Women who have as little as two drinks a day are at an increased risk of breast cancer, a new University of Victoria study has found.
Those women — classified as low-level drinkers — are 8.5 per cent more likely to develop breast cancer than if they had abstained from alcohol, the study says. Hazardous drinkers, who have more than three drinks a day, face a 37 per cent risk increase.
The study confirms what previous studies and authorities around the world have been convinced of, said co-author Tim Stockwell, a researcher at UVic’s Centre for Addictions Research of B.C.
“Alcohol is a major risk factor for breast cancer,” Stockwell said.
“It’s hard to say in any one person that it was just alcohol, but it’s one thing that can tip the probability in the wrong way.”
Stockwell co-authored the study with former UVic associate researcher Cornelia Zeisser and researcher Tanya Chikritzhs of Australia’s Curtin University. The study was published in the journal Alcoholism: Clinical and Experimental Research.
For their study, Stockwell, Zeisser and Chikritzhs gathered existing research on the link between breast cancer and alcohol. They found 60 studies published before 2013 — but there were inconsistent findings of the cancer risk for low-level drinkers.
“Some have failed to find significance,”Stockwell said. “Others have. We believe we’ve found out what makes the difference between those studies.”
At the heart of their findings was a prevalence of bias, which affected all but six of the 60 studies.
Each study depends on comparing drinkers against abstainers, but the definition of abstinence was often faulty, Stockwell said.
For example, some studies asked subjects if they had a drink in the past week. If the answer was no, they might be classified as abstainers, even if they were formerly heavy drinkers or continued to have less than one drink a week. “The studies that didn’t take account of those things tend to have unhealthier ‘abstainers’ to compare the drinker against,” Stockwell said.
“When we controlled and adjusted for those biases, we found that — lo and behold — the lighter drinkers, moderate drinkers and low-risk drinkers still had elevated risk, compared with genuine abstainers.”
About 5,000 women die from breast cancer each year in Canada, according to the Canadian Breast Cancer Foundation. Between five and 10 per cent of breast-cancer deaths are attributable to alcohol, according to a study published in 2009.
That means 250 to 500 Canadian women die annually from breast cancer linked to alcohol consumption.
Alcohol is a risk factor in many types of cancer, including mouth, stomach and colon cancer, Stockwell said. The risk directly increases with the level of alcohol consumed. “The bigger the dose, the bigger the risk.”
Stockwell said it’s important to spread the word. “Seventy per cent of Canadians have been shown to be unaware of an alcohol-cancer connection,” he said.
The Centre for Addiction Research of B.C. is advocating for better labelling that would bring alcohol in line with other carcinogens, he said.
“The good news is this is something we know about and steps can be taken to reduce individual risk and population risk. If we look it straight in the eye, we can do something about it.”
