Comparative amino acid requirements.

It is proposed to look, not at absolute requirements, but rather at the optimum pattern of essential amino acids (EAA) needed and the optimum value for the ratio EAA:total useful crude protein supply (CP; Nx6.25) when the supply is adequate but not excessive. For monogastric mammals and poultry it is clearly of practical value to try to assess requirements in the diet’. However, results may vary greatly (Hegsted, 1973; Bodwell, 1977) and for the ruminant they are valueless because of profound modification of AA supply in the reticulo-rumen. A more useful site for comparing requirements is the small intestine, particularly if available (i.e. absorbed) AA are considered. Estimates of AA requirements for deposition in tissues, secretions or products provide fairly solid ground for making comparative assessments and it is plausible to suppose that they are related to requirements in the small intestine or the diet (Munks et al. 1945; Williams et al. 1954; Hutton & Annison, 1972; Buraczewski, 1973; Hurwitz et al. 1978). However, for reliable predictions it is necessary to understand the ways in which AA are transported and metabolized between the gut and their final destination. In this paper, present knowledge of these processes and the possibilities of using factorial methods to try to identify differences in EAA requirements for a number of different species and physiological states are examined in Table 2. EAA include leucine (Leu), isoleucine (Ile), valine (Val), methionine (Met), phenyl alanine (Phe), threonine (Thr), lysine (Lys), tryptophan (Trp) and histidine (His). In addition consideration is given to total sulphurcontaining amino acids (S-AA; Met and Lys) and to total aromatic amino acids (Ar-AA; Tyr and Phe). Histidine is included although apparently not essential for adult man (Irwin & Hegsted, 1971). An excellent review of qualitative and quantitative comparative AA requirements for a variety of animals has recently been published (Buttery, 1979).