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ORIGINAL RESEARCH |
From the Graduate School of Public Health, University of Pittsburgh, Pittsburgh, Pennsylvania.
Address reprint requests to: Roberta B. Ness, MD, MPH University of Pittsburgh Graduate School of Public Health 130 DeSoto Street 517 Parran Hall Pittsburgh, PA 15261 E-mail: repro{at}vms.cis.pitt.edu
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Abstract |
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Methods: We used data from a population based case-control study. Cases (n = 767) were women 20–69 years of age in whom epithelial ovarian cancer was diagnosed during 1994–1998 and who resided in a defined region of Pennsylvania, New Jersey, or Delaware. Controls (n = 1367) were ascertained by using random-digit dialing and Health Care Financing Administration files and were frequency-matched to cases for age and county of residence. Information on lifetime leisure-time physical activity was obtained during in-person interviews.
Results: Leisure-time physical activity was significantly associated with reduced occurrence of ovarian cancer (P = .01). After adjustment for age, parity, oral contraceptive use, tubal ligation, family history of ovarian cancer, race, and body mass index, women with the highest level of activity had an odds ratio of 0.73 (95% CI 0.56, 0.94) for ovarian cancer compared with women with the lowest level of activity. When the relation was analyzed by various recalled time periods during life, the odds ratios for the highest versus the lowest category of activity at ages 14–17, 18–21, 22–29, 30–39, 40–49, and >50 years ranged from 0.64–0.78.
Conclusion: Leisure-time physical activity is associated with reduced occurrence of epithelial ovarian cancer.
Although physical activity has been associated with a reduced risk of hormone-dependent cancers, including breast1–6 and endometrial cancers,7–9 very little is known about the relation between physical activity and risk for ovarian cancer. Results of the few studies that have examined this relation have been inconsistent. Two studies that used occupational status as a measure of physical activity level found that women in jobs requiring low energy expenditure had a slightly but not significantly elevated risk of ovarian cancer.10,11 A Finnish study reported no significant difference in ovarian cancer risk between physical education teachers and language teachers.12 A retrospective cohort study found that compared with former college athletes, nonathletes had an increased risk (relative risk [RR] 2.5) for cancer of the female reproductive system (n = 6 cases of ovarian cancer).13 In contrast, the only study that assessed leisure-time physical activity levels found a positive association between physical activity and ovarian cancer risk. This large prospective study reported a twofold increase in risk among the most active women compared with the least active women.14
We report the results of a population-based case-control study in which we examined the influence of leisure-time physical activity on risk of ovarian cancer while adjusting for potential confounding factors. Using a historical physical activity instrument, we examined the role of lifetime physical activity on risk of ovarian cancer and explored whether there are critical periods during life during which physical activity has the most impact.
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Materials and Methods |
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Controls 65 years of age or younger were ascertained by using random-digit dialing. They were frequency-matched to cases by 5-year age groups and three-digit exchange. Once telephone numbers were screened for businesses, not-in-service numbers, and households with no women of eligible age, 2314 households were identified as having potentially eligible participants. Of these, 1928 households included women who were willing to be further screened for eligibility. Further screening identified 291 ineligible women: Five were not of eligible age, 11 resided outside of the target counties, 9 had a previous diagnosis of ovarian cancer, 187 had had bilateral oophorectomy, 22 did not speak English or were considered mentally incompetent, 6 were critically ill or dead, and 51 were untraceable. Of the 1637 potentially eligible controls, 1215 (74%) agreed to be interviewed. Controls 65–69 years of age were ascertained through Health Case Financing Administration lists and frequency-matched to cases by county of residence. Of the 423 women initially identified from these lists, 263 were deemed potentially eligible, of whom 152 (58%) agreed to be interviewed. Thus, of 1900 potentially eligible controls, 1367 (72%) were included in the study.
Cases included 616 invasive and 151 borderline epithelial tumors. Pathology slides from a random sample of 120 cases were reviewed by a study pathologist to validate the diagnosis. The study pathologist agreed with the original pathologic review in 95% of the cases for invasiveness and in 82% of the cases for cell type.
