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Working out is not just good for your waistline. It also is good in the fight against breast cancer. A recent study has found that 30 minutes of aerobic exercise per day may reduce the risk of breast cancer in women.

The study was conducted with 391 women between the ages of 18 to 30. All were enrolled in a larger study called Women in Steady Exercise Research, or WISER. None of the women had exercised significantly before; they had body mass indexes between 18 and 40.

According to MedPage Today, the women were divided into two groups. One group continued with their same exercise regime, which is to say, no exercise. The other group needed to perform two and a half hours of moderate to vigorous exercise a week, the standard recommendation of exercise by physicians.

In addition to looking at weight and muscle mass, the researchers looked at the women's level of estrogen, examining the three major classes known as E1, E2 and E3. Time reports that the researchers also looked at levels of nine metabolites in the women's urine. The study found that women who exercised regularly had a 25 percent increase in the ratio between two estrogen metabolites, 2-OHE-1 and 16-alpha-OHE-1. That meant that the women had fewer of the 16-alpha-OHE-1 in the ratio. That is positive, because research indicates that the proliferation of that metabolite encourages the growth of cancer cells.

The Los Angeles Times reports that breast cancer risk is linked to women's exposure to the sex hormone estrogen. Estrogen levels are known to fluctuate with women's fertility, certain medications and, researchers suspect, fat deposits and how sedentary a person's lifestyle is.

In addition to the reduction in breast cancer risk after just 16 weeks, the women who exercised regularly in the study lost fat and had more lean muscle.

The study was published in the journal Cancer Epidemiology, Biomarkers & Prevention.