Ideal Amino Acid Profiles in Diets for Broiler Chickens Remain Elusive Targets

Amino acids are the ‘building blocks’ of protein, including both feathers and whole-body protein, and serve numerous functional roles, including energy provision. Amino acid requirements for broiler chickens are expressed as ideal amino acid ratios relative to digestible lysine. However, ideal amino acid profiles in diets for broiler chickens are likely to remain elusive targets. This review considers the factors contributing to this situation. Just one complicating factor is that amino acids may be deposited as protein in either feathers or whole-body protein. However, in the partitioning of amino acids, it appears that preference may be given to feathering, which requires clarification. A second complicating factor is the transition of amino acids across enterocytes of the small intestinal gut mucosa, and small intestinal uptake of amino acids may be rate-limiting for growth performance. Protein-bound amino acids may be absorbed as oligopeptides or monomeric amino acids, and non-bound amino acids do not require digestion prior to their intestinal uptake. The transition of amino acids across the gut mucosa is involved because amino acids may be diverted into anabolic and/or catabolic pathways and are thereby denied entry to the portal circulation. The metabolic fate of amino acids in the gut mucosa may be subject to manipulation by nutritional strategies, but our comprehension of the relevant metabolism within avian enterocytes is inadequate. The post-enteral availability of twenty proteinogenic amino acids is complex with important implications for the efficiency of utilisation. Importantly, acceptably accurate amino acid profiles will be advantageous for sustainable chicken-meat production into the future.