Here is a brief, somewhat simplified explanation for those of us who aren't quite chemistry wizards:
http://en.wikipedia.org/wiki/Alcohol_flush_reactionhttp://en.wikipedia.org/wiki/Alcohol...nase#In_humans http://en.wikipedia.org/wiki/Acetaldehyde_dehydrogenase.
So—to put it succinctly (this applies broadly to various populations, though it is often discussed regarding those of East Asian descent)...
Inside the liver, an enzyme called alcohol dehydrogenase works to oxidize ethanol into acetaldehyde; subsequently, this substance is converted into harmless acetic acid—essentially vinegar—by
acetaldehyde dehydrogenase —and this is precisely where the complication arises. Acetaldehyde is actually significantly more toxic than the alcohol itself, and it is largely responsible for that dreadful array of hangover symptoms we all know too well... Certain individuals of East Asian heritage carry a dominant
mutation in their acetaldehyde dehydrogenase gene (specifically within the mitochondrial ALDH2 gene), which renders this particular enzyme much less efficient. Consequently, in these individuals, acetaldehyde builds up in the system following alcohol consumption, triggering symptoms akin to acetaldehyde poisoning—most notably that unmistakable facial flushing... Interestingly, those with this deficiency in acetaldehyde dehydrogenase activity are statistically far less likely to struggle with alcoholism...
In short—if you drink too much or perhaps too quickly—your body simply cannot break down the alcohol, or rather the acetaldehyde, with sufficient speed or efficiency. It is a matter of metabolic capacity, really. So, my advice? Drink less, sip more slowly, and please, be mindful of your overall physical condition. If you are already feeling "wiped out," there is certainly no need to pursue a night of heavy drinking.
Now, I should clarify—I don't believe this is quite the same thing as a true allergy in the medical sense of the word. Of course, one might be allergic to other specific ingredients found in certain alcoholic beverages; it isn't always the alcohol itself that acts as the culprit when a reaction occurs. Rather, one is simply more sensitive to the alcohol. Or, perhaps, one is normally fine, but has simply overindulged beyond what the body can handle.
