Metabolic Fate of Dietary Tannins in Chickens

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Abstract
The metabolic fate of tannic acid and its degradation product, gallic acid, was studied using both paper and thin-layer chromatography (TLC) to isolate and identify characteristic metabolic end products in the urine of chickens after the oral administration of these substances. TLC revealed the presence of gallic acid, 4-O-methyl gallic acid and pyrogallol in the urine of hens fed either tannic acid or gallic acid. Tannic acid apparently was hydrolyzed to gallic acid and a large part of this material was O-methylated and excreted in the urine as 4-O-methyl gallic acid. This would explain the beneficial effect of methionine and choline in the diet of chicks fed tannic acid or natural tannins. Decarboxylation of gallic acid accounted for another metabolite, pyrogallol. Paper chromatograms revealed a fourth derivative, pyrocatechol, but the presence of this substance could not be positively identified using TLC.