Biochemical and physiological limitations to efficiency of amino acid utilization for animal growth
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Multiple pathways exist for the utilization of amino acids by the animal. They are building blocks of proteins and essential precursors for the production of biologically important nitrogenous substances (including creatine, dopamine, glutathione, heme, nitric oxide, polyamines, serotonin, and taurine). These physiological processes are obligatory for the reproduction, health and survival of organisms. Additionally, syntheses of large amounts of arginine, proline, aspartate, glutamine, and glycine (whose provision from conventional diets is inadequate for tissue protein synthesis) via inter-organ cooperation necessitate the degradation of nutritionally essential amino acids (EAA). Thus, when diets do not contain adequate NEAA, EAA catabolism fulfils an important purpose in growing animals. Moreover, intestinal microbes extensively utilize both EAA and nutritionally nonessential amino acids (NEAA) for protein synthesis to maintain their normal population and activity. Due to these biochemical events, efficiency of utilization of dietary proteins for animal growth is far below 100%. For example, under current feeding programs, only 70% and 55% of dietary amino acids are deposited as proteins in 14-day-old pigs reared by sows and 30-day-old pigs weaned at 21 days of age, respectively. The remaining amino acids are degraded to CO2, NO, CO, H2S, methane, H2O, ammonia, urea, nitrate, and other nitrogenous metabolites. Because current diets contain substantially more EAA, but less NEAA, than needed for protein accretion in growing pigs, supplementing NEAA and prebiotics to a low-protein diet may be effective in improving efficiency of amino acid utilization and supporting optimal growth performance of pigs.