Insulin resistance (IR) in adolescence is associated with T2DM. The PREVIEW study assesses the effectiveness of a high-protein, low-glycaemic index diet and moderate-protein, moderate-glycaemic index diet to decrease IR in insulin resistant children with overweight/obesity. Inclusion criteria were age 10-17y, HOMA-IR≥2.0 and overweight/obesity. In 126 children (13.6±2.2y, BMI z-score 3.04±0.66, HOMA-IR 3.48±2.28) anthropometrics, fat mass percentage (FM%), metabolic parameters, physical activity, food intake and sleep were measured. Baseline characteristics did not differ between the groups. IR was higher in pubertal children with morbid obesity than in prepubertal children with morbid obesity (5.41±1.86 vs. 3.23±1.86, p=0.007) and prepubertal and pubertal children with overweight/obesity (vs. 3.61±1.60, p=0.004 and vs. 3.40±1.50, p<0.001 respectively). IR was associated with sex, Tanner stage, BMI z-score, and FM%. Fasting glucose concentrations were negatively associated with Baecke Sport (r=-0.223, p=0.025) and positively with daytime sleepiness (r=0.280, p=0.016) independent of sex, Tanner stage, BMI z-score and FM%. In conclusion, IR was most severe in pubertal children with morbid obesity. The relations between fasting glucose concentration with Baecke Sport and sleepiness suggest these might be possible targets for diabetes prevention.