It shows just over 30 minutes of walking a day can cut the chances of breast cancer in women over 50.
Importantly, researchers found the reduced risk only in women who had undertaken exercise in the last four years. Exercising when younger, or more than four years earlier, doesn’t count, according to the team from France.
They analysed data from questionnaires completed by 59,308 postmenopausal women enrolled on the French arm of an ongoing European investigation into cancer and nutrition.
Agnes Fournier, a researcher at the Institut Gustave Roussy in Villejuif, led the study which addressed when women should start regular physical activity and how long the effect lasts.
She said: ‘We found that recreational physical activity – even of modest intensity – seemed to have a rapid impact on breast cancer risk.
‘However, the decreased breast cancer risk we found associated with physical activity was attenuated when activity stopped.
‘As a result, postmenopausal women who exercise should be encouraged to continue and those who do not exercise should consider starting because their risk of breast cancer may decrease rapidly.’
In the study, researchers converted the total amount of recreational physical activity carried out by the women into a measurement called metabolic task hours (MET-h). Postmenopausal women who in the previous four years had undertaken 12 or more MET-h of physical activity each week had a 10 per cent cut in the risk of invasive breast cancer compared with women who were less active.
Women who undertook this level of physical activity between five and nine years earlier, but were less active in the four years prior to the final data collection, did not have a lower risk for invasive breast cancer.
The breast cancer risk-reducing effects of 12 or more MET-h per week of recreational physical activity were independent of body mass index, weight gain and waist circumference.
Dr Fournier, whose study is reported in journal Cancer Epidemiology, Biomarkers & Prevention, said: ‘Twelve MET-h per week corresponds to walking four hours per week or cycling or engaging in other sports two hours per week and it is consistent with the World Cancer Research Fund recommendations of walking at least 30 minutes daily.
‘So, our study shows that it is not necessary to engage in vigorous or very frequent activities; even walking 30 minutes per day is beneficial.’
It is unclear how activity helps but the latest thinking is that aerobic exercise could break down cancer-promoting oestrogen hormones to produce ‘good’ metabolite molecules that lower breast cancer risk.
Sally Greenbrook, of Breakthrough Breast Cancer, said: ‘Breast cancer is most common in postmenopausal women so it is great to see evidence like this which supports the message that physical activity in this age group is beneficial.’