TUESDAY, Nov. 1 (HealthDay News) -- Women who have as few as three alcoholic drinks a week may have a moderately increased risk of developing breast cancer, a new study finds.

Researchers analyzed data from nearly 106,000 women taking part in the U.S. Nurses' Health Study to examine any links between alcohol consumption and breast cancer. The women were followed from 1980 through 2008 and asked about their alcohol consumption about every four years.

"We did see a modest risk [of breast cancer] associated with lower levels of alcohol consumption," said lead study author Dr. Wendy Chen, an assistant professor of medicine at Brigham and Women's Hospital and Harvard Medical School in Boston.

But Chen stressed that women who occasionally over-imbibe on vacation or at a holiday party shouldn't be alarmed; the research measured cumulative alcohol consumption over many years.

During the study period, about 7,700 women were diagnosed with breast cancer. Women who reported drinking 5 to 9.9 grams of alcohol daily (less than half an ounce a day or the equivalent of three to six glasses of wine weekly) were 15 percent more likely to develop breast cancer than women who never or rarely drank alcohol.

Women who drank more -- about two glasses of wine, or 30 grams of alcohol, daily -- had a 51 percent increased risk of breast cancer. (Although the researchers converted grams of alcohol into glasses of wine, the risk was similar whether women drank wine, liquor or beer.)

The study is in the Nov. 2 issue of the Journal of the American Medical Association.

Prior research has also found an association between alcohol consumption and breast cancer. One reason for the connection may be that alcohol raises levels of circulating estrogen, and high levels of estrogen are linked to breast cancer, Chen said.

What made this study unusual is that information was provided about women's alcohol consumption over several decades. Many other studies have asked about alcohol consumption at a single point in time, but drinking patterns may change over a lifetime, Chen said.

Researchers also looked at whether breast cancer risk varied depending on when a woman drank -- either earlier in life (ages 18 to 40) or later in life -- but found it was the cumulative exposure that made the most difference.

Binge drinking per se -- consuming at least six drinks in a single day -- didn't seem to significantly add to breast cancer risk. However, binge drinkers did tend to consume more alcohol overall than other women, which upped the odds of breast cancer.

"It really is a cumulative average over a long period of time that gave the most consistent association with breast cancer risk," Chen said.

Researchers also analyzed average daily alcohol consumption alongside other factors that could impact breast cancer risk, such as family history and age, to make sure they were really getting at the effect of alcohol. They found that women who drank a lot were also more likely to smoke, although most studies have not found a strong link between tobacco and breast cancer, Chen said.

Dr. Steven Narod, research chair in breast cancer at Women's College Research Institute in Toronto, said the study was "well conducted."

"For breast cancer, it does seem the risks [of alcohol] start up at a lower level than we previously thought," Narod said.

But he urged women who drink regularly not to worry too much. "I don't think I would worry about drinking one or two drinks a week. If your average is five or six a week, I'm not sure that I would be particularly worried about that, either. But 10 or more a week, maybe," Narod said.

Previous studies have suggested a glass of red wine daily has cardiovascular benefits, and those findings should not be discounted, said Narod, who wrote an accompanying editorial in the same journal issue.

"Women who abstain from all alcohol may find that a potential benefit of lower breast cancer risk is more than offset by the relinquished benefit of reduced cardiovascular mortality associated with an occasional glass of red wine," he wrote.

Moreover, the study authors said no evidence exists to show that giving up drinking will lower a woman's risk of breast cancer.

More information

The U.S. National Institute on Alcohol Abuse and Alcoholism has more on women and alcohol.