Hydroxycinnamic acids in the digestive tract of livestock and humans

Hydroxycinnamic acids are consumed as water-soluble conjugates and in large r amounts bound to plant cell walls. Bound acids are primarily released by microbial action in the modified forestomach of ruminants and the hindgut o f non-ruminant species, including humans. In the rumen, rapid hydrogenation of p-coumaric, ferulic and caffeic acids, followed by dehydroxylation at C 4 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 ac id), but also as the free acid or glucuronide. In non-ruminants, hydroxy-ci nnamates may be absorbed unchanged in the upper digestive tract via a Na+-d ependent saturable transport system or escape to the hindgut where they are subject to microbial transformations with further absorption of metabolite s, Metabolites of p-coumaric acid found in rat urine are the unchanged comp ound and its glycine conjugate, the reduced derivative and the beta-oxidati on product, 4-hydroxybenzoic acid. Caffeic acid and its methyl ethers (feru lic and iso-ferulic acids) are interconvertable and share metabolites. As i n 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 polysacc haride chains has been little studied although evident differences in the a bility to metabolise such compounds exist between the human and rumen micro flora. (C) 1999 Society of Chemical Industry.