The caffeine molecule is sufficiently hydrophobic to pass freely through biological membranes (24). It is completely absorbed from the gastrointestinal tract and rapidly attains peak plasma levels. Caffeine is equally well absorbed from each of its beverage sources. There are no effective barriers to penetration of any tissue, including placenta and fetus. This characteristic prevents efficient excretion of caffeine by the human kidney since it is readily reabsorbed from the renal tubules.
The half-life of caffeine in plasma varies not only with specie differences but also with age and condition of the individual. The half-life in rodent plasma is 1 to 2 h but it is 6 h in that of the healthy adult human. There is variation depending on smoking habits and the use of some medications. During pregnancy the half-life is increased to 18 h. The immature liver of the newborn human is limited in its ability to metabolize caffeine so that its half-life is three to four days, similar to that of adults with severe liver damage. These differences are significant in extrapolating safety data from animal studies and also in considering diet during pregnancy (24).
The metabolic pathways of caffeine in those mammalian species that have been studied exhibit many similarities (24). The same major reactions take place: demeth-ylation, oxidation at the 8-position on the xanthine ring to form uric acids, and ring cleavage between the 8- and 9-positions to form diaminouricils. AT-acetylation also occurs. The three dimethyl- and the three monomethylxanthines that can be formed are all found in human urine after caffeine ingestion along with all of the corresponding methylated uric acids. Uracils derived from caffeine, theobromine, and paraxanthine as well as large amounts of acetylated products are also present. No xanthine is formed. The human metabolic pathway is shown in Figure 4.
Specie variations in the metabolic pathways of caffeine were first observed in 1900 (1). In humans, about 70% of ingested caffeine is initially converted to paraxanthine, 25% to a mixture of theobromine and theophylline, and about 5% is oxidized without demethylation to form the corresponding uric acid and uracil compounds. Some primates produce theophylline as the predominant initial metabolite. In the human the final product mix is the result of competing reactions, the rates of which vary with gender, dosage, medications in the diet, and individual differences. Minor amounts of uracils derived from caffeine, theobromine, and paraxanthine also occur. Averaged values for caffeine metabolites found in human urine are shown in Table 2.
The acetylated product (A) in Table 2 is believed to be a true metabolite. Product (B) is probably an artifact of analytical procedures.
Reaction mechanisms are not well known. The biochemical changes take place in the liver. Xanthine oxidase mediates the formation of the uric acid derivatives. It is interesting to note that in the human infant, theophylline, often used for treatment of apnea, is methylated to caffeine as the first step in its metabolism (1).
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