Postmenopausal women who gain more than 60 pounds during their adult years are three times as likely to be diagnosed with the most deadly forms of breast cancer as women who gain 20 pounds or less, a study shows.

Weight gain after age 18 was found to be a significant risk factor for all types of breast cancer in a new study conducted by epidemiologists with the American Cancer Society (ASC).

The greater the weight gain, the greater the risk, especially for late-stage disease or disease that had spread to other parts of the body.

The ASC study is one of the first to quantify obesity-related breast cancer risk by tumor type, grade, and stage for older women.

Researcher Heather Spencer Feigelson, PhD, MPH, tells WebMD that she hopes the findings will give women who are worried about breast cancer a clearer picture of the importance of maintaining a healthy weight.

“Women are very concerned about their breast cancer risk, but I don’t think the message has hit home that probably the best thing they can do to lower their risk is be physically active and lose weight if they need to,” she says. “There are not many other modifiable risk factors for breast cancer because the disease is so tied to reproductive events and circulating estrogen levels.”

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Role of Estrogen

Older obese women have more circulating estrogen than thin women do, because the reproductive hormone is produced in the body’s fatty tissue. Estrogen fuels the growth of most breast cancers, and this is believed to be a reason why carrying excess weight increases postmenopausal women’s risk of developing the disease.

The ACS study included 44,161 postmenopausal women who were not taking hormone therapy.

Compared with women who gained 20 pounds or less, women in the study who gained more than 60 pounds during adulthood were found to have twice the risk of ductal breast cancer. They were 1.5 times as likely to have lobular breast cancer.

Women who gained more than 60 pounds had a threefold greater risk of being diagnosed with metastatic disease (spread to areas beyond the breast) than women who gained 20 or less. Risk was about 70 percent greater with a weight gain of 21 to 40 pounds and 1.8 times greater for women who gained between 41 and 60 pounds.

The study is published in the July 1 issue of the ACS journal Cancer.

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Obesity Linked to Other Cancers

Recent studies confirm that almost two out of three women in the U.S. over the age of 60 are either overweight or obese. Experts worry that the obesity epidemic will lead to a dramatic rise in breast cancers among postmenopausal women.

“It looks like gaining weight at any time during adulthood can increase a postmenopausal woman’s breast cancer risk,” says Nancy Potischman, PhD, who is a nutritional epidemiologist with the National Cancer Institute. “Even excess weight gain in pregnancy that isn’t lost seems to increase risk.”

Potischman points out that obesity is associated with an increased risk for many other malignancies including cancers of the colon, rectum, gallbladder, kidney, uterus, and esophagus, as well as chronic diseases like heart disease and type 2 diabetes.

“Being physically active and getting serious about losing weight is something everyone can do to lower their cancer risk and their risk for developing a host of chronic diseases,” she says.

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By Salynn Boyles, reviewed by Louise Chang, MD

SOURCES: Spencer Feigelson, H.Cancer, July 1, 2006; vol. 107: online edition. Heather Spencer Feigelson, PhD, MPH, senior epidemiologist, American Cancer Society, Atlanta. Nancy Potischman, PhD, nutritional epidemiologist, National Cancer Institute.