Researcher Naomi Allen and her colleagues at the University of Oxford analyzed data from the Million Woman Study, which has been collecting health information from 1.28 million women between ages 50 to 64 since 1996. The researchers wanted to see whether the women's self-reported drinking habits were linked to the 68,775 cases of cancer that developed during the study.
They found that drinking just one alcoholic drink per day increased the risk of breast, liver and rectal cancer. For women who also smoked, drinking increased the risk of mouth and throat cancer as well. The type of alcohol didn't matter -- women who drank only wine increased their risk as much as women who drank other kinds of alcohol.
Although it's not clear how alcohol increases cancer risk, Allen says, "there is evidence that moderate alcohol intake-- at the levels studied here-- increase circulating levels of sex hormones, which are known to be associated with an increased risk of breast cancer."
"That's the take-home message," Allen told the Washington Post. "If you are regularly drinking even one drink per day, that's increasing your risk for cancer."
Allen and her colleagues concluded that among women who drink an average of one drink per day, the alcohol is responsible for an extra 11 cases of breast cancer per 1000 women, as well as one case each of mouth, throat and rectal cancer and .7 cases of esophageal, larynx and liver cancer.
The research supports previous smaller studies that have suggested that alcohol consumption increases women's risk of breast cancer, and that heavy use could be linked to other cancers in men and women. However, it's the first such large-scale study to link moderate alcohol use to cancer risk.
Scientists acknowledge that the new finding may confuse people who have been told that drinking one glass of red wine per day is good for heart health.
Right now, the U.S. government's dietary guidelines say that alcohol can have beneficial effects, and allow women to drink up to one drink per day.
But officials say that the guidelines were never intended to recommend drinking that amount of alcohol -- instead, they set an upper limit on what's advisable.
"It's a level of consumption that generally has been found in scientific studies to be associated with a relatively low risk of harms," Robert Brewer, of the Centers for Disease Control and Prevention, told the Washington Post. "But low risk does not mean no risk."
Some researchers say people must balance with their doctors the information about the benefits and risks of alcohol use.
"You have to balance all those things out," Phillip J. Brooks, who studies alcohol and cancer at the National Institutes of Health, told the Associated Press. "This kind of information is important for people to know and to consult with their physician about the various risk factors they have."