According to data published in Cancer Epidemiology, Biomarkers & Prevention, a journal of the American Association for Cancer Research, postmenopausal women who in the past four years had undertaken regular physical activity equivalent to at least four hours of walking per week had a lower risk for invasive breast cancer compared with women who exercised less during those four years.

?Twelve MET-h or metabolic equivalent task-hours [*] 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 Fundrecommendations of walking at least 30 minutes daily,? noted Agn?s Fournier, PhD, a researcher in the Centre for Research in Epidemiology and Population Health at the Institut Gustave Roussy in Villejuif, France. ?So, our study shows that it is not necessary to engage in vigorous or very frequent activities, walking 30 minutes per day is beneficial.?

…Recreational physical activity, even of modest intensity, seemed to have a rapid impact on breast cancer risk…

Postmenopausal women who in the previous four years had undertaken 12 or more MET-h of physical activity each week had a 10% decreased 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 decreased risk for invasive breast cancer.

Decreasing risk after menopause
?Physical activity is thought to decrease a woman?s risk for breast cancer after menopause,? said Fournier. ?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.

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?Our study answers these questions,? Fournier continued. ?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.?

Study results
Fournier and colleagues analyzed data obtained from biennial questionnaires completed by 59,308 postmenopausal women who were enrolled in E3N, the French component of the European Prospective Investigation Into Cancer and Nutrition (EPIC) study. Since its beginnings in 1990, the E3N cohort study has received sustained support from the French National Cancer League and a number of founding partners, including Inserm, Gustave Roussy Institute and MGEN (mutual insurance company). The mean duration of follow-up was 8.5 years, during which time, 2,155 of the women were diagnosed with a first primary invasive breast cancer.

The total amount of self-reported recreational physical activity was calculated in MET-h per week. The breast cancer risk-reducing effects of 12 or more MET-h per week of recreational physical activity were independent ofbody mass index, weight gain, waist circumference, and the level of activity from five to nine years earlier.

[*] The Metabolic Equivalent of Task orMET, which is simply called a metabolic equivalent, is a physiological measure expressing the energy cost of physical activities and is defined as the ratio of metabolic rate, and therefore the rate of energy consumption, during a specific physical activity. A MET also is defined as oxygen uptake in ml/kg/min with one MET equal to the oxygen cost of sitting quietly, equivalent to 3.5 ml/kg/min. The MET concept was primarily designed to be used in epidemiological surveys, where survey respondents answer the amount of time they spend for specific physical activities.

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