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Alcohol flush reaction (also known as Asian flush syndrome, Asian flush, Asian glow, among others) is a condition in which an individual's face or body experiences flushes or blotches as a result of an accumulation of acetaldehyde, a metabolic byproduct of the catabolic metabolism of alcohol.
This syndrome has been associated with an increased risk of esophageal cancer in those who drink. It has also been associated with lower than average rates of alcoholism, possibly due to its association with adverse effects after drinking alcohol.
Flushing, or blushing, is associated with the erythema (reddening caused by dilation of capillaries) of the face, neck, shoulder, and in some cases, the entire body after consumption of alcohol.
Individuals who experience the alcohol flushing reaction may be less prone to alcoholism. Disulfiram, a drug sometimes given as treatment for alcoholism, works by inhibiting acetaldehyde dehydrogenase, causing a five to tenfold increase in the concentration of acetaldehyde in the body. The resulting irritating flushing reaction is intended to discourage alcoholics from drinking.
Persons prone to the condition also have lower blood pressure, perhaps as a result of their much lower levels of drinking alcohol.
For measuring the level of flush reaction to alcohol, the most accurate method is to determine the level of acetaldehyde in the blood stream. This can be measured through both a breathalyzer test or a blood test. Additionally, measuring the amount of alcohol metabolizing enzymes alcohol dehydrogenases and aldehyde dehydrogenase through genetic testing can predict the amount of reaction that one would have. More crude measurements can be made though measuring the amount of redness in the face of an individual after consuming alcohol. Computer and phone applications can be used to standardize this measurement.
Other effects include "nausea, headache and general physical discomfort."
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,  both resulting in an alcohol dehydrogenase enzyme that converts alcohol to toxic acetaldehyde at 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 a less functional 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.
Alcohol flush reaction is best known as a condition that is experienced by people of Asian descent. According to the analysis by HapMap project, the rs671 allele of the ALDH2 gene responsible for the flush reaction is rare among Europeans and Africans, and it is very rare among Mexican-Americans. 30% to 50% of people of Chinese and Japanese ancestry have at least one ALDH2 allele. The rs671 form of ALDH2, which accounts for most incidents of alcohol flush reaction worldwide, is native to East Asia and most common in southeastern China. It most likely originated among Han Chinese in central China, and it appears to have been positively selected in the past. Another analysis correlates the rise and spread of rice cultivation in Southern China with the spread of the allele. The reasons for this positive selection aren't known, but it's been hypothesized that elevated concentrations of acetaldehyde may have conferred protection against certain parasitic infections, such as Entamoeba histolytica.
Facial flushing when drinking is indicative of ALDH2 deficiency, and that an ALDH2-deficient drinker who drinks two beers per day has six to ten times the risk of developing esophageal cancer as a drinker not deficient in the enzyme.
Since the mutation is a genetic issue, there is no cure for the flush reaction. Prevention would include not drinking alcohol.
One hypothesis is that acetaldehyde causes the redness and vasodilation, and because the H2-antagonist class of medicine inhibits the ADH enzyme (the conversion from ethanol to acetaldehyde) both in the GI tract and in the liver, the conversion happens at a much slower pace, reducing the effects acetaldehyde has on the drinker. The idea that acetaldehyde is the cause of the flush is also shown by the clinical use of disulfiram (Antabuse), which blocks the removal of acetaldehyde from the body via ALDH inhibition. The high acetaldehyde concentrations described share similarity to symptoms of the flush (flushing of the skin, accelerated heart rate, shortness of breath, throbbing headache, mental confusion and blurred vision).
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