Original PaperOccupational and leisure time physical activity and the risk of breast cancer
Introduction
Strenuous physical activity in adolescence and young adult life has been inversely related to subsequent breast cancer risk, possibly by delaying menarche and the onset of regular ovulatory activity1, 2. Physical activity may also modify body fat composition and decrease fat stores, and hence reduce breast cancer risk in post-menopause, since oestrogen levels and availability are related to conversion of androgens to oestrogens in the adipose tissue[3].
Epidemiological data on physical activity and breast cancer risk at various ages are, however, still open to debate. In a comprehensive review of studies published up to 1997[4], seven out of nine studies on occupational activity showed some reduction for risk among more active women, at least in some subgroups; 11 out of 16 studies on recreational activity showed also some reduction in risk. However, the strength of the association varied across studies, and the protection tended to be more consistently reported in case–control than in cohort studies. With reference to more recent studies, no association was observed for recreational physical activity in a population-based case–control study including 1668 cases under the age of 45 years in three areas of the US[5], as well as in a report from the Nurses' Health Study II, based on 372 breast cancer cases in young women[6]. However, a significant inverse trend in risk emerged for postmenopausal women among the 109 cases from the College Alumni Health Study[7].
Sociodemographic, reproductive and hormonal correlates of breast cancer[8], which were only partly allowed for in most studies, could explain part of the inverse association between physical activity and breast cancer risk, but this cannot totally account for the different results of various studies. It is also unclear whether the relevant measure of physical activity is some overall measure of lifelong activity, or whether selected periods of life are of specific relevance9, 10. For instance, a 60% reduced risk in the highest exercise level was reported from a case–control study of women under the age of 40 years in California[11], and was interpreted in terms of influence of physical activity on luteal phase hormones[12]. Definition and assessment of physical activity in epidemiological studies is also complex, and this may explain some of the apparent discrepancies[13].
In order to provide further information on the issue, we considered data from a case–control study of breast cancer conducted in the Swiss Canton of Vaud[14], including detailed information on potentially relevant covariates, such as anthropometric, hormonal and reproductive factors and diet.
Section snippets
Patients and methods
A case–control study on breast cancer was conducted between January 1993 and August 1998 in the Swiss Canton of Vaud[14]. Cases were women with incident, histologically confirmed breast cancer, admitted to the University Hospital of Lausanne. A total of 246 women aged between 29–74 years (median age 56) were included in the present analysis. Controls were 374 patients aged between 27–74 years (median age, 58) admitted to the hospitals in the same catchment area as cases for acute,
Results
Table 1 gives the distribution of breast cancer cases and the comparison group according to age and selected characteristics. Cases reported later first birth (OR=1.5, 95% CI 1.1–2.2, for ≥25 versus <25 years) more frequent history of breast cancer in first-degree relatives (OR=4.1, 95% CI 2.1–7.8), personal history of benign breast disease (OR=2.4, 95% CI 1.2–4.8), and higher calorie intake (OR=1.8, 95% CI 1.3–2.5).
Table 2 details occupational and leisure-time physical activity. The
Discussion
The present data, based on one of the few available European datasets, further suggest that physical activity is a favourable indicator of breast cancer risk. An inverse association between both occupational and leisure-time physical activity was observed in adolescence and younger age, this being in agreement with the observation that strenuous physical activity in adolescence may delay menarche, decrease the number of regular menstrual cycles, and consequently be inversely related to
Acknowledgements
This work was partly supported by the Swiss National Science Foundation (Grant No. 32-31330.91) and by the Swiss Cancer Research Fund (KFS, Contracts AKT No. 72 and 700-8). The authors wish to thank the following hospital physicians for their assistance: H. Bossart, P. Burckhardt, G. Chapuis, P. De Grandi, N. De Tribolet, J.-F. Delaloye, D. Egloff, E. Frenk, M. Gillet, S. Krupp, J. Leisinger, F. Lejeune, P.F. Leyvraz, S. Leyvraz, J.-J. Livio, R. Mirimanoff, Ph. Monnier, P. Nicod, R. Panizzon
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