Gut sensing of dietary amino acids, peptides and proteins, and feed-intake regulation in pigs

Amino acids (AA) play key metabolic roles in the regulation of feed intake, growth, and immunity among other physiological functions. Dietary AA supplements (i.e. non-protein-bound synthetic AA) and proteins (through peptides and free AA coming from digestion) regulate appetite by stimulating the release of gut hormones from enteroendocrine cells located throughout the gastrointestinal tract, via activation of transmembrane receptors of the G-protein coupled transmembrane receptor (GPCR) family, including calcium sensing receptor, GPCR of the class C Group 6 Member A (GPRC6A), T1R1–T1R3 of taste receptor family 1 and lysophosphatidic acid receptor 5 (GPR92). In addition, growing evidence supports the role of AA transporters in mediating AA sensing in the gut. While most of the current literature available on the topic relates to research undertaken in laboratory rodents, there has been a growing demand on the confirmation of these physiological mechanisms in pigs. In pigs, limiting essential AA L-tryptophan, and branched-chain AA L-leucine and L-isoleucine were shown to stimulate anorexigenic hormones cholecystokinin (CCK) and glucagon-like peptide 1 mediated by calcium sensing receptor and T1R1–T1R3, using primary tissue cultures. Tryptophan was shown to stimulate the orexigenic hormone ghrelin in pigs. In addition, L-lysine and L-methionine have also been shown to affect feed intake. Other free AA affecting CCK and glucagon-like peptide 1 levels in pigs are L-phenylalanine and L-arginine. Similar functions have been reported for some dietary proteins such as casein, lectin, or whey and for protein hydrolysates. Overall, this review uncovers recent evidence that dietary non-protein-bound AA are efficient modulators of appetite on the basis of their strong effects on orexigenic and anorexigenic hormones in pigs. However, little has been explored to assess the relevance of essential versus non-essential dietary AA impact on feed intake. Further understanding of the role of AA and proteins on appetite modulation and the mechanisms behind gut hormone release could prove to be a relevant tool to improve swine nutrition and performance.