Problematic alcohol use — that is, drinking that is accompanied by unpleasant consequences — is associated with a smaller hippocampus in the brain, according to new research.

“The hippocampus has been implicated in memory and learning processes, as well as emotionality and emotion regulation. Several lines of research find that people with various forms of psychopathology (schizophrenia, posttraumatic stress disorder, depression) have smaller hippocampal volume,” explained study author Sylia Wilson of the University of Minnesota.

“An emerging body of research also finds smaller hippocampal volume among people with problematic alcohol use, but this research hadn’t yet been synthesized using meta-analysis.”

For their study, which was published in the journal Psychological Medicine, the researchers conducted a meta-analysis of 23 brain imaging studies. A meta-analysis is a method of statistically summarizing data from previous research.

The findings confirmed that problematic alcohol use was linked to reductions in hippocampal volume.

“The meta-analysis found that problematic alcohol use is associated with smaller hippocampal volume,” Wilson explained to PsyPost. “We also found larger effects (smaller hippocampal volume) among people with clinically significant alcohol use (alcohol use disorders) and that effects were associated with age. These results are consistent with a neurotoxic exposure effect of alcohol on the hippocampus.”

But the methodology of this particular study doesn’t allow the researchers to make any conclusions about cause and effect.

“The meta-analysis finds an association between problematic alcohol use and smaller hippocampal volume, but cannot speak to the causal nature of this association,” Wilson said. “There’s ample evidence from this meta-analysis and the broader literature that the alcohol addicted brain looks and functions differently than the unaddicted brain.”

“Most models of addiction implicitly or explicitly attribute these differences to a neurotoxic exposure effect of alcohol on the brain,” Wilson told PsyPost. “However, it’s also plausible that people had brain differences even before beginning to misuse alcohol, which might reflect vulnerability toward the development of problematic alcohol use.”

“Because it’s not ethical to randomly assign people to addicted or unaddicted groups, differentiating effects of alcohol-related exposure on the brain from pre-existing vulnerability requires study designs that allow for greater causal inference.”

But Wilson was involved in other research that indicated heavy drinking leads to reductions in hippocampal volume.

“In a follow-up study that took a causally informative approach in a sample of twins, we found that twins who used more alcohol had smaller hippocampal volume compared with their co-twins who used less alcohol,” she explained.

“Because twins share both genes and environmental experiences, the results of this study lend greater confidence to the conclusion that the smaller hippocampal volume found among people with problematic alcohol use reflects an alcohol-exposure related effect.”

The study, “Problematic alcohol use and reduced hippocampal volume: a meta-analytic review“, was also co-authored by J. L. Bair, K. M. Thomas and W. G. Iacono.