Alcohol significantly increases a woman's risk of breast cancer; this is particularly true of estrogen-receptor and progesterone-receptor positive breast cancer, a new study shows.

These findings were presented at the annual meeting of the American Association for Cancer Research in San Diego, along with a second study that found an association between breast cancer risk and two genes involved in alcohol metabolism.

Previous data have suggested that consuming alcohol — even in small amounts — raises the risk of breast cancer, although the precise mechanisms involved had not been clarified.

In some forms of breast cancer, malignant cells have receptors that make them sensitive to hormones such as estrogen. The first study aimed to see if the hormone receptor characteristics of the tumor influenced the relationship between alcohol consumption and breast cancer risk.

A team led by Dr. Jasmine Lew of the U.S. National Cancer Institute followed more than 184,000 postmenopausal women for an average of seven years.

Those who averaged less than one drink a day had a 7 percent increased risk of breast cancer compared to teetotalers, the team reported. Women who drank one to two drinks a day had a 32 percent increased risk, and those who had three or more glasses of alcohol a day had up to a 51 percent higher risk.

Higher risk was seen mostly in the 70 percent of tumors that are classified as estrogen receptor- and progesterone receptor-positive. This suggests that alcohol's effect on breast cancer is related to estrogen.

The second study dug deeper into other possible mechanisms by which alcohol consumption increases breast cancer risk.

"For years, we've known that there's an association between alcohol drinking and breast cancer risk, but nobody knows yet what the underlying biological mechanisms are," said Dr. Catalin Marian, lead author of the study and a research instructor in oncology at the Lombardi Comprehensive Cancer Center at Georgetown University in Washington, D.C. "The logical step was to begin analyzing the alcohol metabolizing genes."

And indeed, two of these genes were associated with a two-fold increase in breast cancer risk.

"This is an association," Marian said. "This type of study is good for generating hypotheses. It's not a definite conclusion. It needs to be replicated by other studies to say for sure that what we found is there."

In other words, the scientists are seeing a relationship — positive correlation — between alcohol consumption and the risk of breast cancer, but it is too soon to tell what it is in alcohol that causes the cancer. For this reason, another researcher urged caution in interpreting the results of both studies.

"These studies are too early for use in a clinical setting or to advance a public health message," said Dr. Peter Shields, co-author of the genetics study and deputy director of the Lombardi Comprehensive Cancer Center. Future studies are needed to determine exactly what may be behind the relationship.