Physical Activity

Regular physical activity has been hypothesized to prevent breast cancer, largely by reducing circulating levels of sex hormones. The mechanisms by which physical activity reduces exposure to hormones vary by period of life. Young girls participating in strenuous athletic training, such as running and ballet dancing, have delayed menarche, which is known to reduce risk of breast cancer, and even moderate-intensity physical activity may delay menstruation. This effect of activity at young ages may be reflected in lower body weight and body fat, both of which are determinants of delayed menstruation. A later menarche is associated with a later onset of regular ovulatory cycles as well as lower serum estrogen concentrations during adolescence. Once menstruation has been established, anovulatory and irregular menstrual cycles may be more frequent among moderately and strenuously active women than among inactive women, although disagreement exists regarding the degree to which the intensity of physical activity influences menstrual abnormalities. Furthermore, a substantial degree of ovarian dysfunction may occur, even among physically active women who appear to have normal menstrual cycles. Among older women, levels of past and current physical activity influence fat stores, and which after menopause are the locus of conversion of androstenedione to estrogen.

Despite the evidence that higher levels of physical activity are associated with lower levels of circulating ovarian hormones, epidemiologic studies relating physical activity to risk of breast cancer show inconsistent results. Methodologic differences in physical activity assessment are likely to have contributed to these inconsistencies. Studies have differed in the ages at which physical activity was assessed; methods for measuring intensity, frequency, and duration of physical activity; definition and categorization of physical activity levels; and age of breast cancer diagnosis. Furthermore, ranges of physical activity typically studied are very limited in comparison with the levels of hard labor typically practiced by women in traditional agrarian societies.

The strongest reduction in breast cancer risk associated with increased physical activity was reported in a population-based case control study of women younger than age 40. The relative risk was 0.42 (95% confidence interval, 0.27 to 0.64) for women with a lifetime average of 3.8 hours or more of physical activity per week compared those with an average of 0 hours per week.

This has been the only study explicitly devoted to the relationship between physical activity and breast cancer, and it used a detailed physical activity assessment instrument to quantify the average number of hours per week of recreational physical activity over the reproductive life span, beginning at menarche. Activities such as housework, gardening, and easy walking not for the explicit purpose of physical exercise were not counted in the measure of physical activity. These researchers concluded from their various analyses that lifelong physical activity is the critical factor of interest with regard to breast cancer risk.