Toronto–Researchers from the Centre for Addiction and Mental Health say alcohol was linked to thousands of cancer cases in Canada last year, and that even mild to moderate drinking poses risk of developing the disease in the future.
Those findings are part of a modelling study from the World Health Organization's International Agency for Research on Cancer (IARC) that was published this week in the journal Lancet Oncology.
The global study estimates the effect of alcohol consumption on cancers worldwide, suggesting 4% of newly diagnosed cases in 2020 may have been associated with drinking alcohol.
In Canada, researchers said alcohol use was linked to 7,000 new cancer cases in 2020, including 24% of breast cancer cases, 20% of colon cancers, 15% of rectal cancers, and 13% of oral and liver cancers.
The study found that most of those alcohol-related cancer cases worldwide were associated with heavier drinking patterns, but researchers estimated that light to moderate drinking _ around one or two drinks per day _ contributed to more than 100,000 cases in 2020, or one in seven.
"People say, 'Well, everything causes cancer,' but I would stress that alcohol is a Class 1 carcinogen defined by IARC,'' study co-author Kevin Shield, a scientist at CAMH's Institute for Mental Health Policy Research, said Wednesday.
"This study establishes that it's a leading cause of cancer globally, and with increases in alcohol consumption (during the COVID-19 pandemic), we are going to see it go up even further.''
IARC includes 121 cancer-causing agents on its list of Class 1 carcinogens on its website, including alcohol, tobacco, ultraviolet radiation and outdoor air pollution. Alcohol can worsen the cancer-causing effects of other substances such as tobacco, the study said.
Shield said there are many ways in which alcohol consumption can lead to cancer, but the main mechanism is by damaging DNA.
"It takes a key number of mutations in your DNA to cause cancer. But over time, what happens is those damages accumulate,'' he said. "So unfortunately, (if) you drink today, you drink tomorrow, you drink the next day, every drinking occasion is increasing your risk.''
The study estimates that globally, men accounted for 77% (568,700 cases) of alcohol-associated cancer cases, compared with 23% for women (172,600). Cancers of the esophagus, liver, and breast accounted for the largest number of cases.
Modelling for the study was based on data from alcohol exposure, including surveys and sales figures, from several countries. The data was combined with relative risk estimates for cancer based on level of consumption.
Shield said researchers looked at epidemiology studies from large international cohorts—including Canada, Europe, China and Australia—and controlled for other possible cancer-causing factors like smoking, obesity and sexually transmitted diseases.
"We're making sure statistically we're taking out those factors so it's only alcohol,'' he said.
The findings led the study's authors to call for greater public awareness of the association between cancer and alcohol, urging for increased government interventions to reduce consumption in worst-affected regions.
Study co-author Jurgen Rehm, a senior scientist at CAMH's Institute for Mental Health Policy Research, said in a press release that the link between light to moderate drinking and cancer is relatively new.
Rehm added that he doesn't think public policy reflects the degree of cancer risk and he recommended interventions including higher taxes on alcohol sales, limiting availability and marketing, and adding cancer-risk warnings to beverage labels.
"With alcohol-related cancers, all levels of consumption are associated with some risk,'' he said.
CAMH said their clinic has treated approximately 3,600 patients in the last year for alcohol use disorder, with Shield adding that Canada saw an increase in consumption, especially during lockdown periods.
While some increases could be temporary due to pandemic-related stress, researchers said that could lead to potentially harmful habits.
"There are people that are very, very at-risk for developing cancer now due to the COVID 19 pandemic,'' Shield said. "We're likely to see an increase in our cancer rates due to our increase in alcohol consumption.''