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For starters, the bloodstream carries most of the ethanol in an alcoholic beverage straight to the liver, where an enzyme called alcohol dehydrogenase breaks it down into acetaldehyde.

An enzyme in the liver called ADH4, or alcohol dehydrogenase 4, breaks ethanol down, which eventually becomes carbon dioxide and water.

And women do not produce as much of an alcohol-metabolizing enzyme called alcohol dehydrogenase, meaning alcohol is more toxic for them, Wakim-Fleming said.

A: In the last 7 million to 21 million years our ancestors evolved a souped-up version of alcohol dehydrogenase, an enzyme that metabolizes alcohol.

Researchers have recently learned that a mutation some 10 million years ago in the alcohol dehydrogenase gene greatly increased alcohol tolerance in the ancestor shared by humans and African apes.