Does physical activity prevent cancer?

Physical activity has marked effects on several functions of the human body that may influence cancer risk. These effects vary according to the mode, duration, frequency, and intensity of the activity and include changes in cardiovascular and pulmonary capacity, bowel motility, endogenous hormones, energy balance, immune function, antioxidant defence, and DNA repair. Although a role for energy balance in cancer causation was advanced almost three centuries ago, it is mainly in the past decade that over 200 population based studies have linked work, leisure, and household physical activities to cancer risk. The most researched cancers are those of the bowel, breast, endometrium, prostate, testes, and lung. Cancer of the large bowel is the most commonly investigated cancer in relation to physical activity.1–4 Meta-analysis1 and systematic reviews 2 3 show an inverse dose-response association between activity and colon cancer such that physically active men and women experience around half the risk of their sedentary counterparts. This observation is seen across populations and study methods, with little indication of publication bias.1 Plausible mechanisms of protection include the favourable …