OBJECTIVE: Early menarche is a risk factor for breast cancer. Since body composition influences age at menarche we decided to estimate the effects of a school-based intervention for the prevention of obesity on the initiation of menses in young girls. METHODS: Ten schools were randomized to a modified curriculum or no curricular changes for 2 school-years. Data of 508 pre-menarcheal girls at baseline (age range 10 to 13 years) were analyzed. RESULTS: Girls attending an intervention school experienced menarche less frequently than girls attending control schools during the intervention period (intervention schools 54%, control schools 59%; RR 0.76; 95% CI 0.66 to 0.87). Attending an intervention school was also associated with lower increase in BMI (-0.3 kg/m2; p = 0.003), lower gains in triceps skinfold thickness (-1.5 mm; p = 0.007), decreased television viewing (-0.6 h/day; p < 0.0001) and increased physical activity (3.1 MET-h/week; p = 0.032). Including these changes as predictors of menarche incidence attenuated the intervention effect (RR 0.94; 95% CI 0.80 to 1.10). CONCLUSIONS: The intervention delayed menarche in this group of girls. The delay was produced by increased physical activity, reduced television viewing and changes in BMI and fat distribution. These findings may have implications for the primary prevention of breast cancer.

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