human digestive system produces approximately 3g of ethanol per day

http://en.wikipedia.org/wiki/Ethanol_metabolism

The average human digestive system produces approximately 3g of ethanol per day merely through fermentation of its contents.[citation needed] Catabolic degradation of ethanol is thus essential to life, not only of humans, but of almost all living organisms. In fact, certain amino acid sequences in the enzymes used to oxidize ethanol are conserved all the way back to single cell bacteria.[1] Such a functionality is needed because all organisms actually produce alcohol in small amounts by several pathways, primarily along the fatty acid synthesis,[2] glycerolipid metabolism,[3] and bile acid biosynthesis pathways.[4] If the body had no mechanism for catabolizing the alcohols, they would build up in the body and become toxic.