Eating more than one serving of red meat every day may double a woman’s risk of developing some forms of breast cancer.

This is the latest finding from an ongoing study of female nurses, conducted by researchers from Boston’s Brigham and Women’s Hospital and Harvard Medical School.

Compared with women who ate three servings or fewer of red meat a week, those who ate more than one-and-a-half servings a day were found to have a 97% increased risk for developing hormone-receptor-positive tumors. The growth of these breast cancers is fueled by hormones; it’s the most common type of the disease.

The same association was not seen for hormone-receptor-negative tumors.

Study participants only included premenopausal women. But researcher Eunyoung Cho, ScD, tells WebMD that the risks may be the same for older women. The researchers will continue to follow the nurses as they age in an attempt to answer this question.

"It may be that diet early in adulthood has a stronger impact on [breast cancercancer] risk than diet later in life," she says. "But we can’t say that from this study."

Although diet has long been suspected of playing a role in breast cancer, most studies have failed to show a strong association between the foods women eat and breast cancer risk.

High-fat diets have been linked to breast cancer in some studies, while others have found no such relationship. Likewise, most studies have failed to show a strong protective effect for fruit and vegetable consumption.

"There are plenty of good reasons for eating a healthy diet, and lowering breast cancer risk might be one of them," American Cancer Society nutritional epidemiologist Marji McCullough, ScD, RD, tells WebMD. "But this has not been proven."

The newly reported study included nearly 91,000 nurses who were in their early 40s or younger in 1989. The women were followed for more than a decade, during which time they completed detailed food-frequency questionnaires designed to determine how often they ate 130 different foods.

Just over a thousand study participants developed breast cancer between 1991 and 2003. The more red meat the women ate, the greater their risk of developing hormone-receptor-positive breast cancer.

Cho and colleagues suggest several possible explanations for the hormone-specific association that they saw.

"Known cancer-causing compounds in cooked or processed red meat increase mammary tumors in animals and have been suspected of causing breast cancer in humans," they write. "In addition, cattle in the United States are treated with hormones to promote growth, which could also influence breast cancer risk."

Cho acknowledges that more studies are needed to confirm the link between eating red meat and hormone-receptor-positive breast cancers.

But she adds that it is not too soon to recommend that women who are concerned about breast cancer limit their red meat consumption.

"There aren’t many modifiable risk factors for breast cancer," she says. "Eating red meat has been strongly linked to other conditions like colon cancer. I think our findings provide another reason for women to reduce their consumption of red meat."

The American Cancer Society already recommends limiting processed and red meats to reduce colorectal cancercolorectal cancer risk. Maintaining a healthy body weight, getting plenty of exercise, and limiting alcohol consumption have all been identified as ways women can reduce their breast cancer risk.

"Being overweight is strongly associated with breast cancer risk," McCullough says. "This has been shown in study after study."