For the study, researchers at the Lawrence Berkeley National Laboratory followed 79,124 women, all of whom were either runners or walkers, for 11 years. None of them had been diagnosed with breast cancer at the start of the study, and they all reported the distances they ran or walked each week, as well as their bra cup size, body weight, and height. Researchers found that the women who met the Centers for Disease Control and Prevention’s current aerobic exercise guidelines (that’s two and a half hours of moderate activity or an hour and 15 minutes of vigorous activity a week) were 42% less likely to die of breast cancer during the study than those whose exercise fell short of the guidelines—even after adjusting for body mass index (BMI). What’s the secret? Exercise reduces estrogen’s effect on cancer by altering how the body breaks down the hormone into either harmful or benign byproducts, according to previous research in Cancer Epidemiology, Biomarkers & Prevention. Also, while the current study controlled for BMI, exercise reduces women’s levels of fatty tissue, which secretes estrogen and is the main source of the hormone in postmenopausal women. So, if you want to nearly halve your risk, the time to beat is two hours—and that’s if you’re walking briskly. If you’re strapped for time, about an hour of running each week will do the job, says lead researcher Paul T. Williams, PhD, who notes that running and walking offer the same breast cancer protection. At higher intensities, it just takes less time to expend the required amount of energy. More from Prevention: Beating Breast Cancer With Zumba
