The researchers found that the greatest effect was in women who exercised recreationally 10 to 19 hours a week: this appeared to reduce their risk of developing breast cancer by around 30%.
However, they also found that substantial weight gain can negate this benefit.
They write about their work in a paper due to be published this week in the journal Cancer.
Study author Lauren McCullough, a doctoral candidate at the University of North Carolina at Chapel Hill's Gillings School of Global Public Health, and colleagues, examined the effect of recreational physical activity, done at different points in life, including after the menopause, on women's risk of developing breast cancer.
While others have already shown that exercise can reduce breast cancer risk, McCullough and colleagues were interested in some of the questions that still remain unanswered. For example, how much exercise, and how often? Does it have to be intense, or does mild physical activity also have an impact? Does this work for all body types or just some? And does it work for all types of breast cancer?
For this study, they examined data on over 3,000 women aged between 20 and 98 years who took part in the Long Island Breast Cancer Study Project: a multistudy effort that began in 1993 to investigate whether environmental factors are responsible for breast cancer.
About half the women (1,504) had breast cancer, and of these the vast majority had invasive breast cancer (1,271).
The researchers found that:
- Women who exercised during their reproductive years or during the menopause had a reduced risk for breast cancer.
- Exercising 10 to 19 hours per week, outside of work activity, was linked to the largest, approximately 30%, reduction in risk.
- All levels of exercise, from mild to intense, were linked to reduction in risk of the most common breast cancers in American women, the hormone receptor positive (ER and PR positive) breast cancers.
- However, active women who gained a significant amount of weight, particularly after the menopause, had an increased risk for breast cancer, suggesting weight gain can wipe out the beneficial effect of exercise.
They conclude that:
"Collectively, these results suggest that women can still reduce their breast cancer risk later in life by maintaining their weight and engaging in moderate amounts of activity."
McCullough told the press:
"The observation of a reduced risk of breast cancer for women who engaged in exercise after menopause is particularly encouraging given the late age of onset for breast cancer."
Written by Catharine Paddock