Just 15 minutes of vigorous activity every day, such as running, in addition to walking and moderate activity, can reduce the risk of developing breast cancer by a fifth in postmenopausal women, a new study has found. Researchers also found that women with the most body fat were 55 per cent more likely to develop the disease than the leanest. Researchers found that being physically active still seemed to help lower breast cancer risk regardless of how fat or thin the women were. Women who did between 15 minutes and 35 minutes of vigorous exercise every day were one fifth less likely to develop breast cancer, compared with those who took no vigorous exercise. 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. "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," Key said. "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," Key added.