Determinants of maximal whole‐body fat oxidation in elite cross‐country skiers: Role of skeletal muscle mitochondria

Abstract

Abstract Elite endurance athletes possess a high capacity for whole‐body maximal fat oxidation (MFO). The aim was to investigate the determinants of a high MFO in endurance athletes. The hypotheses were that augmented MFO in endurance athletes is related to concomitantly increments of skeletal muscle mitochondrial volume density (Mito VD ) and mitochondrial fatty acid oxidation (FAO p ), that is, quantitative mitochondrial adaptations as well as intrinsic FAO p per mitochondria, that is, qualitative adaptations. Eight competitive male cross‐country skiers and eight untrained controls were compared in the study. A graded exercise test was performed to determine MFO, the intensity where MFO occurs (Fat Max ), and . Skeletal muscle biopsies were obtained to determine Mito VD (electron microscopy), FAO p , and OXPHOS p (high‐resolution respirometry). The following were higher ( P < 0.05) in endurance athletes compared to controls: MFO (mean [95% confidence intervals]) (0.60 g/min [0.50‐0.70] vs 0.32 [0.24‐0.39]), Fat Max (46% [44‐47] vs 35 [34‐37]), (71 mL/min/kg [69‐72] vs 48 [47‐49]), Mito VD (7.8% [7.2‐8.5] vs 6.0 [5.3‐6.8]), FAO p (34 pmol/s/mg muscle ww [27‐40] vs 21 [17‐25]), and OXPHOS p (108 pmol/s/mg muscle ww [104‐112] vs 69 [68‐71]). Intrinsic FAO p (4.0 pmol/s/mg muscle w.w/Mito VD [2.7‐5.3] vs 3.3 [2.7‐3.9]) and OXPHOS p (14 pmol/s/mg muscle ww/Mito VD [13‐15] vs 11 [10‐13]) were, however, similar in the endurance athletes and untrained controls. MFO and Mito VD correlated ( r 2 = 0.504, P < 0.05) in the endurance athletes. A strong correlation between Mito VD and MFO suggests that expansion of Mito VD might be rate‐limiting for MFO in the endurance athletes. In contrast, intrinsic mitochondrial changes were not associated with augmented MFO.