The effect of high-intensity training on mitochondrial fat oxidation in skeletal muscle and subcutaneous adipose tissue.

High-intensity interval training (HIT) is known to increase mitochondrial content in a similar way as endurance training [60-90% of maximal oxygen uptake (VO2peak)]. Whether HIT increases the mitochondria's ability to oxidize lipids is currently debated. We investigated the effect of HIT on mitochondrial fat oxidation in skeletal muscle and adipose tissue. Mitochondrial oxidative phosphorylation (OXPHOS) capacity, mitochondrial substrate sensitivity (K(m)(app)), and mitochondrial content were measured in skeletal muscle and adipose tissue in healthy overweight subjects before and after 6 weeks of HIT (three times per week at 298 ± 21 W). HIT significantly increased VO2peak from 2.9 ± 0.2 to 3.1 ± 0.2 L/min. No differences were seen in maximal fat oxidation in either skeletal muscle or adipose tissue. K(m)(app) for octanoyl carnitine or palmitoyl carnitine were similar after training in skeletal muscle and adipose tissue. Maximal OXPHOS capacity with complex I- and II-linked substrates was increased after training in skeletal muscle but not in adipose tissue. In conclusion, 6 weeks of HIT increased VO2peak. Mitochondrial content and mitochondrial OXPHOS capacity were increased in skeletal muscle, but not in adipose tissue. Furthermore, mitochondrial fat oxidation was not improved in either skeletal muscle or adipose tissue.