Amino acid requirements and amino acid supply in the sheep.

The special features of nitrogen metabolism in ruminants, which stem from the extensive breakdown and synthesis of dietary protein in the rumen (see Hungate, 1966), imply the absence of a specific dietary requirement for essential amino acids. Nevertheless, there is considerable theoretical and practical interest in defining the amino acid requirements of ruminant tissues in animals of known nutritional and physiological status. Direct methods of measurement based on the growth or production response of animals to varying intakes of amino acids are clearly inapplicable, and it is necessary to use indirect procedures. The successful development of re-entrant fistulas has made it possible to measure the uptake of amino acids from the small intestine and to use the duodenal fistula to supplement amino acid supply. In this paper methods of measuring amino acid requirement in sheep based on the changes in plasma amino acid (PAA) concentration or oxidation in response to different levels of amino acid supply are discussed in relation to quantitative results of amino acid uptakes obtained using re-entrant intestinal fistulas. Interest in relationships between PAA patterns and amino acid supply arose from observations that post-absorptive changes in PAA concentrations were qualitatively dependent upon the amino acid composition of the ingested protein (see Almquist, 1956). Subsequent studies showed that a PAA reference pattern could be used to determine the first limiting amino acid for growth (Smith & Scott, 1965; Dean & Scott, 1966). Several workers have shown that in non-ruminants a suboptimal dietary content of an essential amino acid results in a low plasma concentration, and that a dietary excess results in a high plasma level (Zimmerman & Scott, 1965; Mitchell, Becker, Jensen, Harmon & Norton, 1968; Keith, Christensen & Owen, 1972). When increasing amounts of the first limiting amino acid were fed, the plasma concentration remained low until the amount required for maximum growth was exceeded, when a sharp and linear increase in concentration was observed. The point of intersection of the two lines obtained when amino acid intake was plotted against PAA concentration has been accepted as a measure of amino acid requirement under the over-all conditions of the experiment. Results of animal performance in the pig provide support for the validity of the procedure (Keith et al. 1972). Wakeling, Lewis & Annison (1970) applied the technique to the measurement of