What does alcohol do to your gut microbiome?

Chronic heavy drinking significantly alters the composition of the gut microbiome. In studies involving people with alcohol addiction as well as in mouse experiments, the balance between bacterial species shifted substantially. That this is not merely a side effect but an actual cause became clear when researchers transferred the altered gut microbiome to healthy mice via fecal transplantation: those mice developed the same disrupted flora.

That shifted gut microbiome has a concrete effect on the heart. Bacteria in the disrupted gut produce more of a substance called phenylacetylglutamine. That substance enters the bloodstream and disrupts calcium regulation in heart muscle cells, causing the heart to relax less effectively. The inner lining of blood vessels also becomes inflamed as a result. In large human studies, phenylacetylglutamine has long been linked to an increased risk of cardiovascular disease, and alcohol now appears to be one of the causes of elevated blood levels.

A second consequence is a leaky gut wall. When the gut microbiome is disrupted, the 'junctions' between intestinal cells become looser. Harmful bacterial products then slip into the bloodstream and reach the liver, where they cause inflammation and scarring. This plays a role in both alcoholic liver disease and fatty liver disease in the absence of alcohol use. In people with a fatty liver, certain gut bacteria can even produce small amounts of alcohol themselves as a byproduct, which can further worsen liver damage.

Alcohol also affects the breakdown of its own toxic intermediate, acetaldehyde. The liver breaks down most of the alcohol, but excretes a portion of the acetaldehyde via bile, after which gut bacteria process it further. The gut therefore participates in the detoxification of alcohol. When the gut microbiome is disrupted, that system functions less effectively, which can result in more acetaldehyde entering the bloodstream.