Although a reduction in colon cancer risk through physical activity is very likely, the magnitude of the benefit is not well quantified. Much evidence suggests that even moderate levels of activity, such as brisk walking three hours per week, could substantially lower risk. More active individuals, such as those jogging or running five hours per week, probably cut their risk by more than half, but even modest increases in exercise are likely to be beneficial. Any increase in activity is good and should be encouraged, but the higher the activity level, the greater the reduction in colon cancer is likely to be. The relationship between physical activity and breast cancer is unclear.
Breast cancer is influenced by estrogens, and exercise appears to affect levels of sex hormones.
Investigators have used several approaches for estimating the relation of physical activity to breast cancer risk. Two studies were based on sports participation in college, other studies have used job title to infer workplace levels of activity, and the remaining studies were based on self reported levels of exposure. Subjects who did not report participation in college athletics were nearly twice as likely to report a history of breast cancer as women who had been on sports teams in college. Findings suggested that college athleticism defined a group of women who lead physically active lives, but the study was limited by the small number of breast cancer cases reported and its ability to control for other potential factors that could have accounted for the association with breast cancer. A number of studies have attempted to estimate workplace activity levels from job titles.