By Melissa Healy, Los Angeles Times

For women past childbearing age, a study finds that a modest amount of exercise — four hours a week of walking or more intensive physical activity such as cycling for just two hours a week — drives down breast cancer risk by roughly 10 percent.

If exercise were a pill, its effectiveness in driving down a woman’s breast cancer risk would occur fairly quickly, the new research says: When women reported at least this modest level of physical activity over the last four years, they were less likely to have developed malignancy in a breast.

But, like a pill, exercise must be continued for the effect to endure: Even if a woman had been physically active earlier in life, when her reported physical activity levels dropped below the equivalent of four hours of weekly walking, her risk of developing breast cancer went back up.

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