Occupational physical activity and the risk of breast cancer

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Abstract

The association between occupational and the risk of breast cancer was analyzed using data from a case–control study of 257 women with breast cancer and 565 control women. Logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for occupations having light and medium activity compared with sedentary ones. After adjusted for potential confounders, women in sedentary occupations had a 29% higher risk, compared to those with the physically medium demanding jobs. For women at age ≥55 years higher occupational physical levels were associated with 53–60% reduction in the risk. There was a significant decreasing trend in the ORs from sedentary to medium work (P=0.001); while no association emerged in younger women. These findings demonstrate that the protective effect of higher levels activity in occupations on breast cancer appears to be confined to older women.

Introduction

Breast cancer is the commonest cause of cancer death among women worldwide and much research has been directed towards its etiology. Most studies have focused on socio-demographic, reproductive, hormonal and dietery factors showing that the risk is increased by early menarche, late menopause, and obesity in postmenopausal women [1], [2], [3], [4]. Many of the established factors of the breast cancer are linked to estrogens. Up to 10% of breast cancer in western countries is due to mutations in certain genes (BRCA1, BRCA2, p53).

Epidemiological data on an association between physical activity and breast cancer has received increasing attention recently. In numerous studies conducted during the past decade breast cancer risk has been reduced among women who reported high levels of occupational or leisure time of lifetime physical activity (for reviews, see [5], [6], [7], [8]). The association finds biological support due to the metabolic effects from increased energy expenditure such as reduced obesity, hormonal and reproductive effects, mechanical effects, and enhancement of the immune system [8], [9].

In recent review [6] of studies dealing with physical activity and the risk of breast cancer published up to 2001, 10 out of 16 studies showed some reduction for risk among women more active in work. The studies reported reduction of the risk ranging from 0.50 to 0.79, at least among one subgroup of women. Two studies found nonsignificant reduction of the risk among certain subgroups, two studies reported no relationship, and two studies provided evidence for a reduced risk among postmenopausal women. These inconsistencies may be partially due to inadequate assessment of occupational activity, uncontrolled confounding by other risk factors, small sample size, analysis of extreme levels of the activity. In addition, in several studies only occupational activity was assessed, while early-life physical activity is also very important in total energy expenditure. It is worth to add here that the association between physical activity and breast cancer is very complex and is a result of multiple biological mechanisms operation.

The purpose of this study was to evaluate the effect of occupational physical activity on breast cancer risk adjusting the risk estimates for potential confounders.

Section snippets

Subjects

A case–control study on breast cancer risk was conducted between October 1997 and October 1998 in Stettin province using questionnaires. Cases were chosen from Cancer Registry. Cases were 257 women aged between 35 and 88 years (median age 55.0±11.3 years) after radical mastectomy; controls were 565 women aged between 35 and 93 years (median age 52.2±12.2 years) admitted to the hospitals or clinics in the same areas of case subjects. Controls were frequently matched to cases on age and place of

Results

Compared with the control group women with breast cancer were somewhat older except age group of 35–44 years, were younger at menarche, had a higher BMI, were more likely to intake of vegetables and fruits very rarely and rarely, and reported a stress experience more frequently.

Table 1 presents number of cases and controls according to occupational categories. Most sedentary subjects were secretaries, office workers; light activity occupations included mainly teachers, scientists, librarians,

Discussion

Despite some limitations of this investigation, e.g. a possible selection bias caused by low response rate among cases, too little accuracy of the sitting time estimation for occupational activity the present study further suggests inverse association between occupational physical activity level and breast cancer risk. Women in sedentary occupations had a 29% higher risk than that of women medium active. The risk reduction was nearly twice increased in the group of women age 55 years and over.

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