The results of a new study have shown that even moderate alcohol intake can have a negative impact on cognitive health.
The study - by the University of Oxford and University College London, both in the U.K. - is published in the BMJ.
Alcohol consumption is a recognized global public health issue. According to the World Health Organization (WHO), "5.1 percent of the global burden of disease and injury is attributable to alcohol."
In 2010, the World Health Assembly passed a resolution urging countries to "strengthen national responses to public health problems caused by the harmful use of alcohol."
They suggest that men and women "are safest not to drink regularly more than 14 units per week, to keep health risks from drinking alcohol to a low level." This is roughly the amount of alcohol contained in four pints of strong beer or five large glasses of 14 percent wine.
However, the researchers behind the new study note that the U.S. guidelines allow a higher limit for men of 24.5 units per week.
'Higher risk of hippocampal atrophy'
In their study paper, in which they discuss the rationale for their investigation, the researchers explain that a link between heavy drinking and adverse brain health - including dementia and degeneration of brain tissue - has already been well established.
However, fewer studies have examined the relationship between moderate drinking and brain health, and their evidence is largely inconsistent.
Therefore, the team decided to investigate whether or not there is a link between moderate alcohol consumption and brain changes by analyzing 30 years worth of data (collected between 1985 and 2015) on 550 healthy men and women who took part in the Whitehall II Study.
The participants were aged 43 on average when they started the study and none of them were alcohol dependent.
The data included information about weekly alcohol consumption and regular measures of brain function and mental performance. The participants also had an MRI brain scan at the end of the study.
When they analyzed the data, the researchers found that higher alcohol intake over the 30-year study period was tied to a higher risk of atrophy or tissue degeneration in the hippocampus, which is a part of the brain that is important for spatial orientation and memory.
They found that the link remained after taking into account factors that might influence it. These included sex, age, years of education, socioeconomic status, social and physical activity, medical history, smoking status, and stroke risk.
Moderate alcohol linked to three times greater risk of atrophy
However, while the participants whose alcohol intake exceeded 30 units per week had the highest risk of hippocampal atrophy (as expected), the analysis also showed a link to moderate alcohol consumption, which they defined as 14 to 21 units per week.
Compared with people who did not drink, people who drank moderately showed a three times higher risk of hippocampal atrophy.
The researchers also found that, compared with abstinence, light drinking - defined as no more than 7 units per week - offered no protective effect against hippocampal atrophy.
The brain scan data also showed evidence of greater deterioration in white matter with higher alcohol consumption. White matter integrity is important for mental ability.
Language fluency also declined faster with higher alcohol consumption. This is tested by asking people to give as many words starting with a particular letter as they can within the space of 1 minute.
However, decline in neither word recall nor semantic fluency was linked to higher alcohol consumption. Semantic fluency is tested by asking people to recall as many words in a particular category as they can within the space of 1 minute.
Questions idea that 'normal' drinking does no harm
While the study was not designed to show cause and effect, the results cannot be taken as proof that moderate drinking hastens brain decline. The authors suggest that further studies should now be done to confirm their findings.
Two strong points that could be argued as placing the study in the robust category are the amount of detailed data on potential influencing factors, and that alcohol consumption was measured regularly over a long period.
The authors suggest that their findings support the idea that alcohol might be a "modifiable risk factor for cognitive impairment, and primary prevention interventions targeted to later life could be too late."
Because alcohol consumption affects a large proportion of the population, the implications for public health could be significant, they conclude.
In an editorial comment about the findings, Killian Welch, a consultant neuropsychiatrist from the Royal Edinburgh Hospital in the U.K., says that they support the "argument that drinking habits many regard as normal have adverse consequences for health."
"With publication of this paper," he adds, "justification of 'moderate' drinking on the grounds of brain health becomes a little harder."
The authors of the study paper conclude:
"Our findings support the recent reduction in U.K. safe limits and call into question the current U.S. guidelines, which suggest that up to 24.5 units a week is safe for men, as we found increased odds of hippocampal atrophy at just 14-21 units a week, and we found no support for a protective effect of light consumption on brain structure."