A new study suggests it's never too late to start an exercise program to help prevent breast cancer. Researchers at the Fred Hutchinson Cancer Research Center and several other institutions found that postmenopausal women who exercised regularly reduced their risk of breast cancer by about 20%.

Unlike some other studies, though, this one found that moderate exercise like walking was enough to provide a benefit. Previous research has suggested women need to engage in vigorous physical activity like jogging, tennis, or aerobics to reduce their risk of breast cancer.

"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 lead researcher Anne McTiernan, MD, PhD. McTiernan is a member of Fred Hutchinson's Public Health Sciences Division, and director of the organization's Prevention Center. Her findings were published in the Journal of the American Medical Association (Vol. 290, No. 10: 1331-1336).

The study is in line with the American Cancer Society's guidelines for physical activity, said ACS deputy chief medical officer Len Lichtenfeld, MD. ACS recommends at least 30 minutes of moderate activity at least five days a week, though more strenuous exercise may be even better for lowering the risk of breast cancer.

McTiernan and colleagues based their conclusion on an examination of more than 74,000 women aged 50-79 who were participating in the Women's Health Initiative Observational Study, a long-term study of disease risk factors in American women. They asked the women about their current exercise habits, and about their exercise levels at ages 18, 35, and 50.

Women whose current physical activity was equivalent to a couple of hours of brisk walking every week had an 18% lower risk of breast cancer than sedentary women. Women who got more than 10 hours of this type of exercise each week lowered their risk by about 22%.

Although a greater amount of vigorous activity also reduced breast cancer risk, the amount of the reduction was not statistically significant. In an editorial accompanying the study, I-Min Lee, MBBS, ScD, said that the numbers may have not have been significant only because too few women in the study group engaged in strenuous activity, making it difficult to spot a statistical trend.

The greatest benefits from exercise were seen in lean women. Women whose body mass index (BMI) was less than 24.13 (normal weight is a BMI between 18.5 and 24.9; more than 25 is overweight, and more than 30 is obese) saw a 30% reduction in their breast cancer risk with a couple of hours of brisk walking each week. Women who were significantly overweight or obese (BMI over 28.44) did not lower their breast cancer risk by exercising.

But that doesn't mean heavy women shouldn't exercise, McTiernan said. "There are many (reasons) 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."

That's because excess weight is thought to increase levels of hormones and growth factors (like estrogen and insulin) that may promote cancer development.

"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," McTiernan explained.

A second study published in the same issue of JAMA suggests that moderate physical activity – 30 minutes, five days a week – is enough to help women lose weight, as long as they watch what they eat, too. But once again, more may be better. The researchers, from the University of Pittsburgh, found that women who got even more exercise (40 minutes or more, five days a week) lost more weight than their moderately-active counterparts.

In McTiernan's study, the protective effect of exercise was seen even in women who were considered to be at higher risk of getting the disease, such as women with a family history of breast cancer, or who take combination estrogen-progesterone hormone therapy.

Her findings indicate that even moderate activity begun later in life can have an effect, "suggesting that physical inactivity may be a modifiable breast cancer risk factor in older women," she said.

Younger women also benefit from physical activity, though. Women who reported getting strenuous exercise at least three times per week at age 35 had a 14% lower risk of breast cancer compared to women who did not exercise vigorously at that age. Women who worked out vigorously at age 50 also had a slight risk reduction, but it was not statistically significant.

All this points to a need for women to get up and moving, McTiernan said. Walking is one of the easiest activities to incorporate into a busy lifestyle because it doesn't require special training or equipment, she noted.

"The main thing is for women to just get out there and do it, and make it something they enjoy."