Abstract

Physically trained individuals exhibit high rates of whole-body fat oxidation and elevated insulin sensitivity. High-intensity interval training (HIT) has been suggested to induce similar metabolic adaptations in a more time efficient manner than traditional endurance training. To widen this claim the authors aimed to investigate whether 6 weeks of HIT improves whole-body fat oxidation and insulin sensitivity in young sedentary males. Eight participants (23±2 years; body mass index 24.5±1.5 kg/m²; VO₂ peak 43.0±1.5 ml/min/kg) with normal glucose tolerance undertook 6 weeks of HIT consisting of between four and six 30-s Wingate tests separated by 4.5 min rest, performed three times per week. Insulin sensitivity (oral glucose tolerance test (OGTT)) and substrate metabolism during a 60-min steady state cycling trial (∼65% pretraining VO₂ peak) was assessed before and after HIT. Dual energy x-ray absorptiometry was also used to measure changes in body composition in response to training. VO₂ peak increased 18% (pre 43.0±1.5 ml/min/kg², post 50.8±1.8 ml/min/kg²; p<0.05). A 5% reduction in relative fat content in the body was observed (pre 19.6±2.2%, post 18.8±2.1%, p<0.05), which was attributed to a 6% decrease in relative fat content in the legs. Absolute carbohydrate oxidation was reduced after HIT (pre 1.75±0.11 g/min, post 1.49±0.11 g/min; p=0.04), while relative fat oxidation showed a tendency to increase (pre 35.2±5.7%, post 43.3±3.6%; p=0.078). Plasma glucose and insulin area under the curve in response to the OGTT was reduced by 17% and 31% (p<0.05) posttraining, respectively, which corresponded with a 27% improvement in the Matsuda index of insulin sensitivity (pre 4.0±0.5, post 5.1±0.6, p=0.066). The results obtained for the first time provide evidence to suggest that HIT is also a time-efficient method to improve whole-body fat oxidation, body composition and insulin sensitivity in young, sedentary males.