The team at Loyola University Chicago Stritch School of Medicine analysed data from more than 365,000 people who took part in 143 studies that were conducted since 1977.
Moderate drinkers were 23% less likely to develop dementia, Alzheimer's disease and other forms of "cognitive impairment", a phrase used to describe a decline in thinking skills. Moderate drinking is generally defined as a maximum of two drinks per day for men and one drink per day for women.
On the other hand, heavy drinking (more than three to five drinks per day) was associated with a higher risk of dementia and cognitive impairment, but the researchers said this finding was not statistically significant.
"We don't recommend that nondrinkers start drinking. But moderate drinking - if it is truly moderate - can be beneficial," study co-author Edward J. Neafsey, a professor in the department of molecular pharmacology and therapeutics, said in a Loyola University Medical Center news release.
Wine better for you
Wine appeared more beneficial than beer or spirits, but that finding was based on a relatively small number of studies, the study authors noted.
The researchers said the protective effect of moderate drinking remained after they factored in age, education, sex and smoking, and that the effects of alcohol were the same in men and women.
They also noted that the association between moderate drinking and reduced risk of dementia and cognitive impairment was statistically significant in 14 of 19 countries, including the United States.
Still, experts in neurological health weren't entirely sold on the findings.
"This study was well-designed and well-executed but falls in the category of epidemiology [population wide, observational studies]," said Dr Sam Gandy, chair in Alzheimer's Disease Research and professor of neurology at the Mount Sinai Alzheimer's Disease Research Center, Mount Sinai School of Medicine in New York City. "There are at least a dozen reports such as this, touting the beneficial neurological effects of alcohol. Each report brings calls and visits from patients, interested in what advice they can take away and apply to their own lives."
Alcohol and dementia
Gandy said that proof that moderate drinking helps curb dementia rates could only come from a prospective, randomised clinical trial - something that hasn't happened yet. "Until there are some randomised clinical trial data, no patient guidance is warranted," he said.
Another expert agreed. Dr James Galvin, director of the Pearl Barlow Center for Memory Evaluation and Treatment at NYU Langone Medical Center in New York City, said that there is growing evidence of a beneficial effect of moderate drinking on dementia, but "we should not rush out to buy bottles of Merlot, Cabernet or Pinot just yet."
And he added that the brain-healthy effects of moderate drinking noted in studies might just be a marker for an overall healthy lifestyle. "The Mediterranean diet with whole grains, fresh fruit and vegetables, olive oil and moderate red wine also reduces the risk of dementia, as does exercise, social engagement, mental activities and an optimistic outlook on life," Galvin said. "It is clear that heart healthy behaviours are also brain healthy behaviours."
For their part, the researchers said that it's not clear why moderate drinking may reduce the risk of dementia and cognitive impairment, but one premise suggests that alcohol might improve blood flow in the brain and thus brain metabolism, the researchers said. And they offered up another theory, that small amounts of alcohol may make brain cells more fit by slightly stressing them and increasing their ability to cope with major levels of stress that can eventually cause dementia.
The review appears in the August issue of the journal Neuropsychiatric Disease and Treatment.
The U.S. National Institute of Neurological Disorders and Stroke has more about dementia.
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