MAXIMUM NUTRITIONAL RESPONSE TO POOR-QUALITY PROTEIN AND AMINO-ACID UTILIZATION

1. Although the theory of responses to amino acids suggests that, prov iding sufficient of the limiting amino acid is fed, it should be possi ble to elicit maximum growth response, maximum response is not usually elicited by poor-quality proteins. 2. It has been suggested that this failure to elicit maximum response is a reflection of poorer limiting amino acid utilisation from poor-quality proteins. This interpretatio n conflicts with the theory of general amino acid imbalance which prop oses that amino acid excesses do not impair the utilisation of the lim iting amino acid. 3. Three protein mixtures of different quality were made by mixing maize gluten meal and soyabean protein concentrate in c onstant proportions, supplementing with tryptophan, threonine and argi nine to adequacy and varying amino acid score (0.62, 0.71 or approxima te to 1.0) by varying additions of free lysine. The 3 mixtures were di luted with protein-free ingredients to produce 3 diet series, each pro viding 3.7, 6.5, 9.2, 12.0, 14.8 and 17.5 g lysine per kg. Each diet w as fed to 4 cages of 2 chickens each from 4 to 14 d of age in a random ised block experiment. Food intake, body-weight and body-nitrogen gain were measured. 4. Differences in protein quality were confirmed by re gression analyses of bodyweight response to protein intake (Net Protei n Ratio) and body-nitrogen response to nitrogen intake (Net Protein Ut ilisation) in the linear range. Regression analyses in the linear rang e of body-weight or body-nitrogen response to lysine intake showed no adverse effect of protein quality on lysine utilisation. Curvilnear an alysis (Reading flock response model) confirmed this finding. 5. Maxim um response could not be obtained with the poorest protein quality. It is illogical to invoke impaired utilisation of the limiting amino aci d to explain this. A small decrease in net energy yield of the diet ma y be sufficient to explain the effect, but it is more likely that the depletion of the limiting amino acid from tissue (muscle) protein whic h results from feeding poor-quality protein explains the effect.