Standardized questionnaires were administered in person by trained interviewers. Detailed information was obtained on demographic characteristics, reproductive history, family history of cancer, anthropometric measures, and physical activity at various ages. Demographic information included age, race, and education. Women were asked about age at onset of menstruation, regularity, and age at menopause. Information was recorded on pregnancies, their length and outcome, and contraception. Self-reported anthropometric measures, including adult height and recent weight (usual weight during the past 5 years), were ascertained and body mass index (BMI) was calculated (kg/m2).
Lifetime leisure physical activity was assessed by using a modified version of the historical leisure physical activity questionnaire developed by Kriska.15 Questions were asked about physical activity in six periods: ages 14–17, 18–21, 22–29, 30–39, 40–49, and >50 years. Women were asked to rank the frequency of the leisure-time physical activities (eg, swimming, hiking, or skating) that they performed during each time period on a scale of 0–4 (0, never; 1, less than 1 hour/wk; 2, 1–3 hours/wk, 3, 4–6 hours/wk; 4, >6 hours/wk). Because few women reported no physical activity, they were combined with women who had a score of 1. The reliability and validity of the original version of the age questionnaire have been established.16–18 After examining the individual questions, we calculated a lifetime leisure physical activity index by averaging activity level rankings (1–4) across all ages in each woman’s life to provide a single estimate of lifetime activity. Women who averaged a ranking of more than 3 were placed in the "high" category. Women with a rank of 2 to 3 were categorized as "moderate," and those with a ranking of less than 2 were placed in the "low" category.
Several studies found that the reliability of leisure activity measurements was lessened when walking was included.16,18 To avoid this problem, we assessed and analyzed walking as a separate variable.
We used multivariable logistic regression to estimate odds ratios (OR) and their 95% CIs after adjustment for potential confounders. Because matching was based on frequencies for only two broad criteria—age within 5-year strata and three-digit telephone exchange (or county of residence)—we did not preserve the match in the analyses. Models were adjusted for age (continuous variable), parity (continuous variable), oral contraceptive use (yes/no), tubal ligation (yes/no), family history of ovarian cancer in any first degree relative (yes/no), BMI, education (high school, or less/more than high school), and race (white, black, other). These variables were significantly related to ovarian cancer in an age-adjusted analyses (data not shown) or were considered important in the literature. Test for trends were obtained by categorizing the exposure variables and treating the scored variables as continuous; the models contained all relevant adjustment covariates. Two sided P values are reported. Statistical analyses included both invasive and borderline ovarian cancer cases. Subsequent analyses excluding borderline ovarian cancer cases did not significantly change the results and are not reported. Only women (n = 2128) for whom information on physical activity was complete were included in the analyses.
Women were asked if they had ever been confined to a chair or bed for more than 1 month. To ensure that illness did not influence the level of physical activity, we reran the logistic regression models excluding women who reported confinement. Exclusion of these women did not change the estimated ORs. To further address this issue, we reran the logistic regression models restricting lifetime physical activity to periods up to but not including the decade before diagnosis. Again, the results did not significantly change the estimated ORs.
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Results |
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shows selected covariates according to level of lifetime leisure physical activity. Thirty-five percent of women reported a low level of lifetime physical activity, 42% reported a moderate level, and 23% reported a high level. Active women were more likely to be younger, white, nulliparous, educated, use oral contraceptives, and have a lower BMI compared with less active women.
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). Leisure physical activity afforded protection during all age periods (Table 3). The ORs were 0.85, 0.69, and 0.71 for increasing levels of leisure-time physical activity at 14–17 years of age compared with the lowest activity level. This association held for advanced age periods as well. Women with the highest physical activity levels compared with the lowest levels at ages 18–21, 22–29, 30–39, 40–49, and 
50 years had ORs ranging from 0.64 to 0.78.
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. Although the relations were not significant, the majority of ORs were below unity.
