IMPACT OF METABOLISM BY EXTRAGASTROINTESTINAL TISSUES ON SECRETORY RATE OF MILK-PROTEINS

This paper reviews the current state of knowledge about the postabsorptive utilization of AA. Data on the duodenal entry of AA and the projected output of these same AA as milk protein demonstrate significant losses. Within the essential AA, these losses range from 70% for Thr to 40% for Met and Lys. For Val, almost half of the loss is due to its use for nonsecretory purposes by the mammary gland; values decrease to 0% for other AA. The liver is the other major organ, apart from the portal-drained viscera, that is responsible for significant AA disposal. Interactions of Met metabolism with other methyl sources and gluconeogenic precursors that can alter the Met requirement also are discussed. Data on the transhepatic and transsplanchnic AA balance in lactating cows, and the coordinated use of these balances with duodenal flow and mammary balance, are needed. Further research also is needed into the functionality of use of AA for purposes other than milk protein synthesis to determine whether excess catabolism of AA occurs; assuming it does, more efforts are needed to identify regulation of AA disposal in the crucial tissues.