CALGARY, Alberta, May 13 -- A physically active life may reduce a woman's breast cancer risk by 25% to 30%, according to a systematic review of the literature.
The protective effect varied, though, by type of activity undertaken, when women were active over a lifetime, and body mass index, found Christine M. Friedenreich, Ph.D., of the Alberta Cancer Board here, and Anne E. Cust, D.Phil., M.P.H., of the University of Melbourne in Australia.

The role of physical activity in breast cancer is becoming increasingly convincing with enough studies now to allow a closer look at the particulars, such as which groups of women benefit most, the researchers wrote in an online report in the British Journal of Sports Medicine

"Since the effect of physical activity may differ within these subgroups, hence, the public health recommendations could vary depending on the population at risk," they wrote.

Exercise may act on breast cancer risk by reduction in circulating levels of and cumulative exposure to sex steroid hormones, changes related to insulin and adipocytokines, reduction in inflammation, modulation of the immune system, and hormonal and cellular metabolism pathways, the researchers said.

Their literature review included 62 studies looking at the impact of physical activity on breast cancer risk. Of these 34 were case-control studies and 28 were cohort studies.
 

Overall, 76% of the studies found evidence that physically active women had lower breast cancer risk. Nearly half of the studies showed a dose-response effect.

The reduction in risk was significant in 30 of 62 studies for the most physically active women compared with those with the lowest active level.

Among these studies with significant findings, physical activity was associated with a 25% risk reduction on average. The benefit was greater among the case-control studies than the cohort studies (average 30% versus 20%).

The magnitude of the reduction in breast cancer risk was even greater (36%) when all studies that showed a risk reduction were included regardless of statistical significance.

High intensity activity was associated with little additional benefit over moderate intensity activity (average decrease in risk 22% versus 26%). Total caloric intake was not a factor.

Recreational or leisure activities had the greatest benefit -- an average 20% decrease in breast cancer risk-- among the types of activity. The benefit for women was similar for physical activity associated with household and occupational work and for walking or cycling as transportation between home and work (average 13% to 14% decreased risk).

Lifetime activity was more consistently associated with reduced breast cancer risk than were measurements of activity around the time of diagnosis. Activity after age 50 also tended to be more strongly associated with decreased risk than activity in adolescence and early adulthood.

Studies also provided a more solid case for risk reductions in postmenopausal women than in premenopausal women, the researchers said.

Thinner women appeared to get the largest benefit with a risk reduction of 27% with activity among women with a BMI less than 22 mg/kg2 that fell to 19% for those with a BMI above 25 mg/kg2 and no decrease for women with a BMI above 30 mg/kg2.

Other factors associated with greater risk reductions were non-white race, lack of a family history of breast cancer, and parity. Physical activity also appeared to impact risk of hormone receptor-negative cancer more than hormone receptor-positive cancer.

However, they cautioned that some of the study subgroups used to determine effect modification were small and will require larger prospective studies to reach more definitive conclusions.
 

Primary source: British Journal of Sports Medicine
 

Source reference: Friedenreich CM, Cust AE "Physical activity and breast cancer risk: impact of timing, type and dose of activity and population subgroup effects" Br J Sports Med 2008.