Women who take 30 minutes of gentle exercise per day are ten per cent less likely to develop breast cancer but once they stop their risk increases quickly, researchers have found.
A study of nearly 60,000 women found those who had exercised for the equivalent of four hours a week were 10 per cent less likely to develop breast cancer than those who were sedentary.
However the effect was only found in women who exercised consistently over four years.
Women who had been active and had stopped more than five years ago, were more than 16 per cent likely to develop cancer than those who continued.
The findings were not affected by the women’s weight or waist circumference showing that the benefits came from being active, not from losing weight.
There are around 41,000 women and 500 men in Britain who are diagnosed with breast cancer annually.
The latest research is published in the journal Cancer Epidemiology, Biomarkers & Prevention.
Lead author Dr Agnès Fournier, a researcher in the Centre for Research in Epidemiology and Population Health at the Institut Gustave Roussy in Villejuif, France, said: “Physical activity is thought to decrease a woman’s risk for breast cancer after menopause.
“However, it was not clear how rapidly this association is observed after regular physical activity is begun or for how long it lasts after regular exercise stops.
“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.”
The study analysed data from questionnaires completed every two years by 59,300 women who had gone through the menopause.
The activity was measured in metabolic equivalent hours, which captures both the intensity and duration in one number, with 12 METs the equivalent of four hours of walking or two hours of sport such as cycling per week.
The effect peaked at 12 METs and exercising for longer or more intensely did not cut the risk of cancer further.
Women who exercised at this level for four years or more were 10 per cent less likely to develop cancer over the eight year study period than those who did little or no exercise.
“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, ” Dr Fournier said.
During the study period 2,155 women were diagnosed with breast cancer.
Women who had exercised at this level and stopped more than five years previously had the same risk of cancer as women who had never exercised.
Sally Greenbrook, Senior Policy Officer at Breakthrough Breast Cancer, said: “Breakthrough Breast Cancer advises that 30 minutes of moderate physical activity a day (or 3.5 hours a week) can reduce breast cancer risk by at least 20 per cent.
“Being physically activity doesn’t need to be running or going to the gym – it can be anything from playing actively with your children, walking or gardening – anything that raises your pulse reduces your risk.
“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.
“In order to help women plan their physical activity, Breakthrough has launched an interactive online resource called BRISK, which includes a weekly tracker tool and provides information about the facts and figures behind physical activity.”