Physical activity and cancer risk

Evidence is accumulating that high levels of physical activity are associat ed with a reduced risk of some cancers. This evidence is most consistent fo r colon cancer, which is reduced by 40-50 % among the most active individua ls, compared with the least active. The effect is evident in men and women, and appears to be independent of important confounding factors. However, t here may be important interactions with body fatness; a high BMI has been r eported to be associated with an increased risk of colon cancer in sedentar y men but not in physically-active men. Whilst the evidence on breast cance r is less consistent, case-control studies typically suggest a reduction of 25-30 % among the most active women, although several studies have found n o effect. potential mechanisms include systemic influences and others relev ant only to site-specific cancers. One unifying hypothesis is that physical inactivity reduces insulin sensitivity, leading to a growth promotional en vironment which may facilitate neoplasia. The non-specific immune system ma y be improved by physical activity, possibly through the summative effects of repeated exercise bouts. Regular exercise, even at a recreational level, probably reduces exposure to oestrogen and thus decreases the risk of brea st cancer. Increased colonic peristalsis, and thus reduced bowel transit ti me, might partly explain the lower risk of colon cancer in active people. P hysical activity emerges as one of the few modifiable risk factors for some cancers and, as such, justifies further study.