Though consumed in small amounts, alcohol seems to increase the risk of breast cancer, particularly estrogen-receptor and progesterone-receptor positive breast cancer in postmenopausal women, a large U.S. study revealed.

Previous studies have suggested that consuming alcohol boosts the risk of breast cancer, although the precise mechanisms have not been clarified.

“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. The logical step was to begin analyzing the alcohol metabolizing genes," 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.

A U.S. National Cancer Institute team followed more than 184,000 postmenopausal women for an average of seven years.

The study showed that 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 increased risk. On the other hand, those having less than one drink a day had a 7 percent increased risk of breast cancer, the team reported.

However, the risk was mostly seen in women whose tumors were positive for both the estrogen and progesterone receptors. About 70 percent of women who are diagnosed with breast cancer have this kind of tumors.

"Our study shows that not only does a small amount of alcohol significantly increase the risk of breast cancer, it increases the risk of the most common type of breast cancer, responsible for around 70 percent of cases," said Jasmine Lew, from the National Cancer Institute (NCI) in America, who also led the study.

She further explained how alcohol use leads to breast cancer by explaining: “alcohol affects estrogen metabolism, which increases risk of hormone sensitive breast cancer. Still, more study is needed to clarify the effect of alcohol on other tumor types.”

According to the American Cancer Society, about 182,000 women in the U.S. will be diagnosed with breast cancer this year.

The findings were presented Sunday at the annual meeting of the American Association for Cancer Research in San Diego.