Gallic acid metabolites are markers of black tea intake in humans

Gallic acid is one of the main phenolic components of black tea. The object ive of this study was to identify urinary gallic acid metabolites with pote ntial for use as markers of black tea intake. In an initial study, nine com pounds, assessed by using gas chromatography-mass spectrometry, were found to increase in concentration in urine after 3 cups of black tea over 3 h. A subsequent study employed a controlled crossover design in which 10 subjec ts consumed 5 cups per day of black tea or water for 4 weeks in random orde r. Twenty-four hour urine samples were collected at the end of each period. Of the 9 candidate compounds identified in the initial study, only 3 were present at higher concentrations in urine of all 10 subjects during tea-dri nking in comparison to water-drinking periods. These compounds were identif ied as 4-O-methylgallic acid, 3-O-methylgallic acid, and 3,4-O-dimethylgall ic acid, all methyl ether derivatives of gallic acid. It is suggested that these compounds have the potential to be used as markers of black tea intak e.