Anytime I drink with my friends, be it wine, liquor, or beer, my skin starts to glow red even after ingesting just a little bit of alcohol. So many people considered me to be drunk once they see the red glow on my face, although I barely feel the effect of the alcohol. This makes me wonder “Why do I flush so easily after drinking alcohol?”
One day, one of my friend mentioned to me that he has read somewhere saying that the red glow does not necessarily mean I was drunk but it has something to do with missing an enzyme or something. That’s right, I have what’s colloquially known as the “Asian Flush”.
So I started digging into this. Actually I had Cath start looking up some scientific reasoning behind this flushing, I join in a bit later.
To understand this, we first have to understand what happens after we ingest the alcohol.
The process can be broken up as follows: (1) absorption of alcohol through the gastrointestinal tracts, (2) equal distribution of the alcohol in the total body water, and (3) metabolism of the alcohol primarily by the liver .
The metabolism of the alcohol is believed to be carried out in two steps [2,3]:
(1) conversion of alcohol to acetaldehyde
(2) conversion of acetaldehyde to acetic acid
Alcohol dehydrogenase (ADH) enzyme is responsible for step (1), whereas acetaldehyde dehydrogenase (ALDH) is responsible for step (2). Note that process in step (1) is also termed alcohol dehydrogenase, and similarly the step (2) process is called acetaldehyde dehydrogenase.
Alcohol is also metabolized with a different system, however this system is only likely to operate at high alcohol concentrations, i.e. in chronic alcoholic . Otherwise, the bulk of the alcohol is metabolized with the alcohol dehydrogenase and acetaldehyde dehydrogenase .
The longer version:
The “Asian Flush” results from a deficiency in one of the isoenzyme in the ALDH enzyme group, specifically the ALDH2 isoenzyme [1,3,4]. ALDH2 is polymorphic, meaning the alleles, i.e. genetic coding, found at the location along the chromosome pair responsible for this enzyme may take several forms. The alleles responsible for the ALDH2 enzyme is normally expressed as ALDH2*1 but becomes ALDH2*2 when it is mutated. ALDH2*2 allele leads to a deficient ALDH2 enzyme, which inhibits the conversion of acetaldehyde to acetic acid . The result of Wall et al.  highly suggests that it is the higher acetaldehyde levels that leads to the flushing response, and not the blood alcohol levels (BAC).
About half of Asian ancestry has this deficient ALDH2 enzyme [3,4,5], but is almost totally absent in Caucasian populations . Asians with only one mutated ALDH2*2 allele in their genes experience only a mild flush response compared to those with two ALDH2*2 alleles.
Moreover, it was found in one study reviewed by Bosron et al.  that the alcohol metabolic rate in Japanese with and without this deficient ALDH2 enzyme is not significantly different. In other words, having this deficiency does not mean one cannot process alcohol as fast.
In many studies, this deficiency in the ALDH2 enzyme has been linked to lower rate of alcoholism [1,5]. However, acetaldehyde is believed to be a mediator of toxicity . Consequently, the elevated acetaldehyde levels may also contribute to increased risk of alcohol related conditions, such as liver diseases.
Bonus: so what determines the alcohol metabolism rate?
The rate of alcohol metabolism is determined by step (1) or alcohol dehydrogenase. In another study reviewed by Bosron et al. , it was found that the variability in the alcohol metabolism rate is largely dependent on genetics (49%), and only somewhat dependent on prior drinking experience (12%) and the combined contribution of age, weight, body fat content and lung volume (<10%).
The metabolism of alcohol is determined by the ADH enzyme. A large variation of the different ADH2 and ADH3 alleles is found across the different races . With 85% of Caucasians having the ADH2*1 allele and 85% of Asians having the ADH2*2 allele . The coding provided by the ADH2*2 have significantly higher reaction rate, i.e. alcohol metabolism rate . In fact, one study reviewed in  have found that the alcohol elimination rate in male Chinese and Japanese subjects were approximately 20% higher (18% and 24% higher to be specific) than that of Caucasian of similar age, body leanness, and alcohol consumption habits.
What I have Learned from all of this…
50% of Asian has a deficiency in their ALDH2 enzyme [3,4,5]. This deficient ALDH2 enzyme cannot process the acetaldehyde resulting from the metabolism of alcohol. It is the higher acetaldehyde level that cause the flushing response not the blood alcohol level (BAC). This deficiency is almost non-existent in Caucasians . This deficiency has little to do with the tolerance of an individual or how fast they metabolize alcohol.
In other words, how red (flushed) a person gets has little to do with how high their BAC is, or how drunk they are. This deficiency in the ALDH2 enzyme also has little to do with the alcohol metabolism rate.
So next time you see someone flushing after a sip of beer or booze, don’t just assume they’re already drunk. They could simply have different genetic coding.
and lastly, medical journal needs less confusing acronyms for so many different things.
 Li, T., Yin, S., Crabb, D., O’Connor, S., & Ramchandani, V. (2001). Genetic and Environmental Influences on Alcohol Metabolism in Humans Alcoholism: Clinical and Experimental Research, 25 (1), 136-144 DOI: 10.1111/j.1530-0277.2001.tb02138.x
 Erik Jacobsen (1952). The Metabolism of Ethyl Alcohol Pharmacol Rev
 Bosron, W., & Li, T. (1986). Genetic polymorphism of human liver alcohol and aldehyde dehydrogenases, and their relationship to alcohol metabolism and alcoholism Hepatology, 6 (3), 502-510 DOI: 10.1002/hep.1840060330
 Tamara L. Wall, Charles M. Peterson, Karen P. Peterson, Mona L. Johnson, Holly R. Thomasson, Maury Cole, & Cindy L. Ehlers (1997). Alcohol Metabolism in Asian-American Men with Genetic Polymorphisms of Aldehyde Dehydrogenase Annals of Internal Medicine
 Ronald C. Johnson, & Craig T. Nagoshi (1990). Asians, Asian-Americans and alcohol. Journal of Psychoactive Drugs