Background Common genetic variants in the insulin receptor substrate 1 (IRS1) gene have been recently associated with insulin resistance and hyperinsulinemia. We examined whether the best-associated variant modifies the long-term changes in insulin resistance and body weight in response to weight-loss diets in Preventing Overweight Using Novel Dietary Strategies (POUNDS LOST) trial. Methods and Results-We genotyped IRS1 rs2943641 in 738 overweight adults (61% were women) who were randomly assigned to 1 of 4 diets varying in macronutrient contents for 2 years. We assessed the progress in fasting insulin, homeostasis model assessment of insulin resistance (HOMA-IR) and weight loss by genotypes. At 6 months, participants with the risk-conferring CC genotype had greater decreases in insulin (P=0.009), HOMA-IR (P=0.015), and weight loss (P=0.018) than those without this genotype in the highest-carbohydrate diet group whereas an opposite genotype effect on changes in insulin and HOMA-IR (P≤ 0.05) was observed in participants assigned to the lowest-carbohydrate diet group. No significant differences were observed across genotypes in the other 2 diet groups. The tests for genotype by intervention interactions were all significant (P&l; 0.05). At 2 years, the genotype effect on changes in insulin and HOMA-IR remained significant in the highest-carbohydrate diet group (P< 0.05). The highest carbohydrate diet led to a greater improvement of insulin and HOMA-IR (P for genotype-time interaction ≤ 0.009) in participants with the CC genotype than those without this genotype across 2-year intervention. Conclusions-Individuals with the IRS1 rs2943641 CC genotype might obtain more benefits in weight loss and improvement of insulin resistance than those without this genotype by choosing a high-carbohydrate and low-fat diet.