Individuals with low levels of alcohol dehydrogenase experience rapid buildup of toxic acetaldehyde when ingesting alcohol, leading to immediate sickness.

“There are actually people who have so little alcohol dehydrogenase that when they ingest alcohol, they get very red and they just feel sick. So if you're somebody that has a sip of alcohol and you just feel horrible, it makes you feel nauseous, chances are you have gene variants that create a situation where you're not making very much alcohol dehydrogenase. You just simply can't metabolize alcohol so you just get a rapid buildup of the toxic effects of alcohol, the acetaldehyde, you're not converting it into those empty calories.”