"Alcohol is a real risk factor, and the more you drink the higher your… (Ed Jones / AFP/Getty Images )
Drinking as few as three to six glasses of wine per week may increase a woman's lifetime risk of breast cancer by 15%, according to an analysis by Harvard researchers.
The study, published Tuesday in the Journal of the American Medical Assn., reaffirms that heavy alcohol use raises breast cancer risk, and it adds that light drinking matters too.
"Alcohol is a real risk factor, and the more you drink the higher your risk," said Dr. Steven A. Narod, the Canada research chair in breast cancer at the University of Toronto, who wrote a commentary accompanying the study.
Whether women should consider abstaining from even light alcohol consumption, however, is not easily answered, preventive health experts said.
On average, a U.S. woman's baseline risk of breast cancer is 1 in 8 over her lifetime, according to the American Cancer Society. The 15% increased risk that was linked to consumption of 5 to 9.9 grams of alcohol per day is modest, similar to the heightened risk associated with using estrogen-progesterone hormone therapy to treat symptoms of menopause. But it's far smaller than the fivefold increased risk that comes from inheriting certain mutations in the BRCA1 and BRCA2 genes.
Moreover, having a few drinks a week has been found to lower the incidence of heart disease by 25% to 40%. The average woman's lifetime risk for heart disease is 1 in 2.
The breast cancer findings were based on data from 105,986 women in the long-running Nurses' Health Study. Participants were assessed at regular intervals from 1980 until 2008 to record alcohol intake and cases of invasive breast cancer, among other things.
The study does not prove that alcohol consumption makes women more susceptible to breast cancer. But the researchers were able to isolate the link between drinking and breast cancer by controlling for a host of other risk factors, such as family history of the disease and age at first pregnancy.
As in previous studies, the analysis found that women who consumed at least 30 grams of alcohol per day — about two drinks — were 51% more likely to develop breast cancer than women who didn't drink at all. But for the first time, researchers were able to see that the risk of breast cancer begins to rise with even modest alcohol intake.
"When you look at this, you see a dose-response effect," said Dr. Wendy Y. Chen, the lead author of the paper and an assistant professor of medicine at Brigham and Women's Hospital and Harvard Medical School in Boston. "That strengthens the fact that what you're finding is real."
Chen and her colleagues also demonstrated that binge drinking, defined as six or more drinks in one sitting, was associated with a 33% increase in cancer risk independent of total alcohol intake over time and that alcohol consumption between the ages of 18 and 40 was linked with higher breast cancer risk later in life no matter what a woman's drinking habits were after age 40.
The study focused only on alcohol's effect on cancer-free women. It's unclear what advice about drinking might be given to women who have been diagnosed with cancer.
Hormones probably play a key role in the alcohol-breast cancer relationship, Chen said, because alcohol intake increases levels of the hormone estrogen, which is known to fuel breast cancer growth.
But modest alcohol consumption also raises HDL, the good cholesterol, and other substances that promote heart health. The cardiovascular benefits probably negate a small increased risk of breast cancer, said Dr. Tim Byers, associate director for cancer prevention and control at the University of Colorado Cancer Center.
"The trade-off makes it very difficult for women to know what to do," he said. "There are not many examples of where there is trade-off for heart disease and breast cancer. Most of the risks for heart disease and breast cancer are the same."
Considering that more women develop heart disease than breast cancer, Byers said, "it's reasonable" for women at average risk for both conditions to drink at low to moderate levels. "But for women at higher risk of breast cancer it becomes a more difficult decision," he said. "The prudent thing for them may be to avoid alcohol."
Chen said she doesn't tell her patients to stop drinking but to limit intake to a few drinks a week.