Refinement of the concept of ideal protein for the growing pig

Abstract

The present review was intended to provide recent information on the relationships between dietary amino acids in growing pigs, through the concept of &dquo;ideal&dquo; protein. The limits of this concept were given, namely with regards to the constant ratios between essential amino acids and lysine, as a reference. The ratios between secondary limiting amino acids (tryptophan, threonine, methionine and sulphur amino acids) and lysine were considered, according to their specific functional role in metabolic and physiological processes. For this purpose, the effects of the deviations between amino acid profiles of dietary and ideal protein (excess protein) on voluntary feed intake and growth performance were investigated, in relation to the most limiting amino acid. Owing to its poor contribution to metabolic processes, lysine is a stable reference for expressing ratios between essential amino acids. Unlike lysine, dietary tryptophan interacts heavily and negatively with excess protein (large neutral amino acids), through brain serotonin dysfunction, in relation to depressed appetite and growth. In practice, the tryptophan / lysine ratio should not be considered as a constant. It should be corrected on the basis of a minimum ratio between tryptophan and large neutral amino acids of 4 %, in order to prevent an eventual risk of their excess in some dietary protein. In the case of threonine, when it is limiting, a positive interaction with non essential amino acids (glutamic acid) was reported. Dietary balance between methionine and cystine was re-evaluated in accordance with the recent data from literature. To conclude, the revised essential amino acid to lysine ratios for the growing pig were given.