Maximum nutritional response to poor-quality protein and amino acid utilisation

1. Although the theory of responses to amino acids suggests that, providing sufficient of the limiting amino acid is fed, it should be possible 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 interpretation conflicts with the theory of general amino acid imbalance which proposes that amino acid excesses do not impair the utilisation of the limiting amino acid. 3. Three protein mixtures of different quality were made by mixing maize gluten meal and soyabean protein concentrate in constant proportions, supplementing with tryptophan, threonine and arginine to adequacy and varying amino acid score (0.62, 0.71 or approximately 10) by varying additions of free lysine. The 3 mixtures were diluted with protein-free ingredients to produce 3 diet series, each providing 3.7, 6.5, 9.2, l2.0, 14.8 and 17.5 g lysine per kg. Each diet was fed to 4 cages of 2 chickens each from 4 to 14 d of age in a randomised block experiment. Food intake, body-weight and body-nitrogen gain were measured. 4. Differences in protein quality were confirmed by regression analyses of bodyweight response to protein intake (Net Protein Ratio) and body-nitrogen response to nitrogen intake (Net Protein Utilisation) in the linear range. Regression analyses in the linear range of body-weight or body-nitrogen response to lysine intake showed no adverse effect of protein quality on lysine utilisation. Curvilinear analysis (Reading flock response model) confirmed this finding. 5. Maximum response could not be obtained with the poorest protein quality. It is illogical to invoke impaired utilisation of the limiting amino acid to explain this. A small decrease in net energy yield of the diet may be sufficient to explain the effect, but it is more likely that the depletion of the limiting amino acid from tissue (muscle) protein which results from feeding poor-quality protein explains the effect.