In women over age 50, losing just four pounds and keeping the weight off can lower breast cancer risk, a new study suggests. Researchers who reviewed data from 180,000 women found the more weight a woman lost - and kept off - the lower her risk of breast cancer, according to the report published in the Journal of the National Cancer Institute.

"We've known for some time that excess body weight (raises the risk) of breast cancer," said lead author Lauren Teras, scientific director of epidemiology research at the American Cancer Society. "In this study we found that losing weight and keeping it off is associated with a lower risk of breast cancer in women aged 50 and older who are not taking hormone replacement therapy. This is particularly important for women who are overweight and in the U.S. about two-thirds of women are overweight or obese."

Women with the most sustained weight loss, 20 pounds (about 9 kilograms) or more, had a 26% lower risk compared to women whose weight remained stable. Those with sustained weight loss of 4.4 to 10 pounds (2 to 4.5 kg) saw a 13% reduction in risk and those who lost 10 to 20 pounds (4.5 to 9 kg) had a 16% reduction in risk.

When the researchers analyzed various subsets of women, they found these benefits of weight loss were confined to women who weren't receiving hormone replacement therapy. The largest effect, a 32% reduction in breast cancer risk, was seen in women who lost 20 pounds or more and were not taking hormones.

Even among those who lost 20 pounds or more and gained some of it back, there was still a lower risk of breast cancer compared to those whose weight remained stable.

"Perhaps just as important, it wasn't too late if a woman gained weight after 50," Teras said. "If she then lost it she had the same risk as someone who remained stable." While the strongest impact was in women who started out overweight or obese, "we did see the same association in normal weight women, just weaker," Teras said.

Data for the new study came from the Pooling Project of Prospective Studies of Diet and Cancer, an international consortium which contains information on 10 cohorts of women and is designed to look at the impact of diet on cancer risk.

The researchers focused on 180,885 women with three or more weight reports - either from a survey or an actual measurement - prior to breast cancer follow-up. Weight loss was cataloged after the first 5.2 years of the study. Then, 4.6 years later, another measurement was made and researchers determined who had kept the weight off. The women were then followed for an average of 8.3 years to check for breast cancer.

Teras hopes the association between weight loss and a lowered risk of breast cancer "will be a motivator for the two-thirds of women who are overweight or obese." While the new study finds an association between weight loss and lowered breast cancer risk, it doesn't prove cause and effect, said Dr. Stephanie Bernik, chief of breast surgery at Mount Sinai West in New York.

"People who lose weight are generally doing other things," Bernik said. "They're usually exercising more and eating better. It could be lifestyle modifications that are reducing the risk of breast cancer." Still, Bernik said, "fat cells are known to produce inflammatory factors that are thought to contribute to an environment that allows cancer to develop and propagate. This study adds to the body of evidence that supports living a healthy lifestyle to help reduce the risk of cancer."