fat oxidation. An incremental exercise protocol in a cycle ergometer with two consecutive phases was performed. The first to determine MFO consisted of 3 min steps of 15W increments with a cadence of 60rpm. The test was stopped when RQ ≥ 1. After 5 min rest, a phase to detect VO₂max began with steps of 15W/min until exhaustion. Muscular strength was assessed by handgrip dynamometry and the standing longitudinal jump test. A strength cluster was calculated with handgrip and long jump adjusted by sex and age. Data were analyzed using multiple linear regression and mediation analyses.

Results: Total lean-mass and leg lean-mass were not associated with MFO. Long jump, relativized by lean-mass and by leg lean-mass have a standardized indirect effect on MFO of 0.50, CI: 0.32-0.70, on MFO/lean-mass 0.43, CI:0.27-0.60 and MFO/leg lean-mass 0.44, CI: 0.30-0.06, which VO₂max mediated, VO₂max/lean-mass and VO₂max/leg lean-mass, respectively (all p < 0.01). The handgrip/arm lean-mass had an indirect effect of 0.25 (CI: 0.12-0.38) on MFO/leg lean-mass, with VO₂max/leg lean-mass as the mediator (p < 0.01). The Cluster/lean-mass and Cluster/Extremities lean-mass have a standardized indirect effect on MFO/lean-mass (0.34, CI: 0.20-0.48) and MFO/leg lean-mass (0.44, CI: 0.28-0.60), mediated by VO2max/lean-mass and VO2max/leg lean-mass (p < 0.01).

Conclusions: Muscular strength and quality have an indirect effect on MFO mediated by VO₂max. These findings suggest the importance of muscle quality on MFO.