Caffeine from coffee or other beverages is absorbed by the stomach and small intestine within 45 minutes of ingestion and then distributed throughout all tissues of the body.
Caffeine is metabolized in the liver by the cytochrome P450 oxidase enzyme system (specifically, the CYP1A2 isozyme) into three metabolic dimethylxanthines: paraxanthine, theobromine and theophylline. Paraxanthine increases lipolysis, leading to elevated glycerol and free fatty acid levels in the blood plasma. Theobromine dilates blood vessels and increases urine volume. Theobromine is also the principal alkaloid in cocoa, and therefore chocolate. Theophylline relaxes smooth muscles of the bronchi, and is used to treat asthma. Each of these caffeine metabolites is further metabolized into various methyluric acids and then excreted in the urine.