The study also showed that the fattest women are 55 per cent more likely to develop the disease than the leanest. But being physically active still seemed to help lower breast cancer risk regardless of how fat or thin the women were.
The findings are based on a study of nearly 126,000 postmenopausal women whose body fat percentage and self-reported physical activity, plus a number of other lifestyle factors, were recorded as part of UK Biobank – a database of medical information and samples for researchers studying how human disease develops.
Around 1,000 women were diagnosed with breast cancer in the follow-up period of around three years, allowing the researchers to study the impact of lifestyle factors on them developing the disease over a relatively short time.
Professor Tim Key, a Cancer Research UK scientist from the Cancer Epidemiology Unit at Oxford University led the study in collaboration with PhD student Wenji Guo.
He said: “We’ve known for some time that exercise may help to reduce breast cancer risk after the menopause, but what’s really interesting about this study is that this does not appear to be solely due to the most active women being slimmer, suggesting that there may be some more direct benefits of exercise for women of all sizes.
“We don’t yet know exactly how physical activity reduces breast cancer risk, beyond helping to maintain a healthy weight, but some small studies suggest that it could be linked to the impact on hormone levels in the body.”
Alison Cox, Cancer Research UK’s director of cancer prevention, said: “This study confirms that the benefits of staying active go beyond just burning calories, sending a clear message to all women about the importance of being physically active throughout life.
“Resources like UK Biobank are providing scientists with greater insights into how our lifestyle choices affect our body’s inner workings, helping us to improve and tailor the advice we can offer people to help them reduce their risk of cancer.”