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Environmental Factor, August 2012

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Study examines the role of timing in breast cancer risk

By Bono Sen

Lauren McCullough

When asked what recommendations she would give to women about physical activity, McCullough said emphatically, “I believe that all women, irrespective of body size, should engage in some activity.” (Photo courtesy of Lauren McCullough)

A new study, cofunded by NIEHS, suggests that women can significantly reduce their risk of breast cancer later in life by being physically active before or after menopause. The researchers analyzed data from women who had participated in the Long Island Breast Cancer Study Project.

In their study, University of North Carolina (UNC) at Chapel Hill epidemiologist Lauren McCullough, and colleagues from UNC, Columbia University, and Mount Sinai School of Medicine looked at the effects of physical activity, weight gain, and body size on breast cancer risk. Their findings, published online in Cancer, suggest that even a moderate level of physical activity can reduce a woman’s risk of breast cancer, as long as there is no significant weight gain after menopause.

"Importantly, we found reduced risk of breast cancer for women who engaged in exercise after menopause," McCullough said in a UNC press release "This is particularly encouraging given the late age of onset for breast cancer."

Greatest benefit from exercise during reproductive and postmenopausal years

That physical activity reduces the risk for breast cancer has been known for sometime. But before this information can be used to design effective public health intervention, several questions need to be addressed. For example, does the age at which activity is performed affect the benefit? How long and how often should women exercise? Is the benefit dependent on the intensity of the activity? Does an individual’s weight affect the reduction in risk, and does exercise reduce the risk equally for all types of breast cancer? McCullough and colleagues designed their study to answer these very questions.

The researchers asked women with breast cancer (1504) and without breast cancer (1555), between the ages of 20 and 98, about their level of physical activity. They found that women who exercised up to two hours each day during their reproductive and postmenopausal years had a 30 percent reduced risk of breast cancer. The reduction in risk was observed irrespective of the intensity of the activity.

Additionally, exercise also reduced the risk of hormone receptor (HR) positive breast cancers by 25 percent, an important observation given that HR positive breast cancer is the more prevalent form of the disease among American women and is on the rise.

Genetics and weight gain also affect risk

Importantly, the researchers noted that women who were moderately active experienced a greater reduction in risk than women at the highest levels of physical activity, suggesting that underlying biologic or genetic characteristics related to exercise physiology may be responsible for this non-linear association and warrants additional investigation. Activity performed during adolescence or early adulthood did not affect a woman’s breast cancer risk, suggesting that the timing of physical activity was important in reducing risk.

When evaluating the three-way effects of weight gain, body mass index, and physical activity, the researchers found that even high levels of physical activity were not sufficient to eliminate the excess breast cancer risk caused by weight gain.

These findings have the potential to inform interventions to reduce breast cancer risk. However, given that the study cohort consisted of a homogenous population of mostly affluent and educated women, further studies will need to be conducted in diverse population before these results are applicable to a wider population.

In addition to NIEHS funding, awards from the U.S. Department of Defense Breast Cancer Research Program and the National Cancer Institute provided support for the research.

Citation: McCullough LE, Eng SM, Bradshaw PT, Cleveland RJ, Teitelbaum SL, Neugut AI, Gammon MD. 2012. Fat or fit: The joint effects of physical activity, weight gain, and body size on breast cancer risk. Cancer; doi:10.1002/cncr.27433 [online 25 June 2012].

(Bono Sen, Ph.D., is the program director for health literacy for the NIEHS journal Environmental Health Perspectives.)

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