In conclusion, HIT was more effective short-term strategy to increase R-R interval variability than aerobic training, most probably by inducing larger increases in cardiac vagal activity. The acute autonomic responses to the single HIT session were not modified by short-term training.

Aims Little is known about the effects of exercise training on brown adipose tissue (BAT) metabolism in humans. We tested the hypothesis that high-intensity interval training (HIIT) and moderate-intensity continuous training (MICT) improve BAT insulin sensitivity. Materials and methods Healthy middle-aged men (n = 18, age 47 years [95% confidence interval {CI} 49, 43}, body mass index 25.3 kg/m2 [95% CI 24.1, 26.3], peak oxygen uptake (VO2peak) 34.8 mL/kg/min [95% CI 32.1, 37.4]) were recruited and randomized into six HIIT or MICT sessions within 2 weeks. Insulin-stimulated glucose uptake was measured using 2-[18F]fluoro-2-deoxy-D-glucose positron-emission tomography in BAT, skeletal muscle and abdominal and femoral subcutaneous and visceral white adipose tissue (WAT) depots before and after the training interventions. Results Training improved VO2peak (P=0.0005), insulin-stimulated glucose uptake into quadriceps femoris muscle (P=0.0009) and femoral subcutaneous white adipose tissue (P=0.02), but not into BAT, with no difference between the training modes. Using pre-intervention BAT glucose uptake, we next stratified subjects into high BAT (>2.9μmol/100g/min; n=6) or low BAT (<2.9 μmol/100g/min; n=12) groups. Interestingly, training decreased insulin-stimulated BAT glucose uptake in the high BAT group (4.0 [2.8, 5.5] vs. 2.5 [1.7, 3.6]) (training*BAT, P=0.02), whereas there was no effect of training in the low BAT group (1.5 [1.2, 1.9] vs. 1.6 [1.2, 2.0] μmol·100g·min−1). High BAT subjects had lower levels of inflammatory markers compared to low BAT subjects. Conclusions Subjects with functionally active BAT have an improved metabolic profile compared to subjects with low BAT activity. Short-term exercise training decreases insulin-stimulated BAT glucose uptake in subjects with active BAT, suggesting that training does not work as a potent stimulus for BAT activation.