“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 or more — had a 26 percent lower risk compared with women whose weight remained stable.
Those with sustained weight loss of 4.4 to 10 pounds saw a 13 percent reduction in risk and those who lost 10 to 20 pounds had a 16 percent reduction in risk.
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 with 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 that 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 — before breast cancer follow-up. Weight loss was catalogued 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 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.”