While the UADT is exposed to acetaldehyde from alcoholic beverages [20] and tobacco smoke, increasing evidence points to the metabolism of ethanol by microorganisms in the oral cavity as an important source of acetaldehyde in saliva and, by extension, in the esophagus. Acetaldehyde levels in saliva are 10–20 times higher than in blood, due to the local formation of acetaldehyde by oral microorganisms [21]. Importantly, ALDH2 heterozygotes had two to three times the acetaldehyde levels in their saliva compared to fully active ALDH2 individuals after a moderate dose of oral ethanol [22].
