Some research has suggested that drinking alcohol in moderation could have protective effects against stroke. However, a large cohort study in a Chinese population shows that this is not the case at all.

We know that alcohol consumption can impact our health in many ways, but some researchers have debated whether the amount and the frequency of consumption could have a bearing on whether drinking is better or worse for health.

Certain studies—such as one published in 2016 in the journal BMC Medicine—have even suggested that moderate alcohol consumption can have a protective effect against stroke. However, other researchers have called such findings into question and decided to conduct their own investigation into this matter.

A new collaborative study—led by teams from Oxford University, in the United Kingdom, and Peking University, the Chinese Academy of Medical Sciences, and the Peking Union Medical College, in Beijing, China—now shows that moderate drinking not only does not protect against cardiovascular events, it actually increases the risk of stroke. These findings, which appear in the journal The Lancet, are based on the analysis of data collected from over 500,000 people in China.

Why the focus on East Asian populations?

The researchers chose to focus on a Chinese population because numerous people of East Asian descent experience something called the "Asian flushing syndrome"—when they consume alcohol, their faces turn red (flushed) and assume a glow.

This, the authors explain in their paper, is because when people with this syndrome drink alcohol, their systems are unable to properly break down some of the components, due to certain genetic variants that are specific to these populations. "The major clearance pathway for blood alcohol is that an alcohol dehydrogenase [...] oxidizes it to acetaldehyde, which causes discomfort at sufficient concentrations," the researchers write.

"An aldehyde dehydrogenase [...] then detoxifies the acetaldehyde, oxidizing it to acetate, which does not cause discomfort," they continue, explaining that "[f]ast clearance of alcohol or, particularly, slow breakdown of acetaldehyde can cause individuals to limit alcohol intake."

While in people of European and African descent, the body breaks down acetaldehyde "quickly enough to maintain tolerably low concentrations in drinkers," the authors explain, in populations from East Asia, this does not happen because of the presence of a certain variant of the ALDH2 gene called rs671. A variant of the ADH1B gene, rs1229984, which is just as common among people of East Asian descent, actually increases blood alcohol clearance rates, thus increasing tolerance to alcohol.

Moreover, according to the study's researchers, both of these genetic variants are associated with less alcohol consumption. In the study, the researchers assessed information provided by 512,715 adults from China who enrolled in the China Kadoorie Biobank initiative, and their first step was to look at whether these participants had the rs671 or rs1229984 genetic variants.

As part of the China Kadoorie Biobank project, the participants also provided information regarding their drinking habits and agreed to provide health data over a follow-up period of 10 years. Using all these data, the investigators who led the current study sought to find out what the relationship actually was between moderate alcohol intake and the risk of stroke.

"Using genetics is a novel way to assess the health effects of alcohol and to sort out whether moderate drinking really is protective or whether it's slightly harmful," says senior epidemiologist and lecturer Iona Millwood, who co-led the study. "Our genetic analyses have helped us understand the cause and effect relationships," she observes.

Moderation does not protect against stroke

"In our population, men drink more than 20 times as much as women, so these two [genetic] variants have large absolute effects on alcohol intake only among men," the researchers write in their paper. Among women, fewer than 2% reported having any alcohol in a given week, and when they did drink, they reported a considerably lower intake than men. Thus, the scientists looked at women as a viable control group in this study.

When looking at the male population, they found that those with the two genetic variants—which were tied to lower alcohol intake—also had a lower risk of high blood pressure and stroke. After performing comparisons, the researchers concluded that alcohol intake—even in moderation—can increase the risk of ischemic stroke by as much as 35% for every four extra alcoholic drinks per day (or 280 grams of alcohol per week). "There are no protective effects of moderate alcohol intake against stroke," emphasizes co-senior author professor Zhengming Chen.

"Even moderate alcohol consumption increases the chances of having a stroke."

—Professor Zhengming Chen

At the same time, he notes, "The findings for heart attack were less clear-cut, so we plan to collect more evidence." Although the researchers acknowledge that they could not reproduce this study with a cohort of European descent, since these populations do not typically have the two genetic variants, they nevertheless argue that the current findings are relevant to all populations.

"Stroke is a major cause of death and disability," notes professor Liming Li, a co-senior author, who adds "This large, collaborative study has shown that stroke rates are increased by alcohol. This should help inform personal choices and public health strategies."