Interactions of physical activity and body mass index with age at menarche: A school-based sample of Chinese female adolescents.

To test the independent and interactive associations of physical activity (PA) and body mass index (BMI) with age at menarche among Chinese adolescence.We conducted a cross-sectional survey in two elementary schools in September 2015 in Shaoxing city, Zhejiang province, China. We used self-administered questionnaires to collect the information of the participators. Analyses were performed with logistic regression models, and the relative excess risk due to interaction (RERI), the attributable proportion because of the interaction (AP), and the synergy index (S) were used to evaluate the biological interaction.A total of 1530 middle school students from grade 1-3 were selected for this study, and we collected 1505 (98.366%) valid questionnaires for the last analyses. The mean age of menarche is 11.603 (SD=0.447)years. Students with overweight/obesity and without SMPA had the highest risks of early age of menarche (OR=3.507, 95%CI: 1.929-6.376) compared with women both with a normal BMI and with SMPA, and the RERI was 1.846 (95% CI: 0.415-4.107), meaning that there was positive interaction on the additive scale.Insufficient PA can significantly modify the association between obesity and early menarche risk. Therefore, the government and society should pay more attention to the promotion of PA throughout childhood and adolescence.