Physical Activity and the Risk of Breast Cancer

Reviewers: Kenneth Blank, M.D. and John Chang, M.D.
Source: The New England Journal of Medicine -- May 1, 1997 -- Volume 336, Number 18

Strong evidence exists tying estrogen exposure to breast cancer. For example, women who have early menopause, late menarche and low parity are at increased risk for breast cancer. In fact, some researches believe that endogenous sex hormones are the key to understanding the etiology of breast cancer. Physical activity may alter the metabolism of estrogen and therefore may influence a women's risk of breast cancer. Several studies have examined the association of exercise and breast cancer risk but no firm conclusions can be drawn from these studies most of which are small and with limited follow up. A recent study reported in the New England Journal of medicine overcomes these shortcomings and helps establish a more definitive link between the risk of breast cancer and physical activity.

From 1974 to 1983 the National Health Screening Service in Norway entered 25,624 women in a national program aimed at better defining factors that contribute to women's health. Participants were aged 20-54 at study entry and were asked to complete health surveys and questionnaires about leisure time and work physical activity. During the follow up period 351 study participants were diagnosed with breast cancer. Analysis of the questionnaires found that greater leisure time activity reduced the risk of breast cancer. Specifically, women who exercised regularly had a 37% reduction in the risk of breast cancer. This association held even after adjusting for other breast cancer risk factors including body mass and parity. The reduction was lowest in lean women, women who continued to exercise for 3-5 years and women under the age of 45. These findings support the conclusions from many smaller reports documenting a the protective effect of physical activity - although a few studies have not found exercise to lower breast cancer risk.

Several mechanisms may explain the study's findings:

• Exercise may reduce fat stores from which estrogen is produced.
• Exercise may contribute to a normal pulsatile variation in hormone release, thereby enhancing fertility which is known to reduce breast cancer risk.
• There is evidence that exercise may protect against insulin resistance, another hormonal factor that may play a role in breast cancer and, finally,
• Exercise is known to bolster the immune system by increasing the number and activity if macrophages, killer T cells and natural killer cells- which may confer a protective effect against breast cancer.

Whether this study provides conclusive evidence of a link between sedentary life style and breast cancer is debatable as the study has several flaws including the failure to account for factors such as smoking, alcohol intake and dietary variations. Exploration of the biologic mechanisms are needed as are further prospective randomized trials. However, it is clear that regular physical activity provides several health benefits including reducing the risk of heart attacks, strokes and diabetes and should be strongly recommended by all physicians to their patients- females and male alike.