The amount of weight a woman gains after age 18 is a strong signal as to whether she will get breast cancer later in life, according to new research released Wednesday by the American Cancer Society.

In one of the largest studies of weight and breast cancer to date, researchers said older women who gained 20 to 30 pounds after high school graduation were 40 percent more likely to get breast cancer than women who kept the weight off.

The risk doubled if a woman gained more than 70 pounds, said Heather Spencer Feigelson, senior epidemiologist with the American Cancer Society.

"Breast cancer is strongly dependent on body weight," Feigelson said. "Even modest amounts of weight gain lead to a significantly increased risk of breast cancer."

Weight gain and body mass have long been known to be risk factors for breast cancer.

The cancer society estimates weight contributes to between one-third and one-half of all breast cancer deaths among older women.

Fat tissue makes estrogen, and estrogen can help breast cancer grow. Weight gain also is the second leading cause of all cancers, according to research the Atlanta-based society published last year in the New England Journal of Medicine.

But the cancer society researchers wanted to examine more specifically the link between weight gain amounts and breast cancer, and this was the first in such a large group.

"The more fat you have - fat cells are capable of synthesizing estrogen - the heavier you are, the higher your estrogen levels," said Dr. Paul Tartter, associate professor of surgery at Columbia University, who was not a researcher in the study. "There's no question that estrogen is the common denominator of most of our risk factors for breast cancer."

The cancer society study included 1,934 breast cancer cases among 62,756 women involved in a separate long-term cancer prevention study.

Post-menopausal women ages 50 to 74 were asked their weight when the study began in 1992 and their weight when they were 18 years old. Surveys were sent to the women in 1997, 1999 and 2001 to inquire about any new cancers.

Women taking estrogen hormones were not included in the study.

Lean post-menopausal women not taking hormone replacement therapy produce very little estrogen and had the lowest cancer risk in the study, Feigelson said.

(Agencies  February 27, 2004)