Physical activity intensity and type 2 diabetes risk in overweight youth: a randomized trial

BACKGROUND: The chronic effects of high-intensity endurance training on metabolic health outcomes in overweight adolescents remains poorly understood. OBJECTIVE: To test the hypothesis that high-intensity endurance training (ET) is superior to moderate-intensity ET for improving risk factors for type 2 diabetes in overweight adolescents. DESIGN AND METHODS: In this randomized trial, 106 overweight and obese adolescents (15.2 years; 76% female; 62% Caucasian) were randomly assigned to high-intensity ET (70-85% of heart rate reserve, n = 38), moderate-intensity ET (40-55% heart rate reserve; n = 32) or control for 6 months (n = 36). The primary and secondary outcome measures were insulin sensitivity assessed using a frequently sampled intravenous glucose tolerance test and hepatic triglyceride content with magnetic resonance spectroscopy. Exploratory outcomes were cardiorespiratory fitness, physical activity and MRI and dual x-ray absorptiometry-derived measures of adiposity. RESULTS: The study had 96% retention and attendance was 61 +/- 21% and 55 +/- 24% in the high-and moderate-intensity ET arms. Intention-to-treat analyses revealed that, at follow-up, insulin sensitivity was not different between high-intensity (-1.0 mU kg(-1) min(-1); 95% confidence interval (CI): -1.6, +1.4 mU kg-1 min-1) and moderate-intensity (+0.26 mU kg(-1) min(-1); 95% CI: -1.3, +1.8 mU kg(-1) min(-1)) ET arms compared with controls (interaction, P = 0.97). Similarly, hepatic triglyceride at follow-up was not different in high-intensity (-1.7% fat/water (F/W); 95% CI: -7.0, +3.6% F/W) and moderate-intensity (-0.40% FW; 95% CI: -6.0, +5.3% F/W) ET compared with controls. Both high intensity (+4.4 ml per kg-FFM (fat-free mass) per minute; 95% CI: 1.7, 7.1 ml kg-FFM-1 min(-1)) and moderate intensity (+4.4 ml kg-FFM-1 min(-1); 95% CI: 1.6, 7.3 ml kg-FFM-1 min(-1)) increased cardiorespiratory fitness, relative to controls (interaction P < 0.001). CONCLUSIONS: ET improves cardiorespiratory fitness among obese adolescents; however, owing to lack of compliance, the influence of exercise intensity on insulin sensitivity and hepatic triglycerides remains unclear.