International Journal of Epidemiology

Abstract

Available data suggest that sex steroids, particularly androgens, play an important role in the carcinogenesis of prostate cancer.1 Although essential for normal growth and development, androgens can induce prostate cancer in animal models when given in large amounts.2,3 In a nested case-control study of 612 male physicians (222 cases of prostate cancer), high levels of circulating testosterone and low levels of sex hormone-binding globulin (SHBG) were associated with subsequent development of prostate cancer.4 On the other hand, prostate cancer often regresses when androgen stimulation is withdrawn through anti-testosterone therapy, bilateral orchidectomy, or adrenal ablations.5 Exercise has beneficial effects on the immune system and can suppress testosterone level; thus, it has been hypothesized to lower risk of prostate cancer. Some studies have shown that basal testosterone levels are lower among men who have recently engaged in physical activity.6 Additionally, male athletes appear to have lower levels of circulating testosterone when compared to non-athletes.7 Despite plausible biological mechanisms for an association between physical activity and decreased risk of © International Epidemiological Association 2000 Printed in Great Britain A prospective study of physical activity and risk of prostate cancer in US physicians