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 modiΓΌed forestomach of ruminants and the hindgut of non-ruminant species, including humans. In the rumen, rapid hydrogenation of p-coumaric, ferulic and caΓΎ eic acids, followed by dehydroxylation at C4 and more slowly at C3 yields 3-phenylpropionic acid. Phenylpropionate is absorbed and undergoes boxidation 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, hydroxycinnamates 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 b-oxidation product, 4-hydroxybenzoic acid. CaΓΎ eic acid and its methyl ethers (ferulic and iso-ferulic acids) are interconvertable and share metabolites. As in the rumen, reduction of the side-chain, demethylation of C 3 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 compounds have been detected in human urine. Metabolism of C 6 -C 1 hydroxycinnamate dimers found as cross-links between polysaccharide chains has been little studied although evident diΓΎ erences in the ability to metabolise such compounds exist between the human and rumen microΓ½ora.