Exercise lowers breast cancer risk; now researchers learn how

April 12, 2013 | by Roberta Nichols

For nearly two decades, researchers have theorized that physical activity helps prevent breast cancer – particularly in older women – by lowering their estrogen levels.

Exercise and breast cancer risk. Exercise has been known to lower the risk of breast cancer. Now researchers are honing in on precisely why that's the case. The answer is in the hormones.

Now, a new study offers clues into how exercise may provide this protection.

The preliminary findings were presented April 9 at the American Academy of Cancer Research (AACR) meeting in Washington, D.C., by Cher Dallal, Ph.D., a National Cancer Institute prevention fellow.

Researchers evaluated the influence of physical activity on the breakdown of estrogens in postmenopausal women.  Known as estrogen metabolism, this process produces molecules called metabolites that break down the estrogen.

Researchers studied the cases of 540 healthy, postmenopausal women between the ages of 40 and 74 who were enrolled as control patients in the National Cancer Institute Polish Breast Cancer Study from 2000-2003. None of the women took hormone therapy.

For seven days, study participants wore small devices called “accelerometers” around their waists while they were awake to record their varying degrees of physical activity.  The devices are believed to be a more accurate accounting of physical activity than subjects' memories.  The women also collected samples of their urine during a 12-hour period.

Researchers tested the urine for estradiol and estrone, two “parent” estrogens, as well as 13 different metabolites.

“This is the first study to consider the relationship between accelerometer-measured activity and a panel of estrogen metabolites measured in urine,” said Dallal in an AACR press release. “We hoped by using direct measures to examine this relationship that we could improve our knowledge of how these factors may influence cancer risk among postmenopausal women.”

“By using these new tools to study the relationship between activity and estrogen metabolism, we hope to get closer to uncovering the combination of parent estrogens, metabolites and metabolism pathways that are related to a lower-risk profile of breast cancer,” Dallal continued.

One of the first scientists to make the connection between physical activity and breast cancer risk was Leslie Bernstein, Ph.D., professor and director of the Division of Cancer Etiology at City of Hope.

“We know for breast cancer [risk], hormones are important,” Bernstein told Healthday.

“This is the first time we have strong evidence that measured physical activity reduces hormone levels.  It helps us understand what’s going on and how it’s working.”

She lauded the study’s use of the accelerometer, which provides a more objective measure than asking women to recall how much energy they expended.

Bernstein has long championed exercise’s ability to lower estrogen exposure and breast cancer risk, not to mention insulin levels and weight.  Women often gain weight after menopause, and estrogen lurks in fat tissue, increasing women’s risk of breast cancer.

As researchers continue to explore exercise’s potential benefits – such as whether physical activity can actually repair DNA – Bernstein urges women to get a move on.

Women who have been estranged from exercise for awhile, she said, should first obtain their doctor’s blessing, then begin physical activities like brisk walking that “puts some stress on the body.”

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