Low levels of physical activity are associated with increased risks of obesity, heart disease, and certain cancers, however physical activity significantly decreases during puberty, especially for girls. The puberty studies of the Breast Cancer and the Environment Research Programs (BCERP) are longitudinal observational studies employing Tanner staging combined with serial clinical and paper based measurements, including detailed physical activity questions. Clinical data have been used in survival statistical models to estimate a date of thelarche for each girl.
PURPOSE: 1) Quantify physical activity level change at different time windows post-thelarche (breast development). 2) Determine an optimal time for an intervention to maintain physical activity level during adolescence.
METHODS: Sliding time windows post-thelarche (estimated date) were identified from 0-1 year to 1-2 years. Statistical analyses using mixed models incorporating the repeated measurements of physical activity were conducted to determine the effect of age, at each time window, on annualized change in time spent performing total, moderate, or vigorous physical activity. Covariates included age (at time of questionnaire), race, BMI, and socioeconomic status.
RESULTS: 1,071 girls with pubertal staging from three sites (Ohio, New York, California) were included in the analysis. The analysis covers data from girls aged 6 to 14 obtained over a 7-year period and included 6,496 physical activity volume measures (questionnaire records). Overall, total hours per week increased 0.05% with each month of age (p=0.03). When the cohort was stratified by the median value of pre-thelarche activity, girls with lower physical activity levels increased their activity during the year after thelarche (p=0.0009) and those with higher physical activity decreased their activity in the window 1.75-2.75 years after (p=0.04).
CONCLUSIONS: Girls vary in the pattern of physical activity change with age and thelarche. Supported by NIEHS & NCI to BCERP # U01ES012770, U01ES012771, U01ES012800, U01ES012801, U01ES019435, U01ES019453, U01ES019454, U01ES019457; NEIHS # R827039, P30ES06096 & P01ES009584; NCI #CSTA-UL1RR029887.