Determinants of maximal whole-body fat oxidation in elite cross-country skiers: Role of skeletal muscle mitochondria.
Level 4 - case-series / case-control
Cross-sectional comparative physiological study between two small cohorts
PubMed 30218613 · doi:10.1111/sms.13298
What was done
Eight competitive male cross-country skiers and eight untrained male controls underwent a graded exercise test to measure maximal fat oxidation (MFO), the exercise intensity eliciting MFO (FatMax), and maximal oxygen uptake (VO2max). Skeletal muscle biopsies were analyzed using transmission electron microscopy for mitochondrial volume density (Mito VD) and high-resolution respirometry for mitochondrial fatty acid oxidation capacity (FAOp) and oxidative phosphorylation capacity (OXPHOSp), both normalized to muscle wet weight and expressed intrinsically per mitochondrial volume.
What was found
Compared to untrained controls, cross-country skiers had significantly higher (P < 0.05): - MFO: 0.60 g/min (95% CI: 0.50–0.70) vs 0.32 g/min (0.24–0.39) - FatMax: 46% VO2max (44–47) vs 35% VO2max (34–37) - VO2max: 71 mL/min/kg (69–72) vs 48 mL/min/kg (47–49) - Mito VD: 7.8% (7.2–8.5) vs 6.0% (5.3–6.8) - FAOp: 34 pmol/s/mg wet weight (27–40) vs 21 pmol/s/mg (17–25) - OXPHOSp: 108 pmol/s/mg wet weight (104–112) vs 69 pmol/s/mg (68–71) Intrinsic mitochondrial metrics were not significantly different between athletes and controls: intrinsic FAOp was 4.0 vs 3.3 pmol/s/mg/Mito VD, and intrinsic OXPHOSp was 14 vs 11 pmol/s/mg/Mito VD. Within the endurance athlete group, Mito VD correlated with MFO (r^2 = 0.504, P < 0.05).
Why it matters
These findings suggest that elevated fat burning capacity in elite endurance athletes is primarily limited by total mitochondrial mass (quantitative expansion) rather than enhanced intrinsic catalytic efficiency per mitochondrion (qualitative adaptation).
Limits
The study evaluated a very small, exclusively male sample (8 athletes, 8 controls). The cross-sectional design cannot establish causality between training, mitochondrial expansion, and fat oxidation changes, nor can findings be generalized to female athletes or other endurance disciplines.
Cited by
- supports Zone 2 exercise improves the mitochondria's ability to oxidize fat as its energy source because it is performed at the intensity of maximal mitochondrial fat oxidation.