Earlier studies have linked excess weight with an increased risk for breast cancer in postmenopausal women, but this is one of the few studies that traces the risk as a function of weight gain over time.

"Among women who had never used postmenopausal hormone therapy, those who had a body-mass index (BMI) gain between age 20 and 50 had a doubling of breast cancer risk," said lead researcher Laura Sue, a cancer research fellow at the U.S. National Cancer Institute.

Sue was expected to present the findings Tuesday at the American Association for Cancer Research's annual meeting, in Washington D.C.

For the study, Sue's team collected data on more than 72,000 women who took part in the Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial. When the study began, the women were between 55 and 74 years old. Among these women, 3,677 had developed a postmenopausal breast cancer.

The researchers looked only at women who had had breast cancer and had never taken hormone replacement therapy to reduce menopausal symptoms. Hormone therapy can boost the risk for developing breast cancer, so by looking at women who had never taken the therapy, the researchers were able to better isolate weight as an individual risk factor.

Compared with women who maintained about the same weight at 50 as they had at age 20, women who gained about 30 pounds over the years increased their risk for breast cancer twofold, the study found.

Among the women in the study, almost 57 per cent had increased their BMI by five kilograms per meter squared (kg/m2) over 30 years. That's akin to a women 5 feet 4 inches tall putting on about 30 pounds, Sue said.

An increase in BMI of 5 kg/m2 or more over 30 years increased the risk of developing postmenopausal breast cancer by 88 per cent, compared with women whose BMI remained stable over the same period.

Among women whose BMI increased 5 kg/m2 or more from the age of 50 onwards, their risk for breast cancer increased 56 per cent, compared with women whose BMI remained the same. That means that jumps in weight before and after age 50 boost a woman's odds for postmenopausal breast cancer, the researchers noted.

The increased risk for breast cancer was tied to the weight gain itself, not to becoming obese, Sue added.

The rise in risk may be due to an increase in the production of estrogen in the body's excess fat cells, which in turn may increase the number of cells produced in the breasts, upping the risk for cancer, Sue said.

The bottom line: "We believe healthy BMI maintenance throughout adulthood is important in terms of breast cancer risk," she said.

More information

For more information on breast cancer, visit the http://www.cancer.org/docroot/cri/cri-2x.asp?sitearealrn&dt5 American Cancer Society.

SOURCES: Laura Sue, M.P.H., cancer research fellow, U.S. National Cancer Institute; April 20, 2010, presentation, American Association for Cancer Research annual meeting, Washington D.C.

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