As an equal mix of Chinese and Irish, I had a 50/50 shot at enjoying the stereotypical Irish drinking culture. Unfortunately, I was never able to fully participate due to my inability to handle a large amount of alcohol. Curiously, this has a little less to do with my lifestyle, and a lot to do with my Asian ancestors’ solution to clean water hundreds of years ago.

Alcohol flush response (AFR), more familiarly known as the ‘Asian glow,’ affects as many as half of all people of East Asian descent according to Sharon Moalem, a researcher and doctor at New York’s Mount Sinai School of Medicine, in his book Survival of the Sickest. From rising temperatures to a bright-red face, this bodily response to alcohol makes it difficult for some to consume even one alcoholic beverage.

Evident from its nickname, AFR is most prevalent within East Asian communities. However, it is actually highly uncommon in just about every other population group. The discrepancy begs the question: what determines our ‘drinking gene’?

Although people blame factors such as weight, dehydration, or lack of nourishment, the main cause of the Asian glow lies not in our diet but in ancient water purification systems.

When people drink alcohol, the body detoxifies alcohol and extracts calories from it through a complex process that involves multiple organs and many different enzymes. The majority of these reactions occur in the liver, and it is here where our ‘drinking gene’ plays a role.

After alcohol consumption,  your liver takes several steps towards metabolizing your drink by converting it into a chemical known as acetaldehyde. This process is facilitated by the enzyme alcohol dehydrogenase. A second enzyme, acetaldehyde dehydrogenase, converts the acetaldehyde into acetate. Finally, the acetate is converted into fat, carbon dioxide, and water.

The culprit behind the redness is the second enzyme in this series of reactions. Most people who experience AFR have a genetic variation, ALDH2*2, which causes them to produce a less powerful form of acetaldehyde dehydrogenase—one which cannot convert acetaldehyde into acetate as effectively. As a result, these people accumulate acetaldehyde up to 10 times the normal concentration. Considering that acetaldehyde is 30 times as toxic as alcohol, any accumulation can result in a reaction, where one of the symptoms is, of course, the flushing response.

However, a red face isn’t the only side effect of this genetic variation. One drink is all an ALDH2*2 carrier needs to experience heightened heart rate, headache, extreme dizziness, and nausea.

So, it’s clear that the cause of AFR is a less powerful form of the enzyme acetaldehyde dehydrogenase, but what is the link between flushing when you drink and clean water?

As humans began to settle in cities and towns in ancient times, clean water became a challenge. According to Moalem, some theories suggest cultures came up with different solutions to purifying their water.  In Europe, the solution was fermentation. This method was based on producing alcohol to kill the microbes contaminating drinking water. On the other side of the globe, people in East Asia boiled water to produce tea as their main mode of purification.

“As a result, there was evolutionary pressure in Europe to have the ability to drink, break down, and detoxify alcohol, while the pressure in Asia was a lot less,” explains Moalem.

Based on this theory, it was necessary for Europeans to develop a better ‘drinking gene’ than those of Asian descent, as their water purification system required them to frequently break down alcohol while having a drink—and not just the alcoholic type.

While AFR makes consuming excessive alcohol a challenge, Moalem points out an upside to ALDH2*2. “You’re highly resistant to alcoholism,” he says. “It’s just too unpleasant to drink!”