Women who drink just a few glasses of wine or beer a week may have a slightly increased breast cancer risk, researchers said Tuesday.
Their findings are based on more than 100,000 nurses followed over three decades and add weight to earlier studies linking alcohol to breast cancer and other tumors.
"Even at low levels of alcohol consumption, three to six drinks per week, we found a modest increase in risk," said Dr. Wendy Chen of Brigham and Women's Hospital in Boston, whose findings appear in the Journal of the American Medical Association.
And, she added, "There wasn't a particular period in which it was safe to drink alcohol."
But before you put away the Pinot, there are some important caveats to consider.
First, it wasn't a huge effect -- about 15 per cent higher risk among drinkers compared with teetotalers. For example, even among women who sipped three to six glasses of wine per week, only 3.3 per cent would develop breast cancer over 10 years. That compares to 2.8 per cent of abstainers and 3.5 per cent of women having up to 13 drinks a week.
Second, there is still no ironclad proof that alcohol itself is to blame, even though the researchers did their best to rule out competing explanations such as smoking or older age. They also adjusted for other influences on breast cancer risk, like whether or not a woman has had children and breastfed.
"This is an observational study, so we really can't say anything definite about cause and effect," Chen, also of Harvard Medical School, told Reuters Health.
Still, she believes the link between drinking and increased breast cancer risk is likely to be causal. Alcohol raises estrogen levels, which play a role in the development of many breast tumors.
Overall, the researchers estimate that if drinking really does promote breast cancer, it might be responsible for 10 per cent of all cases in the US
"The recommendation would not be to stop drinking altogether, but to keep it below the range of three to six glasses a week," Chen said, adding that going over that limit occasionally -- say, during vacation -- would be alright.
But that's not the end of the story because some research suggests a drink a day may be beneficial for the heart.
Just recently, one study of women showed that both light and heavy drinkers lived longer after a heart attack than abstainers. (See Reuters Health story of October 27, 2011.)
"One drink a day is a really good target, assuming that a person can be disciplined about that," Dr. James O'Keefe, a cardiologist at St. Luke's Health System in Kansas City, Missouri, told Reuters Health last week.
In an editorial published along with the latest findings, Dr. Steven Narod of the Women's College Research Institute in Toronto said the results probably aren't relevant for women with breast cancer. And even for women without the disease, the picture is murky.
"There are no data to provide assurance that giving up alcohol will reduce breast cancer risk," writes Narod. "Furthermore, 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."
Chen acknowledged those shortcomings and said her group is currently studying the link between drinking and death from any cause, as well as whether people who stop drinking have a lower risk than those who don't.