One in four cases of breast cancer in post-menopausal women who have not used hormone replacement therapy is caused by weight gain, but the risk can be substantially lowered by losing weight, researchers reported today.
Researchers found that if the women lost at least 22 pounds, they could reduce their risk of breast cancer by about 40 percent. If they kept the weight off for at least four years, the risk was reduced by 60 percent.
"Weight is one of the few risk factors for breast cancer women can do something about," said lead author A. Heather Eliassen, an epidemiologist at Brigham and Women's Hospital and Harvard Medical School in Boston.
The report, published in the Journal of the American Medical Association, is the first to look at the effects of losing weight after menopause.
Previous studies have established the connection between fat and breast cancer. Fat tissue produces estrogen, which fuels tumor growth in some kinds of breast cancer.
Other risk factors for breast cancer include a family history of the illness, early onset of menstruation, smoking and alcohol.
The report, which followed more than 87,000 women for 26 years, confirmed that gaining weight increases the chance of breast cancer.
Women who gained 55 pounds or more after age 18 increased their breast cancer risk 45 percent. Gaining 22 pounds or more after menopause increased the risk of breast cancer about 20 percent, the report said.
Among the women who gained 55 pounds or more throughout adulthood, those who did not use hormone replacement therapy increased their breast cancer risk by 98 percent. Those who did use the therapy increased their risk by 20 percent.
Eliassen said the use of hormone replacement therapy, which replaces the estrogen women lose during menopause, probably masks the effects of excess fat on breast cancer because estrogen levels are already elevated in those women.
"Although this study ... suggests that it is never too late to lose weight to decrease risk," Eliassen said, "women should avoid gaining weight both before and after menopause to decrease their ... breast cancer risk."