These malignancies are associated with low levels of physical activity and other factors common in the American lifestyle. Exercise influences many of the metabolic processes and the levels of various hormones, some of which are likely to be related to specific cancers, so it is quite plausible that exercise may reduce cancer risk. The precise ways hormones and other biochemical processes are related to cancer, however, are complex and much remains unknown. Thus, the best evidence regarding exercise and risk of cancer comes from studies that directly examine the association between physical activity and risk of specific cancers. Level of physical activity has been assessed in several ways in epidemiologic studies.
Some studies rely on questionnaires based on self-reported levels of specified activities.
Other studies have used occupational status to estimate physical activity on the job, as the average level of energy expenditure varies by occupation. Others have studied certain groups, such as participants of high school athletics, who presumably have high activity levels. No method is perfect, but each can yield important information. The evidence for a protective influence of physical activity is most clear for colon cancer, although exercise does not appear to protect against rectal cancers. Cancers of the colon make up about three-quarters of total colorectal cancers.