More evidence that keeping physically active is among the best tactics for prevention.

Yet another study has found that being physically active reduces breast cancer risk. That's the good news. The bad news is that weight gain can cancel out the benefits of physical activity, according to a recent study designed to try to sift out what women can do to improve their chances of avoiding breast cancer and to quantify just how much exercise can help reduce a woman's risk.

The researchers studied data from about 3000 women between the ages of 20 and 98 years old. Of those women, 1,504 had breast cancer and 1,555 did not have the disease. The women were enrolled in the Long Island Breast Cancer Project, which was initiated to look for a link between the environment and breast cancer.

Women who were physically active, but then gained a significant amount of weight, had an increased risk of breast cancer, particularly if they put on weight after going through menopause.

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Women who exercised had a reduced risk of breast cancer, whether or not they had gone through menopause. Those who exercised between 10 to 19 hours per week reaped the most benefit, with a 30 percent reduction in breast cancer risk.

The level of exercise intensity did not affect risk reduction. Exercise appeared to selectively reduce the risk of hormone receptor-positive breast cancer, the most common breast tumor diagnosed among American women.

The researchers, from the University of North Carolina, Chapel Hill, found that women who were physically active, but then gained a significant amount of weight, had an increased risk of breast cancer, particularly if they put on weight after going through menopause. This observation suggests that the benefits of exercise in terms of breast cancer risk are negated by weight gain.

The study was published in the journal Cancer.

This article originally appeared on TheDoctorWillSeeYouNow.com, an Atlantic partner site.