How might exercise during adolescence influence breast cancer risk?
Several studies have reported a decrease in breast cancer risk associated with regular exercise. Exercise may reduce a girl’s risk of breast cancer by decreasing the level of estrogen in her body, decreasing her weight, decreasing her insulin resistance, strengthening her immune system, or increasing her age at menarche. Generally, athletes have a later age at menarche compared to non-athletes. The influence of moderate exercise on age at menarche is less clear. Some researchers have reported that girls who are moderately active (non-athletes) have differences in hormone levels and a delay in the onset of menstruation compared to inactive girls. Others have reported no effect of moderate physical activity on the age at menarche. Differences in the way the researchers assessed physical activity or problems with recall of earlier exercise practices may account for the differences in results. Later age at menarche in athletes may also be due to heredity. Does obesity during childhood and/
