Causes
It is commonly thought that the flush reaction is caused by an inability to metabolize alcohol. To the contrary, around 80% of Asian people (less common in Thailand and India) have a variant of the gene coding for the enzyme alcohol dehydrogenase called ADH1B, and almost all Chinese and Korean people have a variant of the gene called ADH1C. These genes result in an alcohol dehydrogenase enzyme which converts alcohol to acetaldehyde with a much higher efficiency than other gene variants (40 to 100-fold in case of ADH1B). In about 50% of Asians, the increased acetaldehyde accumulation is worsened by another gene variant, the mitochondrial ALDH2 allele, which results in an inhibited acetaldehyde dehydrogenase enzyme, responsible for the breakdown of acetaldehyde. The result is that affected people may be better at metabolizing alcohol, often not feeling the alcohol "buzz" to the same extent as others, but show far more acetaldehyde-based side effects while drinking.
Read more about this topic: Alcohol Flush Reaction