Alcohol Flush Reaction

This reaction to alcohol is called alcohol flush reaction and is most commonly seen in individuals of East Asian ancestry, which is why it is sometimes referred to as the "Asian Flush ".

Alcohol is toxic to human bodies. In people who do not experience alcohol flush reaction, alcohol is broken down (metabolized) in the liver into substances which can be either used or excreted by the body. This breakdown occurs in several steps (see figure). First the enzyme alcohol dehydrogenase (ADH) converts alcohol to acetaldehyde. Acetaldehyde is a substance even more toxic to the body than alcohol and contributes largely to the adverse effects of alcohol generally known as a “hangover”. Second, acetaldehyde is broken down into the harmless acetic acid (or vinegar) by another enzyme called aldehyde dehydrogenase-2 (ALDH2). A third enzyme finally breaks the acetic acid into fat, carbon dioxide, and water. The impaired function of any of these critical enzymes disrupts alcohol metabolism, leading to varying degrees of discomfort depending on the amount of alcohol ingested and which enzyme is affected.

Alcohol flush reaction is largely due to a genetic variant that affects one of the enzymes responsible for breaking down (metabolizing) alcohol. This genetic variant is found in the ALDH2 gene and is known as the ALDH2*2 allele. The ALDH2 gene regulates the production of the enzyme aldehyde dehydrogenase 2 that has the role of transforming acetaldehyde into acetic acid. The ALDH2*2 allele is dominant. This means that even one copy of it interferes with the formation of a fully functional ALDH2 enzyme, with the result that acetaldehyde cannot be broken down and builds up in the body when drinking alcohol, leading to the symptoms described above.