Acetaldehyde is an intermediate metabolite of alcohol:
It's the first stop for ethanol on the way to benign acetate. Aldehydes tend to be short-lived and toxic species because of their reactivity, and acetaldehyde is no different. On the way to your hangover, though, alcohol goes from ethanol to acetaldehyde (via alcohol dehydrogenase) to acetate (via aldehyde dehydrogenase). Many east Asians, interestingly, lack an adequate supply of aldehyde dehydrogenase, and they experience a sort of accelerated hangover on consumption of even moderate amounts of alcohol.
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"Chocolate?" It seems like a random word at the end of your post title rototiller.
One imagines acetaldehyde permeating the blood brain barrier, going Schiff base with endogenous primary amine neurotransmitters, then reduction to eth-amphetaminoid goodies. One drink is one too many, a dozen drinks are not nearly enough.
Asia boiled its drinking water as tea. Arabia boiled its drinking water as coffee. Europe used fermentation, enjoying centuries of massive hangovers and fetal alcohol syndrome. Evolution is a hoot if you are one of the survivors.
Why do east asian lack of aldehyde dehydrogenase?
Uncle Al: That seems very likely. I know for a fact that consuming both cocaine and alcohol at the same time results in a modified cocaine molecule - it becomes a the ethyl ester. I would also imagine that the cocaine could become demthylated and go all schiff base with acetaldehyde (this is known to occur with nicotine).
Hean Y: Apparently a popular theory for this is a common mutation of the allele ALDH2-2 (http://www.jem.org/cgi/content/abstract/194/5/571). This mutation lowers the activity of aledhyde dehydrogenase. In the brief literature review I did, there seems to be an unsettled debate as to whether or not this mutation has become so widely spread due to natural selection or random selection. Intriguing nonetheless though.
I often sneeze when I eat dark chocolate, sniff hard alcohol, eat strong mints or look at the sun (sun sneeze). If I do all of these at once, it's a guaranteed sneezing fit. I don't have any other allergies or asthma or other diseases. Do I have (partially) the Asian allele mutation, or some other factor? I don't react oddly to acetone, acetate or acetamorphin or aspirin. Just curious.
Why do east asian lack of aldehyde dehydrogenase? Hean Y.
About 50% do. My hypothesis: As Zach said, but furthermore, east Asians had no need for alcohol, they used coal and wood to boil/cook. Mongols herding on plains were nomadic, so they often lacked fuel, instead they ate yogurt, cheese curds, and drank fermented milk, not fresh milk, so both Mongols and east Asians (Chinese, Korean) had lactose intolerance, but only east Asians had alcohol intolerance. Europeans had fermented cheese for long winter storage, (non-fermented) fresh milk, fermented grape/grain/roots, and so developed tolerance (= selection) for both lactose and alcohol.