Partitioning of nutrients for growth and other metabolic functions: efficiency and priority considerations.

The raw materials for proteinaceous growth in the avian are amino acids supplied from protein ingestion and from body protein degradation. Maximal efficiency of utilization (i.e., retention) of absorbed nitrogen approximates 76%. This occurs when a well-balanced mixture of amino acids is ingested. The utilization efficiency of individual dietary amino acids varies around the 76% figure, with slow-turnover amino acids such as lysine being used more efficiently (80%) than fast-turnover amino acids such as isoleucine (61%). A paucity of information exists on metabolic priorities for amino acids at various levels of amino acid deficiency. It is clear, however, that dietary histidine is used with priority for protein synthesis before it is shunted toward carnosine or anserine biosynthesis. Likewise, cysteine use by chicks for growth has priority over cysteine use for glutathione (GSH) biosynthesis. With a cysteine-free diet containing the minimal methionine requirement for maximal growth, hepatic GSH is depressed and remains low and unchanged until about 30% of the growth need for cysteine is satisfied, at which point GSH begins accumulating rapidly. In rats, liver GSH begins accumulating rapidly even with very small doses of cysteine. This suggests that the metabolic priority for cysteine in rats is as great for GSH synthesis as it is for growth.