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

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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>˙</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>Max</mml:mi></mml:mrow> </mml:math> . 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% <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>˙</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>Max</mml:mi></mml:mrow> </mml:math> [44-47] vs 35 [34-37]), <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>˙</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>Max</mml:mi></mml:mrow> </mml:math> (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 (r² = 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.