Last Updated: 2003-08-19 16:26:31 -0400 (Reuters Health)

WASHINGTON (Reuters) - Older women who are obese have a much higher risk of breast cancer because their fat cells release too much estrogen, researchers said on Tuesday.

The international study comparing obese women to women of normal weight confirms what doctors have long suspected - that fat cells release the hormone into the blood, allowing it to help turn normal cells cancerous.

"There was clear hypothesis that the mechanism for the effect of obesity might be high blood estrogen levels, but no one has been able to test that directly," said Dr. Tim Key of the Cancer Research U.K. Epidemiology Unit at Britain's Oxford University.

The researchers, who report their findings in this week's issue of the Journal of the National Cancer Institute, said they are good news - giving women a way to reduce their risk of breast cancer.

"Women's risk is affected by many fixed factors - a family history of the disease, the number of children they have, the age they have their children, when they start their periods and when they stop," Key said.

"But obesity is something that women have a level of control over. Put simply, maintaining a healthy weight avoids extra breast cancer risk for these women."

Key and colleagues in Britain, Italy, Japan and the United States studied eight different groups of women who were past menopause - when the risk of breast cancer rises dramatically.

None of the women had cancer and none were taking hormone replacement therapy when their blood samples were first taken. The researchers then watched the women for between two and 12 years to see which ones developed breast cancer.

MORE WEIGHT EQUALS MORE CANCER

During the study, 624 women developed breast cancer. Hormones in their blood were compared with the hormones from 1,640 cancer-free women of the same age.

The more the women weighed, the higher their risk of cancer. And the more the women weighed, the higher their levels of a form of estrogen called estradiol.

A woman who was obese, with a body mass index of 30 or more, had an 18 percent higher chance of developing breast cancer than a woman with a BMI of 25 - just on the border of being overweight.

"If we had made a comparison at a BMI of 22, which is relatively slim, you'd have a bigger effect," Key said in a telephone interview.

Body mass index compares weight to height, giving a broad range of healthy weights. A BMI of 22 is considered optimal, while over 25 is overweight, 30 is obese, meaning the risk of diseases is greatly increased, and 40 is morbidly obese.

With 40 percent of U.S. women now obese, breast cancer, already the No. 2 cancer killer of women after lung cancer, could become an even worse problem.

"Obesity is a risk factor for other diseases such as heart disease and diabetes," Joanne Dorgan, an epidemiologist at Fox Chase Cancer Center in Philadelphia who worked on the study, said in a statement.

Researchers have found in other studies that a low-fat diet - especially a diet low in animal and other saturated fats - can reduce the risk of breast cancer.

A woman's lifetime risk of breast cancer is one in nine. Heart disease is an even bigger killer, killing more than 500,000 U.S. women a year.

According to the World Health Organization, 1.2 million women globally will develop breast cancer. It will kill 40,000 people in the United States this year.