Some studies have shown that alcohol consumption may increase the risk of breast cancer, but researchers are unclear as to what exactly causes the increased risk. Researchers led by Joanne Dorgan, MPH, PhD, and her colleagues at the Fox Chase Cancer Center in Philadelphia, investigated whether small amounts of alcohol could boost levels of hormones thought to increase breast cancer risk.

Dorgan’s team studied 51 healthy postmenopausal women who were not taking hormone replacement therapy (HRT), i.e. synthetic estrogens. Each participant went through three 8-week periods in which she consumed 15 or 30 grams of alcohol (the equivalent of one or two drinks) per day, or an alcohol-free drink. At the end of each 8-week period, the researchers measured levels of sex hormones — including estrogen, testosterone, and progesterone — in the women’s blood.

Women who consumed 15 grams of alcohol per day showed a 7.5% increase in a breakdown product of estrogen called estrone sulfate, compared to women not drinking alcohol. Women who consumed 30 grams of alcohol per day showed an even greater increase (10.7%), compared to women not drinking alcohol. Similar increases were seen in another hormone called dehydroepiandrosterone sulfate (DHEAS).

"Our results, suggest that even one drink a day could increase breast cancer risk," Dorgan says, pointing out that hormone levels appeared to increase as the number of drinks increased.

"Most people agree that at higher levels of intake — such as more than three drinks a day — alcohol increases breast cancer risk," says Dorgan. "At lower levels, there is some controversy, which is one of the reasons we did this study," she says.

The study authors note that this is the first study to evaluate under controlled conditions the effects of chronic, moderate alcohol ingestion on levels of serum estrogens and androgens in postmenopausal women.

Dorgan recommends that women might want to consider discussing with their physicians whether they should or should not drink at all. "A physician can advise a woman by taking into account her individual situation," she says.

Michael Thun, MD, vice president of epidemiology and surveillance for the American Cancer Society (ACS), advises "the most important thing to understand is that even at levels of moderate drinking — not exceeding one drink of alcohol daily — the current evidence suggests that there is some increase in breast cancer risk."

"One drink of alcohol a day, however, may reduce the risk of heart disease, so the net effect on a woman’s health depends to a large extent on her age and her risk of breast cancer, and of heart disease," Thun says, noting that alcohol is the one case in which cancer and heart guidelines are in conflict.

"The risk of breast cancer and other cancers increases as alcohol consumption increases," Thun notes. "But after menopause, the reduction in risk of heart attacks and strokes associated with one or two drinks a day is greater than the increase in breast cancer risk for nearly all women," he qualifies.

Thun warns, "for some people, beginning to drink causes them to lose inhibition against further drinking, and no health agency recommends that non-drinkers should start drinking." He adds that "if you drink, then you should do so in moderation — not exceeding one comparable amount of beer, wine, or spirits a day."