An analysis of six long-term studies conducted among a total of more than 300,000 women has confirmed that drinking alcohol can raise a woman's risk of developing breast cancer. But the increase in risk is very small for those who consume no more than one drink a day.
The findings, being published Wednesday in The Journal of the American Medical Association, showed that a woman's risk of breast cancer rises with the amount of alcohol she regularly consumes, strongly suggesting that by drinking moderately or not at all a woman could reduce her chance of getting breast cancer.
But breast cancer is not the only disease influenced by alcohol intake. Excessive alcohol consumption increases the risk of several digestive-tract cancers. On the other hand, moderate drinking has been linked in numerous studies in several countries to a reduced risk of heart disease and increased longevity overall.
In the new study, the researchers found that women who were relatively heavy drinkers, consuming two to five alcoholic drinks each day, were 41 percent more likely to develop breast cancer than nondrinkers. But for moderate drinkers who consumed three-fourths to one drink, or 10 grams of alcohol, a day, the risk was only 9 percent higher than among nondrinkers.
Put another way, a woman who lives to age 85 in the United States has a 12.5 percent chance of having breast cancer at some time in her life. If she consumes one drink a day, the new analysis suggests, her chance of having breast cancer would increase to 13.6 percent.
By contrast, cigarette smoking can raise a woman's risk of developing lung cancer by as much as 1,000 percent. Lung cancer is the leading cancer killer of women in the United States, causing 67,000 deaths among women annually, 23,500 more deaths than are caused by breast cancer.
Among the women studied, the risk of developing breast cancer rose by an average of 9 percent for each additional 10 grams of alcohol consumed daily. No risk difference was found for various forms of alcoholic drinks; comparable amounts of beer, wine and hard liquor had similar effects. One shot of hard liquor contains 15 grams of alcohol, 12 ounces of beer contain 13 grams and 4 ounces of wine, 11 grams.
In speculating on reasons for the relationship between alcohol and breast cancer, the authors of the new report noted that alcohol raises estrogen levels, and estrogen is known to stimulate breast-cancer growth. They said that drinking two to five drinks a day results in a breast-cancer risk comparable to that associated with having a family history of breast cancer or starting to menstruate before age 12.
The authors also pointed out that alcohol consumption is one of the few factors linked to breast cancer that are within a woman's control. They suggested that in deciding whether and how much to drink, women should consider alcohol's benefits to the heart and their own risks of developing heart disease and breast cancer.
"Our study suggests that the link between alcohol and breast cancer risk applies to most women," said Dr. Stephanie Smith-Warner, lead author of the new study. She is a research fellow in nutrition at the Harvard School of Public Health.
In an interview Dr. Smith-Warner added, "Women should consult with their personal physicians to evaluate their cardiac and breast-cancer risk factors and determine if moderate alcohol consumption is advisable for them."
If, for example, a woman was at high risk for heart disease but low risk for breast cancer, consuming one drink a day might be better for her health than no drinks. But drinking alcohol may be more hazard than help to a woman who is free of the known risk factors for heart disease.
Deaths from heart disease greatly exceed those from breast cancer. Each year more than 500,000 American women die of heart disease, compared with 43,500 who die from breast cancer. According to a large long-term study published in December by the American Cancer Society, having one drink a day raises a woman's risk of dying of breast cancer by 11 percent but diminishes overall mortality by 20 percent because of alcohol's protective effects on the heart.
Dr. Smith-Warner and her colleagues noted, however, that there are ways other than drinking alcohol to reduce a woman's cardiac risk, including getting regular exercise, maintaining a normal body weight, controlling blood pressure and cholesterol, not smoking, and taking aspirin.
There are also some things a woman can do to reduce her breast-cancer risk. Women who smoke, for example, have higher breast-cancer rates, and those who get regular physical exercise before menopause face a breast-cancer risk that is 30 percent to 50 percent lower than that among sedentary women.
The new study is the largest ever to examine the relationship between diet and breast cancer. Dr. Smith-Warner and her colleagues pooled data from six "prospective" studies in four countries -- the United States, Canada, the Netherlands and Sweden.
In a prospective study, information is gathered at the outset about diet and other characteristics that might influence the risk of disease; the subjects are then followed for years to see what happens to their health. In the six studies, the women were followed for up to 11 years.
More than 50 percent of American women age 18 and older are current drinkers, according to data gathered by the National Center for Health Statistics. Of those who drink, during the two weeks before being surveyed, 29 percent consumed no alcohol and 46.4 percent drank "lightly," consuming no more than three drinks a week.
If the trend shown in the new analysis applies to this low consumption rate, these women would have about a 5 percent greater risk of developing breast cancer than women who never drink.
Another 21 percent of women who drink reported consuming four to 13 drinks a week, which would confer an added risk of about 7 percent to 18 percent. Only 3.4 percent reported drinking 14 or more drinks a week, which would be associated with a breast-cancer risk up to 40 percent higher than among nondrinkers.
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