Women who gain weight throughout adulthood may have an increased risk for breast cancer, according to a study in the Archives of Internal Medicine.

Interestingly, the increased risk was only noticed in those women who did not take hormone replacement therapy after menopause.

Obesity has long been known to be a risk factor for developing breast cancer after menopause. It's thought that the estrogens that accumulate in fat tissue may either initiate or promote the growth of cancerous cells in the breast.

But few studies have looked at whether having a healthy weight early in life and then putting on weight in later years raised the cancer risk.

A team led by Jiyoung Ahn, Ph.D., of the National Cancer Institute in the U.S. analyzed data from 99,039 postmenopausal women who were part of the huge National Institutes of Health-AARP Diet and Health Study.

In 1996, the women reported their current body measurements and weight, plus their weight at ages 18, 35 and 50. After calculating the women's body mass index (BMI), the researchers then classified the women as underweight, normal weight, overweight or obese.

By 2000, 2,111 of the women had developed breast cancer.

Women who were not obese or overweight at age 18 but were overweight at ages 35 and 50 had 1.4 times the risk of developing breast cancer compared with women who maintained a normal weight.

Women who lost weight had the same breast cancer risk as those whose weight remained stable.

The researchers speculate that the weight gained in later life is mostly the depositing of fat mass not lean body mass, and therefore represents the age-related metabolic change that seems to be important in breast cancer development.

"These findings may reinforce public health recommendations for the maintenance of a healthy weight throughout adulthood as a means of breast cancer prevention," the authors write.