The Biology Behind the Flush
When you drink alcohol, your body metabolizes it in two steps. First, alcohol dehydrogenase (ADH) converts ethanol into acetaldehyde, a toxic compound and known carcinogen. Second, aldehyde dehydrogenase 2 (ALDH2) converts acetaldehyde into harmless acetate. The entire process normally happens so quickly that acetaldehyde barely accumulates. But for roughly 560 million people worldwide, the second step is broken.
The ALDH2*2 Variant: rs671
The rs671 SNP causes a single amino acid substitution: glutamic acid to lysine at position 504 (Glu504Lys). This seemingly small change destabilizes the ALDH2 tetramer's active site, dramatically reducing its ability to convert acetaldehyde to acetate. The result is a bottleneck in alcohol metabolism that causes acetaldehyde to accumulate in the blood.
Genotype and Enzyme Activity
- GG (ALDH2*1/*1): Normal enzyme function. Acetaldehyde is cleared rapidly. No flush reaction. Found in virtually all non-East-Asian populations and about 60–65% of East Asians.
- GA (ALDH2*1/*2): Enzyme activity reduced to approximately 17% of normal due to the dominant-negative tetramer effect. Acetaldehyde accumulates, causing facial flushing, nausea, tachycardia, and headache after drinking. About 30–35% of East Asians.
- AA (ALDH2*2/*2): Near-zero enzyme activity. Extremely severe reaction to even small amounts of alcohol. Most homozygotes avoid alcohol entirely due to the unpleasant response. About 3–8% of East Asians.
The Cancer Risk That Cannot Be Ignored
The flush reaction is more than an inconvenience. Acetaldehyde is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC). It damages DNA by forming adducts, crosslinks, and causing chromosomal aberrations. For ALDH2-deficient individuals who drink regularly, the cancer risk is substantial:
- Esophageal cancer: ALDH2*1/*2 heterozygotes who consume alcohol have a 6–10 fold increased risk of esophageal squamous cell carcinoma compared to *1/*1 drinkers.
- Head and neck cancers: 2–3 fold increased risk in ALDH2-deficient drinkers.
- Gastric cancer: Elevated risk, particularly in heavy drinkers with the deficiency.
- Compounding factor: Many East Asian populations also carry a fast-acting ADH1B variant (rs1229984) that produces acetaldehyde more rapidly, further increasing the acetaldehyde burden.
Population Genetics and History
The ALDH2*2 allele is concentrated almost exclusively in populations of East Asian descent. Its frequency peaks in Southeast China, Taiwan, and Japan (30–40%) and decreases with distance from this center. The allele is essentially absent in European, African, and Native American populations.
The high frequency despite its obvious disadvantage has puzzled geneticists. One hypothesis suggests that ALDH2 deficiency may have conferred a selective advantage in rice-cultivating societies, possibly by reducing heavy drinking and its social consequences, or through some unrelated metabolic benefit. Others propose genetic drift or founder effects in ancient East Asian populations. The question remains open.
Cultural Context
In many East Asian cultures, social drinking is deeply embedded in business and personal relationships. The term “Asian glow” is widely recognized but often treated as trivial or humorous. This cultural normalization can lead ALDH2-deficient individuals to push through the discomfort and drink anyway, unaware of the cancer risk they are incurring. Public health efforts in Japan, South Korea, and China increasingly emphasize genetic testing and awareness of ALDH2 status as a preventive health measure.
Beyond Alcohol: Other ALDH2 Roles
ALDH2 also detoxifies other aldehydes encountered in daily life, including 4-hydroxynonenal (4-HNE), a lipid peroxidation product generated during oxidative stress. ALDH2-deficient individuals may have reduced capacity to clear these endogenous toxins, which has been linked to increased risk of cardiovascular disease and Alzheimer's disease in some studies. Nitroglycerin (used to treat angina) also requires ALDH2 activation, meaning the drug may be less effective in carriers of the *2 allele.