Recently, an editorial in the British Medical Journal explored the relationship physical activity and cancer. Epidemiologists David Batty from London and Inger Thune from Norway reviewed some of the evidence regarding the wide range of potential cancer-protective effects of exercise. Below are some excerpts and important points from the editorial.
To start, they note that exercise has significant effects on several functions of the human body that may influence cancer risk. These effects include changes in the following:

cardiovascular capacity	energy balance
pulmonary capacity	immune function
bowel motility	antioxidant defence
hormone levels	DNA repair

They note that in the past decade alone, over 200 population based studies have linked physical activities to cancer risk. The most researched cancers are those of the:

• Bowel

• Breast

• Endometrium

• Prostate

• Testes

• Lung

Bowel Cancer
Cancer of the large bowel is the most commonly investigated cancer in relation to physical activity (1-4) with physically active men and women experiencing around half the risk of their sedentary counterparts.
Plausible mechanisms of protection include the beneficial effects of exercise on levels of the following substances, which influence the growth and proliferation of cells in the colon:

• Insulin

• Prostaglandins

• Bile acid

In addition, exercise reduces bowel transit time and thereby the duration of contact between fecal carcinogens and the mucosal lining of the colon.
Breast and Endometrial Cancer
Concerning these strongly hormone-dependent cancers the editorial states:

Endogenous sex hormones are strongly implicated in the development of breast and endometrial cancer. Physical activity may modulate the production, metabolism, and excretion of these hormones, so an association with these cancers is biologically possible. Physical activity may also reduce the risk of cancer through its normalising effect on body weight and composition. Evidence from population based studies suggests that occupational, leisure, and household activities are associated with about a 30% reduction in breast cancer rates (5) ... Those studies that have explored the link between physical exertion and the risk of endometrial cancer suggest a negative association (1,3).

Prostate Cancer
The editorial notes that since athletes show lower levels of circulating testosterone than non-athletes, and testosterone influences the development of prostate cancer, this has led to the hypothesis that physical activity may protect against this cancer. Most, but not all, studies suggest a protective effect (3).
Lung Cancer
The editorial notes that:

Although physical activity improves pulmonary ventilation and perfusion, which may reduce both the concentration of carcinogenic agents in the airways and the duration of agent-airway interaction, the association of activity with lung cancer has received relatively little attention. Findings from most, but not all, studies suggest a negative relation ... (1,3)

Exercise as a Cancer Treatment
The editorial notes that, in addition to the apparent role of exercise in the prevention of some cancers, there is growing interest in and evidence of its use in the treatment and rehabilitation of patients with cancer (12, 13). Physical activity may reduce the likelihood of recurrence and enhance survival through its capacity for doing the following:

• Improving bodily movement

• Reducing fatigue

• Enhancing immune function

Conclusion
The editorial notes that since exercise seems to have protective effects against some types of cancers and does not increase the risk of any cancer, it should be more actively encouraged. They state that "in light of the decreasing population prevalence of total physical activity, doctors should advocate moderate endurance-type activity, such as walking and cycling. As well as reducing the risk of chronic diseases such as coronary heart disease and non-insulin dependent diabetes, such physical activity does seem to protect against some cancers." British Medical Journal December 9, 2000;321:1424-1425