The heavy use of alcohol among adolescents and young adults may slow down the rate of brain growth, according to a new study on non-human primates.

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Researchers at the Oregon National Primate Research Center measured brain growth through magnetic resonance imaging in 71 rhesus macaques (Macaca mulatta) that consumed ethanol or alcoholic beverages. In order to rule out other possible factors, the researchers monitored the animals’ precise alcohol intake, diet, daily schedules, and general health.

The results suggest that heavy alcohol consumption (the equivalent of 4 beers per day for humans) reduced the rate of brain growth by approximately 0.25 millimeters per year for every gram of alcohol consumed per kilogram of body weight. Normal brain growth in adolescent rhesus macaques is 1 millimeter per 1.87 years, the authors reported in the journal eNeuro. 

The adolescent brain is more sensitive than the adult brain to alcohol because connections between brain cells are not yet as robust, which makes them more easily disturbed. One part of the brain that is affected by alcohol is the hippocampus — the seahorse-shaped area deep inside your brain that is responsible for learning and memory. Alcohol is known to damage or even destroy brain cells in this brain area, which isn’t surprising knowing that many people experience fuzzy memories or even ‘blackouts’ after consuming alcohol.

Human brain imaging studies showed that the volume of white matter — which is important for pathways connecting neurons located at farther distances from each other — increases during adolescence, presumably reflecting enhanced brain connectivity and improved communication between areas. However, macaque monkeys that consumed alcohol experienced a slower growth of cerebral white matter, the researchers wrote.

“Human studies are based on self-reporting of underage drinkers,” said co-author Christopher Kroenke, Ph.D., an associate professor in the Division of Neuroscience at the primate center. “Our measures pinpoint alcohol drinking with the impaired brain growth.”

The brain is plastic, which in the context of this study means that it should recover at least partly after a person stops drinking alcohol. However, it’s not clear whether there are long-term effects on cognitive abilities and mental functions once the adolescent brain ends this growth phase. In the future, the researchers would like to investigate this question.

“This is the age range when the brain is being fine-tuned to fit adult responsibilities,” lead authors Tatiana Shnitko, a research assistant professor in the Division of Neuroscience at the primate center, said in a statement. “The question is, does alcohol exposure during this age range alter the lifetime learning ability of individuals?”