The role of physical activity in the prevention of breast and endometrial cancer
Abstract: The aim of this thesis was to explore through epidemiologic studies the role of physical activity in preventing breast and endometrial cancer in women. First, we assessed risk for endometrial (Paper I) and breast (Paper II) cancer in relation to occupational physical activity in a large nationwide cohort generated through linkage between census data in 1960 and 1970 and the Cancer Register 1971-1989. We focused on women with the same level of estimated occupational physical activity in 1960 and 1970. During the nineteen years of follow-up, we observed a total of 1, 949 cases of endometrial and 8,261 cases of breast cancer. We found a regularly increasing risk for endometrial cancer with decreasing level of occupational physical activity notably among women aged 50-69. Socioeconomic status did not appear to confound the risk estimates. The risk of breast cancer increased with decreasing level of occupational physical activity and with increasing socio-economic status. After adjustment for socioeconomic status, a 30% gradient in risk remained only among women 50-59 years old. Second, we investigated the role of both leisure-time and occupational physical activity on endometrial (Paper III) and breast (Paper IV) cancer risk in a nation-wide case-control study among Swedish women aged 50-74 years. Information on leisure-time physical activity during childhood, at ages 18-30 and in recent years as well as anthropometric, reproductive, and hormone-related factors was assessed through a detailed questionnaire. We estimated occupational physical activity through record linkage to the census data (19601990). Among 709 women with endometrial cancer and 3,368 population controls, women with sustained low leisure-time physical activity were at a 50% higher risk of endometrial cancer than the most physically active women. Low occupational physical activity level during peri-menopausal years increased the endometrial cancer risk notably among women who were non-obese or smoked. Among 3,347 women with breast cancer and 3,455 population controls, high leisuretime physical activity at ages 50-74 was associated with risk reduction. Women with sedentary occupations during their reproductive years had a 50% higher risk compared to those with the physically most demanding jobs. In comparison with active women, those with the combination of sedentary jobs and lack of leisure-time exercise had a 3-fold higher risk. The protective effect of occupational physical activity on breast cancer risk was limited to leaner women and more pronounced in those who never used hormone replacement therapy. Third, we evaluated the effect of leisure time physical activity during most period of reproductive years and occupational physical activity on breast cancer risk in a cohort of 11,408 female twins born 1886 through 1925 (Paper V). Information on physical activity, anthropometric measures, reproductive and social factors were assessed through a questionnaire. A total of 506 breast cancer cases occurred during follow-up from 1967 to 1997. Women with regular leisure-time physical activity had 20% lower risk for breast cancer compared with inactive women. We found some evidence that the level of risk reduction was modified by birth cohort, age and body mass index in that the reduction in risk was more pronounced among older women with no overweight.
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