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Discussion |
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The relation between walking and ovarian cancer risk was not significant. Walking measurements, however, are notoriously unreliable, and our results may reflect the difficulty in assessing an activity that is performed so frequently throughout the day.16–18 Alternatively, walking expends less energy than many other activities and as a result may exert less influence on risk.
Several biological mechanisms may explain the protective effect of physical activity on the risk of ovarian cancer. Physical activity may prevent ovarian cancer by reducing the number of lifetime ovulatory cycles. This theory is based on evidence that physical activity in young women is associated with delayed menarche,23–26 amenorrhea,25,26 and anovulatory cycles.27,28 However, regular strenuous exercise may be needed to produce some of these effects, and it is doubtful that many women in our study exercised to that extent. For example, age at menarche in our study was similar among women regardless of their early participation in physical activity (data not shown).
The positive effects of physical activity on risk of ovarian cancer may be mediated in part through alterations in metabolic pathways. In recent studies, central obesity has emerged as a risk factor for ovarian,14 breast,29,30 and endometrial cancer.31,32 Central obesity is associated with hyperinsulinemia,33,34 decreased levels of sex hormone–binding globulin, and increased levels of serum estrogens and androgens.35 These metabolic variations have been hypothesized as risk factors for ovarian cancer.36,37 Physical activity preferentially reduces visceral adiposity.38 Thus, physical activity may decrease ovarian cancer risk by reducing levels of insulin and insulin-like growth factor or by lowering levels of circulating endogenous sex hormones. Physical activity can also decrease insulin levels by enhancing insulin sensitivity independent of body size.39,40
Another mechanism by which physical activity might protect against ovarian cancer involves alterations in prostaglandin levels. There is evidence linking ovarian cancer with inflammation, particularly prostaglandin metabolism.41 Exercise can increase levels of prostaglandin F2
42 (an inhibitor of tumor growth) and may decrease levels of prostaglandin E243 (a potent mediator of inflammation and known cancer promoter).
To our knowledge, only one ovarian cancer study was designed to measure physical activity. The Iowa Women’s Health Study,14 which followed 31,396 postmenopausal women for 7 years (n = 97 cases of ovarian cancer), found a twofold increase in risk of ovarian cancer among the most active women compared with the least active. Differences in assessment of physical activity between the Iowa Women’s Health Study and our study may account for the discrepancy in findings. In the Iowa Women’s Health Study, women were asked how often they participated in moderate and vigorous leisure activity. Women who participated in vigorous physical activity two or more times per week and those who participated in moderate physical activity more than four times per week were considered to have a high physical activity level, regardless of the duration of the activity. In our study, women were asked how many hours per week (less than 1, 1–3, 4–6, or more than 6) they engaged in leisure-time physical activity. Therefore, in the Iowa Women’s Health Study, women engaging in 20 minutes of vigorous activity twice weekly would be considered to have a high level of physical activity, whereas in our study those women would be placed in the low category of lifetime physical activity (less than 1 hour/week).
The periods during which physical activity was measured also differed between the two studies. The Iowa Women’s Health Study was a prospective cohort study that assessed current physical activity. In contrast, our study measured physical activity over a lifetime. Chronic diseases, such as cancer, may have long developmental periods, and physical activity habits may vary over time.44 To adequately assess the influence of physical activity on cancer risk, a lifelong physical activity measurement may be needed.
We did not collect information on diet, nutritional supplements, or other lifestyle variables. We therefore cannot rule out the possibility that physical activity may be a surrogate marker for a healthy lifestyle.45,46
Every attempt was made to minimize the biases inherent to case-control studies. Selection bias was reduced by the high participation rate among eligible, incident cases. Women were not aware of the study hypotheses and interviewers were trained to standardize interview questions, thus reducing recall bias. Moreover, our finding that physical activity is related to age, race, education level, and BMI agrees with those of other studies47,48 and reflects the internal validity of the physical activity measure.
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Footnotes |
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Received December 21, 1999. Received in revised form March 20, 2000. Accepted March 30, 2000.
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