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

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 V ˙ O 2 Max . 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% V ˙ O 2 Max [44-47] vs 35 [34-37]), V ˙ O 2 Max (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.
(© 2018 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)