Obesity poses cancer risk for older women.

Women trying to lose weight have gained yet another incentive: A recent survey establishes the strongest evident so far of a link between obesity and breast cancer. Postmenopausal women who have gained at least 45 pounds since age 18 face nearly twice the risk of getting the disease as women who add less than 5 pounds, researchers find.

Earlier studies had shown that women who receive estrogen as part of hormone replacement therapy generally face a higher risk of breast cancer than those who don't. Such therapy has obscured the role of obesity.

"Hormone use masks the association between weight gain and cancer risk," says epidemiologist and study coauthor Zhiping Huang of the Harvard School of Public Health in Boston.

Moreover, as long ago as the 1970s, scientists had suggested that because estrogen stored in fat can be released into the bloodstream, obese women might have higher estrogen concentrations after menopause than lean women.

To clarify the role of weight gain in breast cancer, researchers recently analyzed surveys from 95,256 female nurses in 11 states between 1976 and 1992. They identified 1,000 who developed breast cancer before menopause and 1,517 who got the cancer after menopause. In the former group, weight shows no correlation with cancer rate, they report in the Nov. 5 Journal of the American Medical Association.

Until now, no study had distinguished clearly between the effects of extra weight and the use of hormone supplements in postmenopausal women. According to the new study, weight gain seems to pose an added breast cancer risk for postmenopausal women not taking estrogen. Gaining 5 to 22 pounds after age 18 boosts breast cancer risk by 20 percent; adding 22 to 44 pounds hikes it by 60 percent.

Lean women who take estrogen face an increased risk of breast cancer, compared to lean women who don't take hormones. Obese women face a heightened risk whether they are taking estrogen or not.

Although obesity doesn't seem to add to the risk of breast cancer in young women, it does increase the chances that the disease will be fatal. Excess weight tends to delay detection of tumors, Huang says. In postmenopausal women, obesity may also speed the growth of tumors because fat releases estrogen into the body, she speculates.

Although epidemiological studies have linked hormone replacement therapy to the incidence of breast cancer, laboratory proof that estrogen causes cancer is still lacking, says Trudy L. Bush, an epidemiologist at Johns Hopkins Medical Institutions in Baltimore.

Doctors often prescribe estrogen for symptoms of menopause, such as hot flashes and temperament changes, and to fend off osteoporosis and heart disease. Although studies link estrogen and breast cancer, other factors may be equally important in determining whether postmenopausal women get the disease, cautions Bush. "My guess is that obesity is a marker for a lifestyle and that women who exercise have a lower breast cancer rate."

The new study measures obesity by using total body weight, a less accurate gauge than, for example, waist-to-hip ratio, Bush says. The data are also self-reported, and people sometimes lie about their weight, she notes.

Nonetheless, the study offers an element of control to women in the fight against breast cancer, says Christine Swanson, an epidemiologist at the National Cancer Institute in Bethesda, Md. "This is a nice contribution because women are searching for something they can modify, and weight is modifiable. [Breast cancer] is a disease they fear, so this might be an added motivation to change their physical activity and eating habits."