Just one alcoholic drink a day can boost a woman's risk of breast cancer by about 5%, according to a new review of existing research.
Heavier drinking - three or more drinks a day - can increase risk up to 50%, according to researchers from Germany, France and Italy.
"Alcohol consumption is causally related with breast cancer," the study authors concluded after reviewing 113 prior studies. They attributed 2% of breast cancer cases in Europe and North America to light drinking alone, and about 50,000 cases worldwide to heavy drinking.
The research seems to confirm the expert advice for women to minimise drinking, said study leader Dr. Helmut Seitz, professor of medicine, gastroenterology and alcohol research at the University of Heidelberg in Germany.
What the study found
The findings suggest that healthy women at average risk of breast cancer should not consume more than one alcoholic drink a day, the authors said.
"Women at an elevated risk for breast cancer should avoid alcohol or consume alcohol only occasionally," the researchers wrote. Those at increased risk include those with a family history of breast cancer.
The link between alcohol and breast cancer was first suggested in the early 1980s, the authors said. To update the research, they searched for studies published before November 2011. They found more than 3,400 studies in all and narrowed their focus to 113 that examined the effects of light drinking on breast cancer risk.
The review will be published in the journal Alcohol and Alcoholism.
In the US, one in eight women will develop breast cancer in her lifetime, experts estimate. The increased risk associated with drinking is added to that starting risk.
Alcohol is thought to increase oestrogen levels, in turn, perhaps, increasing the risk of breast cancer. Several studies have found alcohol more strongly linked to cancers known as oestrogen receptor positive, which require oestrogen to grow.
Seitz said the team's research controlled for various other factors that might affect risk, such as obesity.
Two American experts put the new report into perspective.
Alcohol linked to breast cancer
The association between moderate alcohol use and a slightly increased risk of developing breast cancer has been reported before, said Dr. Joanne Mortimer, director of Women's Cancer Programs at the City of Hope Comprehensive Cancer Center in Duarte, Calif.
"This is an update of the evidence linking alcohol to breast cancer," said Susan Gapstur, vice president of the epidemiology research program at the American Cancer Society.
By including both newer studies and older ones, Gapstur said, "we are basically getting to the point where we can more precisely estimate the risk of light alcohol consumption."
As the link between alcohol and breast cancer strengthens, women may wonder how to strike a balance between breast health and heart health, since moderate alcohol has been found to be heart-healthy.
Follow the American Cancer Society guidelines, Gapstur said.
"Our guidelines say, for overall health, if you don't drink, don't start," she said. "If you do, it's best to limit your consumption to one drink a day if you are a woman."
If you are at high risk of breast cancer, limiting consumption to even less may be wise, she said.
Mortimer, however, said many women are at increased risk of breast cancer because of genetic factors.
"Lifestyle changes won't impact much," she said.
Seitz has another opinion. "The heart benefits hold true only for a subgroup of individuals," he said. "Those who have more than one risk factor for coronary heart disease and especially the elderly may benefit from small amounts of alcohol. Younger people do not."
Warning signs of breast cancer
To learn more about alcohol and cancers, visit American Cancer Society.
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