Increased physical activity, even when begun later in life, reduces overall breast-cancer risk by 20 percent among women at all levels of risk for the disease, according to a study led by researchers at Fred Hutchinson Cancer Research Center. Their findings appear in the Journal of the American Medical Association. The activity need not be strenuous but it should be done consistently, such as taking a brisk, 30-minute walk five days a week, said lead investigator Anne McTiernan, director of the center's Prevention Center.
"We thought it was important to determine if moderate-intensity physical activities, such as walking, biking outdoors or easy swimming, when initiated later in life, can reduce the risk of breast cancer, since these types of activities are achievable for most women," said McTiernan, who is also the lead author of "Breast Fitness: An Optimal Exercise and Health Plan for Reducing Your Risk of Breast Cancer".
"Our results suggest that indeed, moderate activity, even when started in a woman's postmenopausal years, can cut her risk of breast cancer by about 20 percent, suggesting that physical inactivity may be a modifiable breast-cancer risk factor in older women."
In addition, the researchers found that regular exercise also causes a similar reduction in overall breast-cancer incidence among women considered to be at highest risk for the disease, such as those with a strong family history of breast cancer, those who've never had children and those who take combination estrogen/progestin hormone-replacement therapy.
"The good news is that even though HRT increases the risk of breast cancer, exercise is something women can do to lower this risk if they choose to continue taking HRT to manage the symptoms of menopause or to prevent osteoporosis," McTiernan said.
McTiernan and colleagues also found that the less a woman weighs, the more regular, moderate exercise appears to have a protective effect. Women of low to normal weight - and even those who were moderately overweight - who exercised the equivalent of 10 hours of walking each week experienced breast-cancer risk reductions of more than 30 percent. Breast-cancer risk didn't budge, however, among women exercisers who were significantly overweight or obese.
The weight status of the participants was determined by calculating their body-mass index, or BMI, a measure of body fat based on height and weight. The World Health Organization divides BMI into the following categories for both men and women: underweight (18.5 or lower), normal (18.5 to 24.9), overweight (25 to 29.9) and obese (30 or greater).
"This finding certainly shouldn't be an exercise disincentive for obese women," McTiernan cautioned. "There are many benefits for women of any weight to start exercising, like reducing their risk of heart disease and diabetes. But in terms of breast-cancer risk, obese women will see most benefit once they start getting their weight down."
Weight plays a role in breast cancer, researchers believe, because fatty tissue produces hormones and growth factors, such as estrogen and insulin, which may promote cancer development.
"We think that exercise works to lower cancer risk by lowering body fat, which in turn lowers the levels of circulating cancer-promoting hormones. So even if a woman is exercising, if she's overeating and her body fat stays high, she's not going to get the same cancer-fighting protection as a woman with less body fat."
The best fat-burning formula involves low- to moderate-intensity exercise that is done over a longer period as opposed to short, intense bursts of activity, McTiernan said. "The bottom line is that duration of activity is more important than intensity. The turtle beats the hare when it comes to preventing breast cancer through exercise."
To get the optimum breast-cancer protection, women should aim for at least 30 minutes of moderate-intensity exercise a day, five days a week, she said, stressing that sedentary women should start gradually and work their way up to the recommended minimum activity level.
"For most, walking is probably going to be the easiest thing to do because it doesn't require training or special equipment, just a good pair of shoes. The main thing is for women to just get out there and do it, and make it something they enjoy," said McTiernan.
The study participants were part of the Women's Health Initiative Observational Study, an arm of WHI that is following women over time to identify predictors of disease. The women were tracked for nearly five years to examine the association between current and past recreational physical activity and the incidence of breast cancer.
"When we looked at the women's total activity throughout their adulthood, including light, moderate and strenuous exercise, and added it all together, those with the highest total amount of activity seemed to be the most protected. It wasn't necessarily important regarding how much of that activity was strenuous," said McTiernan.
