Dietary protein - from amino acid supply to bioactive peptides
Over the years, research in animal nutrition has moved from describing the protein content of feedstuffs to the amino acid content, through to the absorption and metabolic utilisation of amino acids. Recently, attention has turned towards describing peptides that can be derived from food proteins during natural digestion.
Dr Paul Moughan, the author of this article, intends to draw largely upon results from his own laboratory at Massey University, New Zealand, in an attempt to highlight developments in amino acid and peptide nutrition.
Current understanding on how to describe the amounts of amino acids in diverse feedstuffs and their 'availability' to monogastric animals will be outlined, leading on to a discussion of bioactive peptides, which is an area that may very well herald a 'new frontier' in animal research.
The work of W.C. Rose and co-workers in the early 1950s led to the definition of essential (indispensable; must be ingested) and non-essential (can be synthesised in the body) amino acids. Consequently, much emphasis has been placed on essential amino acids considered to be the most limiting in formulated diets.
More recently, however, the essential/non-essential classification has been queried, as it does not allow for gradations of essentiality brought about by different physiological circumstances, and the distinction is highly dependent upon evaluation criteria.
Whereas the formulation of diets based on gross amino acid content was a major advance over crude protein, so too was a move in the early 1980s towards formulation based on digestible amino acids.
Not only does amino acid content of feeds vary, but also amino acid digestibility varies, often quite markedly, both within and across feedstuffs.
Traditionally, amino acid digestibility measurement was based on analysis of the faeces or in the chicken, excreta. However, because of the profusion of microorganisms in the hindgut that metabolise protein entering from the small intestine, faecal output does not accurately reflect excretion of unabsorbed dietary and endogenous (of body origin) amino acids.
The flow of amino acids at the terminal ileum is more representative of the unabsorbed dietary amino acid flow; and development of ileal digestibility measures has been another milestone in animal nutrition.
Little is known of the potential effect of microorganisms present in the upper tract on the derivation of ileal digestibility coefficients; and this is a topic urgently requiring investigation.
When dietary amino acid digestibility is determined at the end of the small intestine, it is important that correction is made for endogenous amino acids, as endogenous protein makes up a sizeable proportion of the protein present in ileal digesta.
Absorption (measured at terminal ileum) of reactive lysine has been determined in a recent study (Moughan et al., 1996) with the growing pig. The FDNB method allowed accurate assessment of the amount of chemically reactive lysine, which was grossly overestimated by conventional amino acid analysis (acidhydrolysed lysine), but the reactive lysine was not completely absorbed.
In conclusion, the ability to formulate diets in such a way as to supply the animal with a balanced array of available amino acids for body protein synthesis is central to efficient animal production.
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Article made possible through the contribution of Nottingham University Press.