Gallic acid metabolites are markers of black tea intake in humans

Gallic acid is one of the main phenolic components of black tea. The objective of this study was to identify urinary gallic acid metabolites with potential for use as markers of black tea intake. In an initial study, nine compounds, 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 subjects consumed 5 cups per day of black tea or water for 4 weeks in random order. 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-drinking in comparison to water-drinking periods. These compounds were identified as 4-O-methylgallic acid, 3-O-methylgallic acid, and 3, 4-O-dimethylgallic 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 intake.