OBJECTIVEWe reported previously that low-saturated-fat dietary counseling started in infancy improves insulin sensitivity in healthy children 9 years of age. The aim of this study was to evaluate the effect of lifelong dietary counseling on insulin sensitivity in healthy adolescents between 15 and 20 years of age. In addition, we examined dietary fiber intake and the polyunsaturated fatty acid (PUFA) + monounsaturated (MUFA)-to-saturated fatty acid (SFA) ratio in the intervention and control adolescents and the association of these dietary factors with homeostasis model of insulin resistance (HOMA-IR).RESEARCH DESIGN AND METHODSThe study comprised adolescents participating in the randomized, controlled Special Turku Coronary Risk Factor Intervention Project (STRIP) study, which aims to guide the study participants toward a diet beneficial for cardiovascular health. HOMA-IR was assessed annually between 15 and 20 years of age (n = 518; intervention, n = 245; control, n = 273), along with diet, BMI, pubertal status, serum cotinine concentrations, and physical activity. Dietary counseling was given biannually during the follow-up.RESULTSHOMA-IR was lower (7.5% on average) in the intervention group than in the control group between 15 and 20 years of age (P = 0.0051). The intervention effect was similar in girls and boys. The PUFA+MUFA-to-SFA ratio was higher (P < 0.0001) and the dietary fiber (g/MJ) intake was higher (P = 0.0058) in the intervention group compared with the control group. There was no association between the PUFA+MUFA-to-/SFA ratio and HOMA-IR, whereas dietary fiber intake (g/MJ) was associated with HOMA-IR in girls (P < 0.0001).CONCLUSIONSDietary counseling initiated in infancy and maintained until 20 years of age was associated with improved insulin sensitivity in adolescents.