Hydroxycinnamic acids in the digestive tract of livestock and humans

Hydroxycinnamic acids are consumed as water-soluble conjugates and in larger amounts bound to plant cell walls. Bound acids are primarily released by microbial action in the modified forestomach of ruminants and the hindgut of non-ruminant species, including humans. In the rumen, rapid hydrogenation of p-coumaric, ferulic and caffeic acids, followed by dehydroxylation at C4 and more slowly at C3 yields 3-phenylpropionic acid. Phenylpropionate is absorbed and undergoes beta-oxidation in the liver to benzoic acid which is then excreted predominately (75-95%) as its glycine conjugate (hippuric acid), but also as the free acid or glucuronide. In non-ruminants, hydroxy-cinnamates may be absorbed unchanged in the upper digestive tract via a Na+-dependent saturable transport system or escape to the hindgut where they are subject to microbial transformations with further absorption of metabolites, Metabolites of p-coumaric acid found in rat urine are the unchanged compound and its glycine conjugate, the reduced derivative and the beta-oxidation product, 4-hydroxybenzoic acid. Caffeic acid and its methyl ethers (ferulic and iso-ferulic acids) are interconvertable and share metabolites. As in the rumen, reduction of the C-3 side-chain, demethylation of ferulate and dehydroxylation at C4 are products of microbial action. Dehydroxylation at C3 is more rarely encountered. The resulting 3-hydroxyphenylpropionic acid is commonly found in the urine of all species and is the major metabolite in rats where relatively little chain-shortening occurs. A larger range of metabolites including C-6-C-1 compounds have been detected in human urine. Metabolism of hydroxycinnamate dimers found as cross-links between polysaccharide chains has been little studied although evident differences in the ability to metabolise such compounds exist between the human and rumen microflora. (C) 1999 Society of Chemical Industry.