Walking at least one hour a day may lower the risk of breast cancer in postmenopausal woman, a large, long-term study suggests.

Previous studies have suggested that women who get physical activity regularly have a 25 per cent lower risk of breast cancer compared to those who were the most inactive. Now researchers have looked at the question in more detail, focusing on whether walking helps in the absence of more vigorous exercise.

Epidemiologist Alpa Patel of the American Cancer Society and her co-authors compared exercise levels for 73,615 postmenopausal women, including 4,760 who were diagnosed with breast cancer over 17 years.

"Women who engage in an at least seven hours of walking over the course of a week may reap a modest benefit, even in the absence of more vigorous exercise," Patel and her co-authors concluded in Thursday's online issue of the journal Cancer Epidemiology, Biomarkers and Prevention.

"Given that breast cancer is the most common cancer affecting women, and that walking is a common activity among postmenopausal women, the finding of a possible lower risk with an average one or more hours/day of walking is of considerable public health interest."

Those who walked at a leisurely pace of 4.8 km/h for an hour a day reduced their breast cancer risk by 14 per cent, which translates to about one fewer woman in 1,000 getting breast cancer each year, Patel said.

Walking more vigorously at a pace of  7.2 km/h for one hour a day reduced the risk by 25 per cent compared with those who were least active, meaning 1.6 cases of breast cancer per 1,000 would be avoided per year.

"Physical activity may act favourably on hormonal and non-hormonal pathways to lower breast cancer risk," such as weight control, the study's authors said.

The association between physical activity and breast cancer did not differ based on tumour factors like hormone receptor status, body mass index or the use of postmenopausal hormones, which the authors said were unclear until now.

Unlike two other studies that suggested sitting time may increase breast cancer risk, Patel's team did not find any evidence of that. The authors noted that most of the women in the study were homemakers with an average age of 63 when the study began, so the findings may not reflect total physical activity of those whose occupations involve manual activity.

The researchers conducted a careful study with followup every two years, said Christine Friedenreich, a cancer epidemiologist at Alberta Health Services Cancer Care. Her own review of more than 70 studies found similar benefits for physical activity and breast cancer.

"This is a very empowering message for women that this is something that they can do themselves to reduce their risk of developing breast cancer," said Friedenreich. "We can't say that we can ever prevent breast cancer completely but we can say you can reduce the risk."

Current guidelines recommend that Canadian adults get at least 2.5 hours per week of moderate to intense activity, but fewer than half get that, according to the Canadian Health Measures Survey.

In Toronto, Darlene McKee, 67, walks a total of two hours every day to run errands and for exercise.

"It lifts my mood," McKee said, adding the findings keeps her motivated to continue pounding the pavement.

With files from CBC's Kas Roussy