A single glass of wine a night could lead to cognitive decline

Heavy drinking and dependence on alcohol are known to cause cognitive impairment, including memory loss and problems with verbal fluency, verbal learning, processing speed, attention and problem solving. However, the effects of moderate drinking on brain function have not been investigated. Considering that most people who consume alcohol probably fall into this category, such a study is well overdue. 

A team led by Anya Topiwala of the University of Oxford recently explored the relationship between alcohol consumption and cognitive function in more than 20,000 participants from the UK Biobank. Not only did participants report their own alcohol consumption, they also undertook various cognitive tests that assessed executive function, fluid intelligence and reaction time.

The researchers went a step further and measured the levels of iron present in the participants’ brains and livers, in an attempt to establish whether iron levels represent a potential pathway to alcohol-related cognitive deficits. Iron is involved in many fundamental biological processes in the brain including oxygen transportation, DNA synthesis, mitochondrial respiration, myelin synthesis, and neurotransmitter synthesis and metabolism. But high levels of this element have also been found in the brains of people with neurodegenerative conditions such as Alzheimer’s and Parkinson’s diseases.

In the largest study to date of moderate alcohol consumption and iron accumulation, the researchers recruited 20,965 participants who had completed their UK Biobank questionnaires and had undergone MRI imaging of the brain. Approximately 7,000 of these also underwent scanning of the liver, with both types of scans being used to identify levels of iron in the cells.  The mean age of participants was 55 years, and 48.6 percent were female. Although 2.7 percent classed themselves as non-drinkers, average intake was around 18 units per week, which translates to about 7½ cans of beer or 6 large glasses of wine. 

This level of alcohol consumption, classed as moderate, was higher than current UK low-risk guidelines (<14 units per week), but within low-risk guidelines at the time for men (not more than 21 units per week, pre-2016). Despite this, the researchers found links between level of alcohol consumption, presence of elevated iron concentrations in certain brain areas, and deterioration in cognitive function.

The team found that alcohol consumption of above seven units per week was associated with markers of higher iron in several brain nuclei, including the bilateral putamen, caudate and substantia nigra. These basal ganglia, and the substantia nigra in the mesencephalon, are associated with the control of motor movements, procedural learning, eye movement, cognition, emotion and other functions. 

The significantly higher levels of iron accumulated in these brain areas had also been identified in previous research on people with Alcohol Use Disorder, also known as alcohol addiction, but in the present study the result was seen in people who were only moderate users of alcohol.

In addition, higher alcohol consumption was associated with increased levels of iron in the liver, which is considered a good indicator of the levels of iron in the body in general, and the researchers found that iron levels were also the best indicators of alcohol-related liver damage. Alcohol suppresses the production of hepcidin, an important hormone in the liver that regulates iron balance. As a result, alcohol consumption leads to increased absorption of iron from digested food and can be expected to result in imbalances in iron levels in body organs such as the brain.

High iron levels in some brain regions were also associated with worse outcomes on cognitive tests, such as trail-making tests that assess executive function, and fluid intelligence tests that assess reason and logic. Reaction times were also slower in cases where certain brain regions were shown to have higher levels of iron.

There are some limitations to this large study of moderate alcohol consumption and iron accumulation. Drinking was self-reported and could have been underestimated, by participants. In addition, MRI-derived measures are indirect representations of brain iron, and could conflate other brain changes observed with alcohol consumption and that have changes in iron levels.

In conclusion, Topiwala states, “In the largest study to date, we found drinking greater than 7 units of alcohol weekly associated with iron accumulation in the brain. Higher brain iron in turn linked to poorer cognitive performance. Iron accumulation in the brain could thus underlie alcohol-related cognitive decline.”

Given the prevalence of moderate drinking in today’s world, even small links between alcohol and reduced brain function could have substantial impacts across whole populations. Thus, these results support the introduction of interventions to reduce alcohol consumption in the general population.

The research is published in the journal PLoS Medicine.

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By Alison Bosman, Earth.com Staff Writer