Heavy drinking ‘risky’
Because few participants were deemed to be heavy drinkers, the power to identify an association between heavy drinking and cognitive function was limited. Dr. Zhang acknowledged, though he noted that heavy drinking is “risky.”
“We found that, after the drinking dosage passes the moderate level, the risk of low cognitive function increases very fast, which indicates that heavy drinking may harm cognitive function.” Limiting alcohol consumption “is still very important,” he said.
The associations of alcohol and cognitive functions differed by race/ethnicity. Low to moderate drinking was significantly associated with a lower odds of having a consistently low trajectory for all four cognitive function measures only among white participants.
A possible reason for this is that the study had so few African Americans (who made up only 14.8% of the sample), which limited the ability to identify relationships between alcohol intake and cognitive function, said Dr. Zhang. “Another reason is that the sensitivity to alcohol may be different between white and African American subjects.”
There was a significant U-shaped association between weekly amounts of alcohol and the odds of being in the consistently low–trajectory group for all cognitive functions. Depending on the function tested, the optimal number of weekly drinks ranged from 10-14.
Dr. Zhang noted that, when women were examined separately, alcohol consumption had a significant U-shaped relationship only with word recall, with the optimal dosage being around eight drinks.
U-shaped relationship an ‘important finding’
The U-shaped relationship is “an important finding,” said Dr. Zhang. “It shows that the human body may act differently to low and high doses of alcohol. Knowing why and how this happens is very important as it would help us understand how alcohol affects the function of the human body.”
Sensitivity analyses among participants with no chronic diseases showed the U-shaped association was still significant for scores of total word recall and vocabulary, but not for mental status or total cognition score.
The authors noted that 77.2% of participants had at least one chronic disease. They maintained that the association between alcohol consumption and cognitive function may be applicable both to healthy people and to those with a chronic disease.
The study also found that low to moderate drinkers had slower rates of cognitive decline over time for all cognition domains.
Although the mechanisms underlying the cognitive benefits of alcohol consumption are unclear, the authors believe it may be via cerebrovascular and cardiovascular pathways.
Alcohol may increase levels of brain-derived neurotrophic factor, a key regulator of neuronal plasticity and development in the dorsal striatum, they noted.
Balancing act
However, there’s also evidence that drinking, especially heavy drinking, increases the risk of hypertension, stroke, liver damage, and some cancers. “We think the role of alcohol drinking in cognitive function may be a balance of its beneficial and harmful effects on the cardiovascular system,” said Dr. Zhang.
“For the low to moderate drinker, the beneficial effects may outweigh the harmful effects on the small blood vessels in the brain. In this way, it could preserve cognition,” he added.
Dr. Zhang also noted that the study focused on middle-aged and older adults. “We can’t say whether or not moderate alcohol could benefit younger people” because they may have different characteristics, he said.
The findings of other studies examining the effects of alcohol on cognitive function are mixed. While studies have identified a beneficial effect, others have uncovered no, minimal, or adverse effects. This could be due to the use of different tests of cognitive function or different study populations, said Dr. Zhang.
A limitation of the current study was that assessment of alcohol consumption was based on self-report, which might have introduced recall bias. In addition, because individuals tend to underestimate their alcohol consumption, heavy drinkers could be misclassified as low to moderate drinkers, and low to moderate drinkers as former drinkers.
“This may make our study underestimate the association between low to moderate drinking and cognitive function,” said Dr. Zhang. In addition, alcohol consumption tended to change with time, and this change may be associated with other factors that led to changes in cognitive function, the authors noted.