Prospective study of recreational physical activity and ovarian cancer

Background: It has been hypothesized that physical activity may reduce the risk of ovarian cancer by decreasing estrogen levels, reducing body fat, an d reducing the frequency of ovulation. Epidemiologic studies of this relati onship have obtained inconsistent results. The only prospective study to da te reported a positive association between frequent vigorous exercise and o varian cancer risk. We further evaluated this relationship in the Nurses' H ealth Study cohort. Methods: Participation in recreational physical activit y was assessed by questionnaire in 1980, 1982, 1986, 1988, 1992, and 1994, with questions assessing exercise frequency, duration, and intensity. Resul ts were adjusted for age, parity, oral contraceptive use, tubal ligation, a nd other risk factors for ovarian cancer. All statistical tests were two-si ded. Results: During a 16-year follow-up (from 1980 to 1996), 1.2 million p erson-years were accrued by 92 825 cohort members, and 377 cases of epithel ial ovarian cancer were confirmed. The relative risk (RR) of ovarian cancer for women engaging in recreational physical activity for 7 hours or more p er week compared with those reporting less than 1 hour per week was 0.80 (9 5% confidence interval [CI] = 0.49 to 1.32; P-trend =.59). When both the fr equency and intensity of activity were taken into account, activity level w as also not associated with a reduced risk of ovarian cancer. Compared with inactive women, participants reporting high activity in terms of metabolic equivalent task hours (MET hours) were at greater risk of ovarian cancer ( RR for 20 to <30 MET hours/week = 1.84 [95% CI = 1.12 to 3.02]; RR for,30 M ET hours/week = 1.27 [95% CI = 0.75 to 2.14]). Conclusions: Overall, result s did not suggest an inverse association between recreational physical acti vity and ovarian cancer. The possibility of a modest increase in risk with frequent vigorous activity requires further investigation.