ARTICLE FEED & ADDITIVE MAGAZINE April 2023 77 all physiologically and nutritionally essential AAs. Improved patterns of AAs in diets for swine and chickens as well as zoo and companion animals have been proposed in recent years. Animal-sourced feedstuffs supply abundant EAAs and AASAs (including glutamate, glutamine, glycine, proline, 4-hydroxyproline, and taurine) for diets of swine, poultry, fish, and crustaceans to improve their growth, development, reproduction, and health, while sustaining global animal production. The design of the study, the chosen response criterion, and the statistical model affect the requirement estimate. Considerable experimental work has been carried out to determine the requirements for Lysine, Metionine, Threonine, and Trptophon in growing pigs. STRUCTURES AND ROLES OF AMINO ACIDS Providing high quality animal-derived proteins for human nutrition is an essential role of animal production. Amino acids are the building blocks of protein, which are composed of an amino group (-NH2), a carboxyl group (-COOH) and a side chain specific for each AA. Proteins are polymers of AA where the carboxyl group of one AA reacts with the amino group of another AA. In the 1930s, William Rose at the University of Illinois brought forward the idea that “some elements in proteins” are essential constituents of the diet and he later discovered Thr as being one of these elements. The AA Lysine, Metionine, Threonine, Trptophon, Phenylalanine, Histidine, Valine, Ile, and Leucine are (dietary) essential or indispensable AA because the pig does not have the metabolic capacity to synthesize the carbon chains of these AA. The carbon chains of Ser, Glycine, Arginine, Alanine, Proline, Glucine can be synthesized de novo and these AA are therefore called (dietary) non-essential AA. Although the pig has the potential to synthesize these AA, this does not mean that the synthesis capacity is sufficient to fulfil the requirements. Arginine is often considered as one of the non-essential AA for which the synthesis capacity may be insufficient in (young) pigs. The total dietary supply of protein must be sufficient to provide the necessary nitrogen required for the synthesis of non-essential or semi-essential amino acids. Because of their role in protein synthesis, the requirements for AA for growth depend, quantitatively, to a large extent on protein deposition. Ideal protein concept refers to a situation where all essential AA are co-limiting for performance so
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