What this means

ALDH2 codes for aldehyde dehydrogenase 2, which clears acetaldehyde — the toxic intermediate produced when ethanol is broken down. The rs671 A allele (E487K) creates an inactive subunit; because the enzyme is a tetramer, even one bad copy disables most of the complex, so heterozygotes already show much-reduced clearance. About 30–40% of East Asian people carry at least one A allele, which is why the "Asian flush" is so visible. The variant is genuinely protective against alcohol-use disorders but raises risk of upper aerodigestive cancers among people who drink despite the flush.

When your body breaks down alcohol, it first becomes a substance called acetaldehyde — unpleasant and slightly toxic. The ALDH2 enzyme normally clears it quickly. One common version of this gene makes an enzyme that barely works, so acetaldehyde builds up after even a small drink. That causes the "Asian flush": red face, nausea, headache. About 30–40% of East Asian people carry at least one of these slow copies. The variant strongly protects against developing alcohol problems, but raises the risk of certain cancers in people who drink despite the flush.

Caveats

• The flush is a warning signal — drinking despite it raises cancer risk meaningfully.
• The variant is very rare outside East Asian populations.
• Antihistamines mask the redness but do nothing about acetaldehyde build-up.
• Tolerance to the flush builds with repeated exposure but the underlying chemistry is unchanged.

References

• Higuchi et al. — Aldehyde dehydrogenase genotypes in Japanese alcoholics (Lancet, 1994)
• Brooks et al. — The alcohol flushing response: an unrecognized risk factor for esophageal cancer from alcohol consumption (PLOS Medicine, 2009)