Researchers saw the benefits in as little as 4.5 pounds lost.

Doctors who say weight loss can reduce the risk for breast cancer finally have some pretty solid science on their side: A study published Tuesday in the JNCI: Journal of the National Cancer Institute found that postmenopausal women may decrease their likelihood of getting breast cancer if they lose weight––even a small amount.

“Excess body weight is a known cause of postmenopausal breast cancer,” lead study author Lauren Teras, PhD, scientific director for epidemiology research at the American Cancer Society, tells Health. “An important question is, can you reverse it? Believe it or not, this was not something we knew for certain. We had hoped it was true, but the scientific evidence was not there.”

Until now, that is. The researchers analyzed data from 10 separate prospective studies, involving 180,000 women over age 50 who were tracked for a decade. Nearly 7,000 of the women received a breast cancer diagnosis during that time, according to the data.

Compared to postmenopausal women who maintained a stable weight over approximately 10 years, the researchers found that women who lost weight—even as little as 4.5 pounds—and then kept it off for another 5 years, had a 13 percent lower risk of developing breast cancer. And more weight lost led to an even greater impact on cancer risk—at 20 pounds of sustained weight loss, women cut their risk of breast cancer by about 25%, Teras told NBC News. (The researchers accounted for other risk factors that can impact obesity and/or breast cancer risk, like physical activity levels and whether the women were on hormone replacement therapy.)

“This is particularly important for the 40 percent of women worldwide, and more than two-thirds of US women, who are overweight or obese and therefore at higher risk for breast cancer,” says Teras. “Given that breast cancer is the most commonly diagnosed cancer in women, the question of whether weight loss can reduce breast cancer risk is of great public health importance.”

Although the study was restricted to women aged 50 years and older, Teras stresses that it is important for women of all ages to maintain a healthy body weight to avoid a variety of negative health outcomes. Another important takeaway from this study is that the results suggest that if you do gain weight, it is not too late to lower your risk of breast cancer by losing weight—in women who lost 20 or more pounds, and gained some (but not all) back, their breast cancer risk was still lower than if they hadn't lost any weight at all.

While it should be noted that this study only showed an association between breast cancer risk and weight loss, not a cause-and-effect relationship, it's still an important finding in the realm of breast cancer diagnoses—especially since breast cancer is still second deadliest cancer in women in the US, after skin cancer. (The American Cancer Society estimates the average woman's risk for developing breast cancer is about 13 percent.)

As well as reaching and maintaining a healthy weight, the American Cancer Society recommends regular exercise (at least 150 minutes of moderate intensity or 75 minutes of vigorous intensity activity each week) and limiting or avoiding alcohol to reduce your risk for breast cancer.

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