Exercising for twice the recommended weekly amount is likely to help reduce the risk of breast cancer for postmenopausal women, scientists have said.

According to the recent study, published in JAMA Oncology, postmenopausal women who exercised 300 minutes per week were better at reducing total fat and other adiposity measures, than those who exercised for the recommended 150 minutes. The findings are significant as body fat, abdominal fat and adult weight gain have been associated with increased risk of postmenopausal breast cancer.

The one-year study by Christine M. Friedenreich of Alberta Health Services, Canada, and colleagues compared 300 minutes of exercise per week with 150 minutes per week of moderate to vigorous aerobic exercise for its effect on body fat in 400 inactive postmenopausal women, who were evenly split into the two exercise groups.

The women, who had body mass index (BMI) 22 to 40, were asked not to change their usual diet. Any aerobic activity that raised the heart rate to 65-75 per cent of maximum was permitted, and most of the supervised and home-based activities involved the elliptical trainer, walking, bicycling and running.

Average reductions in total body fat were larger in the 300-minute group, which also saw greater decreases in subcutaneous abdominal fat, abdominal fat, BMI, waist circumference and waist-to-hip ratio. Some of the effects were stronger for obese women (BMI greater than or equal to 30) for change in weight, BMI, waist and hip circumference, and subcutaneous abdominal fat, according to the results.

"A probable association between physical activity and postmenopausal breast cancer risk is supported by more than 100 epidemiologic studies, with strong biologic rationale supporting fat loss as an important (though not the only) mediator of this association,” wrote the authors.

“Our findings of a dose-response effect of exercise on total fat mass and several other adiposity measures including abdominal fat, especially in obese women, provide a basis for encouraging postmenopausal women to exercise at least 300 minutes/week, longer than the minimum recommended for cancer prevention."

The case for increasing the recommended weekly amount of exercise, at least for postmenopausal women at risk of breast cancer, has been supported by numerous studies. Previous research has shown that exercise can slow the growth of breast cancer tumours, while separate research has highlighted the role that exercise can play in recovery after sufferers enter remission.