In 2003, a study published in the prestigious Journal of the American Medical Association reported that having two or more alcoholic beverages per day was associated with a 22 per cent increased risk of dementia. But people who drank less than that had up to a 54 per cent lower risk, compared with people who didn’t drink at all.

“I remember reading that JAMA paper 20-some odd years ago and thinking, I really ought to try and have a glass of wine every day,” said Dr Joel Gelernter, a professor of psychiatry, genetics and neuroscience at the Yale University School of Medicine.

That study wasn’t the only one to suggest that moderate alcohol consumption was beneficial for the brain: Observational research has consistently shown that light-to-moderate drinkers have a lower risk of dementia compared with nondrinkers.

“This idea of the J-shaped curve was really pretty darn reproducible in many studies,” said Dr Kristine Yaffe, a professor of psychiatry, neurology and epidemiology at the University of California, San Francisco. But, she added, “I think more recently, people have debunked that J.” Indeed, researchers have come to understand that the health benefits attributed to a daily glass of wine are largely a mirage. People who drink in moderation tend to have more advantages in life than people who abstain (such as having higher socioeconomic status, more years of education and generally better health), and experts think that explains their lower risk for dementia, not the alcohol itself.

The evidence that heavy drinking (often defined as more than two drinks a day) can damage the brain and increase the risk of dementia has also continued to accumulate. But experts are still learning exactly how it happens and at what amount harms start to occur.

HEAVY DRINKING AND THE BRAIN Experts have a few theories about how chronic excessive alcohol use hurts the brain. Research, much of it conducted on animals and human cells in a lab, indicates that alcohol can induce oxidative stress in the brain and disrupt the brain’s immune system, triggering harmful inflammation. Both oxidative stress and inflammation are associated with dementia.

Alcohol affects the brain indirectly, too. Heavy drinkers have an increased risk of high blood pressure and stroke, and blood vessel damage is a key contributor to vascular dementia. People who drink a lot also tend to have other unhealthy habits that can harm the brain, like smoking or a poor diet.

At the most extreme, some people with alcohol use disorder develop Wernicke-Korsakoff syndrome, a form of dementia caused by a severe thiamine (Vitamin B1) deficiency stemming from chronic heavy alcohol use.

“There’s multiple different pathways” through which alcohol can damage the brain, said Louise Mewton, an associate professor studying alcohol use at the University of Sydney. And, she added, “it may be all of these acting at once. The literature certainly is not settled.”

MODERATE DRINKING AND THE BRAIN With lower levels of alcohol consumption, it’s less clear when and how the brain is affected. And because most studies on people’s drinking habits find that misleading J-curve, scientists have had to get creative with their research methods.

A few studies looking at brain volume have indicated that damage may happen even with light-to-moderate drinking. One large 2022 study from the United Kingdom found that having an average of eight drinks a week was associated with slightly smaller brain volume than having an average of four per week. And the greater the alcohol consumption, the smaller the brain.

Dr Charlies DeCarli, a professor of neurology at the University of California, Davis, said there’s no evidence that moderate amounts of alcohol directly harm neurons. “What these newer studies indicate, though, is that there is some relationship” between small doses of alcohol and the brain.

But, he added, “brain volume does not necessarily translate directly into cognitive performance.”Other research suggests that light-to-moderate drinking may be associated with some loss in cognitive functioning, even if it doesn’t qualify as dementia. Another UK study found that people who had as little as four drinks a week experienced a greater decline in language abilities over the course of 30 years compared with people who were abstinent. Again, the more people drank, the worse their cognition got.

Newer research looking at genetic predisposition to drinking alcohol has also produced sobering results. A study published in January found that when researchers used people’s self-reported drinking habits to assess dementia risk, the typical J-curve emerged. But when they repeated the analysis using genes that predict drinking behaviour (whether someone is genetically more likely to drink a lot, a little or not at all), dementia risk increased even at low levels of predicted alcohol consumption.

Dr Gelernter, who was involved in the genetics study, said he now thinks there is a linear relationship between alcohol consumption and risk for cognitive decline. But the research hasn’t prompted him to abstain entirely. “I certainly no longer try to have a glass of wine every day, but I haven’t stopped,” he said. Although “it was much nicer when we said, ‘I’m doing it for my health,’” he added.

By Dana G Smith © The New York Times Company The article originally appeared in The New York Times