Just a Few Drinks a Week Boosts Breast Cancer Risk

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A new study should give women pause before filling their wine glass. Researchers found that women who regularly drink a small amount of alcohol — less than a drink a day — may increase their lifetime risk of breast cancer.

Dr. Wendy Chen, assistant professor of medicine at Harvard Medical School and Brigham and Women’s Hospital, and her colleagues report this week in the Journal of the American Medical Association (JAMA) that women who consumed as little as three to six drinks a week over several decades were 15% more likely to develop breast cancer than women who didn’t drink at all. Heavier drinking, defined as at least two drinks per day, was associated with a 51% increase in risk.

The effect was cumulative, with the risk of breast cancer rising 10% with each 10 g (about a third of an ounce) increase in alcohol consumption per day. While that may sound worrisome, it’s worth remembering that on average the overall risk of breast cancer remains modest. As Dr. Steven A. Narod of the Women’s College Research Institute in Toronto noted in an accompanying editorial in JAMA, based on the study’s data women who routinely have a drink a day could expect their 10-year risk of breast risk to increase from 2.8% to 3.5%. For women who have two drinks a day, that risk would rise from 2.8% to 4.1%.

The new findings add to an increasing body of evidence that has associated drinking with breast cancer. But while previous studies have shown that heavy drinking can boost the risk, Chen’s study also linked cancer to small amounts of alcohol — the same level of consumption that is commonly suggested to reduce the risk of heart disease. Having a few drinks a week could lower the risk of heart disease by 25% to 40%; for the average 50-year-old woman, the risk of heart disease is 39%. Whether the risks of breast cancer outweigh the benefits for the heart, says Chen, is a decision that only individual women, based on their individual risk factors, should make with help from their doctor.

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The current research involved more than 105,000 women enrolled in the long-running Nurses’ Health Study. The participants were asked about their alcohol consumption every four years between 1980 and 2008. While other analyses have asked women only about their recent drinking habits — in the past month or past year, for example — Chen’s decades-long investigation allowed her team to track drinking patterns over time, offering stronger data on the cumulative effect of alcohol on breast cancer.

“We were really looking at average intake over a long period of time,” says Chen. “That gave us the most consistent measure of alcohol exposure, rather than just focusing on what the women did in their 40s or 50s or more recently.”

Even after adjusting for other factors that contribute to breast cancer, such as age, weight, smoking and family history of the disease, the relationship between alcohol consumption and breast cancer remained. The relationship was also strongest in women who reported drinking more either early or late in life, suggesting that women may be more vulnerable to the effects of alcohol on breast tissue during those periods. Also, type of alcohol — red wine, beer or hard liquor — did not change women’s risk.

Exactly how alcohol may trigger tumors isn’t clear, but experts theorize that it activates estrogen production in the body; higher circulating levels of the hormone can promote breast cell growth. That theory seems to be supported by other data from Chen’s study: most of the breast cancers diagnosed in the study’s participants were positive for the estrogen receptor, meaning that the tumors were driven by and dependent on the hormone.

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So women face a difficult decision. Do the heart benefits of drinking outweigh the breast cancer risks? Younger women, who are at lower risk of heart disease than older women, may want to focus on their risk of developing cancer, Chen says, while older women or those with a family history of heart disease may choose to weight the cardiovascular benefits more heavily. But these risks will be different for every woman. “For each individual woman, whether or not she decides to change her drinking habits depends on her own personal risk factor profile to decide what is best for her,” says Chen. If only every decision were as simple as choosing between red or white.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.