“More than 100 epidemiological studies have looked at the risk of breast cancer and physical activity,” says Heather Neilson, a Canadian exercise researcher at Alberta Health Services in Calgary.
“The majority have found that women who are the most physically active have a 10 to 25 percent lower risk than women who are the least physically active.”
Most studies have tracked postmenopausal women, but some have looked at younger women. And others have looked at those who already have breast cancer.
“The evidence is growing that women have a lower risk of dying of breast cancer if they are more active after diagnosis,” notes Neilson.
Of course, something else about women who exercise might explain their lower risk of getting or dying of the disease. Only trials that randomly assign women to exercise or not can find out.
A recent year-long trial—the Nutrition and Exercise for Women (NEW) study— was a surprise.
The trial assigned 439 overweight or obese postmenopausal women to:
■ a weight-loss diet, or
■ aerobic exercise (45 minutes a day for five days a week), or
■ diet plus exercise, or
■ a control group (which was told not to make diet or exercise changes).
The results: estradiol, insulin, inflammation, and other markers of risk improved (just about equally) in both the diet and diet-plus-exercise groups (which lost an average of 20 pounds). However, the exercise-only group’s markers were not significantly different from the control group’s.
“The NEW trial strongly implies that for overweight or obese postmenopausal women, most of the benefit of exercise comes from weight loss,” says Neilson.
Still, exercise could lower breast cancer risk by some mechanism that wasn’t examined in the NEW study.
“The epidemiological evidence that exercise lowers the risk of breast cancer is quite strong, so there’s probably something about physical activity that we don’t understand, or different pathways that we’re not measuring,” says Kristin Campbell, an associate professor of physical therapy at the University of British Columbia who co-authored the NEW trial.
For example, says the National Cancer Institute’s Regina Ziegler, “we’re looking at whether physical activity changes how the body metabolizes estrogen.”
In the meantime, they all recommend exercise, whether you lose weight or not.
“Physical activity helps prevent weight gain in the future and helps maintain weight loss,” says Campbell. “And it improves cardiovascular health, type 2 diabetes, mental health, and quality of life, so it’s obviously a great benefit.”
Sources: Curr. Nutr. Rep. 3: 22, 2014; Breast Cancer Res. Treat. 137: 869, 2013; J. Natl. Cancer Inst. 104: 1, 2012; J. Clin. Oncol. 30: 2314, 2012.