Physical activity and cancer prevention -- is 'no pain, no gain' passé?

Physical activity is important in the prevention of a Ž variety of diseases and health conditions US DHHS, . 1996 . It is known to exert marked effects on several functions of the human body that may influence cancer risk, such as endogenous hormone levels, immune function, antioxidant defence, DNA repair and bowel motility. These effects vary according to the mode, duration, frequency and intensity of the activity. The interest in the association between physical activity and cancer is a relatively recent phenomenon: it is mainly in the past decade that well over 100 epidemiological studies have linked work, leisure and household physical activities to cancer risk. The most researched cancers are those of the colon, breast and endometrium. Cancer of the colon is the most commonly studied cancer in relation to physical activity, with approximately 50 studies. A recent review by an international group, which met in Lyon in February 2001, showed that physically active men and women experience around half the risk of their sedentary counŽ . terparts IARC, 2001 . The protective effect is seen across populations and study methods. Increasing levels of activity, whether in intensity, frequency or duration, generally seem to be associated with greater reductions in cancer risk. Most of the more than 30 epidemiological studies conducted in North America, Asia and Europe also demonstrated lower breast cancer risk among the most physically active women. The decrease in risk of breast cancer was, on average, 20 40%. In addition to concluding that there is sufficient evidence for causality for the association between physical activity and prevention of cancer in colon and breast, the Working Group considered the evidence to be limited for a possible protective effect of physical activity against cancers of the endometrium and prostate. For endometrial cancer, only a small number of studies reported on physical activity, but these were quite consistent in suggesting a moderately strong protective effect. For prostate cancer, about 20 epidemiological studies on the relationship with physical activity levels showed somewhat inconsistent results, but a majority of them found an inverse association with physical activity, which generally was only moderately strong and sometimes only in subgroups. Endogenous sex hormones are strongly implicated in the development of breast, endometrial and prosŽ . tate cancer IARC, 2001 . Physical activity may modulate the production, metabolism and excretion of the endogenous hormones, such as sex steroids, glucocorticoids, insulin-like growth factor 1 and its binding proteins, so an association with the hormonally related cancers is biologically plausible. Physical activity may also reduce the risk of cancer through its normalizing effect on body weight. For colon cancer, the plausible mechanisms of protection include the favourable effect of physical exertion on insulin, prostaglandins and bile acid levels, all of which may promote the growth and proliferation of colonic cells. Moreover, physical activity reduces colon transit time and thereby the duration of contact between faecal carcinogens and colonic mucosa. It is less clear what kinds and intensity of physical activities are associated with lower risk. It is important to clarify this issue in future studies, in light of recent recommendations that call for at least 30 Ž minutes of moderate-intensity physical activity e.g. . brisk walking at 5 6 km h most days of the week Ž . US DHHS, 1996 . Recent reports suggest that physical activity at a level that is easily achievable for Ž almost everyone at least one hour walking per week, . regardless of pace was associated with lower coroŽ . nary heart disease rates Lee et al., 2001 . This is a level which is less than that suggested by the current guidelines. Whether the slogan ‘no pain, no gain’ is