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Insulin may affect breast cancer (BC) risk and prognosis. Exercise reduces insulin in obese BC survivors. We designed a randomised controlled trial to test the effect of an aerobic exercise intervention (AEI) on insulin parameters and body composition in non‐obese BC women without insulin resistance. Thirty‐eight BC women were randomised into an intervention group (IG = 18) or control group (CG = 20). IG participated in a structured AEI for 3 months, while CG received only the Word Cancer Research Fund/American Institute Cancer Research (WCRF/AICR) recommendation to be physically active. Fasting insulin, homeostasis model assessment of insulin resistance (HOMA‐IR) index, metabolic parameters and body composition were collected at baseline and after the AEI. IG reduced insulin and HOMA‐IR index by 15% and 14%, while CG increased these parameters (+12% and +16%). Insulin changed differently over time in the two randomised groups (pinteraction = .04). The between‐group differences in the change of insulin (IG = −1.2 μU/ml versus CG = +0.8 μU/ml) and HOMA‐IR index (IG = −0.26 versus CG = +0.25) were respectively significant (p = .04) and non‐significant (p = .06). IG significantly improved lower limb muscle mass in comparison with CG (p = .03). A structured AEI may improve insulin, HOMA‐IR index and body composition in non‐obese BC survivors without insulin resistance. [ABSTRACT FROM AUTHOR]