An analysis of the hypothesis that amino acid requirements for chicks should be stated as a proportion of dietary protein

Abstract

This is a review of previously published data from experiments designed to measure the chicks' requirement for an essential amino acid in diets containing surplus protein (generally in the range 220–300 g crude protein (CP)/kg diet). The evidence shows that, within this range, the requirement for a first limiting amino acid increases nearly in direct proportion to the CP content of the diet. To explain this, the following possibilities are considered: (1) that energy supply was limiting the response to a critical amino acid, (2) that the experiments were conducted in environments which limited heat disposal by the chicks, (3) that results from the trials have been misinterpreted because growth rate rather than protein deposition was used as a response measure, (4) that the availability of amino acids in the diets was lower than had been assumed, (5) that nutrients other than the amino acid under study were limiting performance, and (6) that there was an imbalance of amino acids in the diet. It is concluded that only the last explanation fits the facts. The imbalances reported have led to a decline in the efficiency of the utilisation of the first limiting amino acid in some instances. The depressed growth observed with a high-protein diet limiting in one essential amino acid could not be attributed to lowered feed intake in most cases. The imbalance is, in these respects, different from classical imbalances which result from loading the diet with a mixture of essential amino acids. From the evidence reviewed, some doubt is cast on the validity of the dilution method for estimating amino acid requirements; nevertheless it its concluded that the dilution method remains the most trustworthy procedure for making estimates to be used in practical diet formulation. It is recommended that, in future, programmes for diet formulation should be modified to prevent surplus protein being permitted in the solution, unless a proportional adjustment is made to the minima prescribed for amino acids likely to be present in limiting proportions.