Muscle fat oxidative capacity is not impaired by age but by physical inactivity: association with insulin sensitivity

The study aimed at determining whether aging and/or sedentariness impairs muscle fat oxidative capacity (OXFA) and whether this was associated with increased risk to develop insulin resistance. We first examined muscle mitochondrial functions, OXFA and insulin sensitivity (ISI; evaluated during an oral glucose tolerance test) in a cross-sectional study with 32 sedentary (S) and endurance-trained (T), young (Y) and elderly (E) men (24.2 +/- 2.6 vs. 66.6 +/- 3.2 yr). As for mitochondrial functions, OXFA was higher in T than in S but similar between age groups (SY 41.8 +/- 11.3, TY 68.0 +/- 17.7, SE 40.1 +/- 14.1, TE 73.1 +/- 20.1 palmitate.min(-1).g wet tissue(-1); activity P < 0.0001, age P = NS, activity x age P = NS). Similar results were obtained with ISI (SY 6.2 +/- 2.2, TY 11.4 +/- 4.4, SE 5.9 +/- 1.5, TE 11.0 +/- 3.5, activity P < 0.001, age P = NS, activity x age P = NS). Stepwise regression showed that, among body composition, VO2max and muscle biochemical characteristics, OXFA was the main predictor of ISI (r = 0.60, P < 0.001). We subsequently showed in eight sedentary elderly subjects (63.5 +/- 3.3 yr) that OXFA and insulin sensitivity (measured using insulin clamp) improved in parallel after 8 weeks of endurance training (r = 0.79, P < 0.01). We concluded that mitochondrial functions, OXFA and ISI, are not impaired by age but by physical inactivity and are closely correlated.