Effects of supplementing pig diets with tryptophan and acidifier on protein digestion and deposition, and on brain serotonin concentration in young pigs

The objective of the study was to find out whether feeding acidifying preparation with diets containing crystalline tryptophan (Trp) affects availability of this amino acid for protein and serotonin (5-HT) synthesis in young pigs. Two experiments were performed. Supplementation of Trp-deficient diet with 3 g/kg of acidifier (A) (mixture of phosphoric, citric and fumaric acids) did not affect the coefficients of ileal apparent digestibility (CIAD) of protein and protein-bound amino acids except CIAD of Trp which was increased from about 0.65 to 0.73. In pigs of initial body weight 16 kg, the increase of digestible dietary Trp level from 0.77 to 1.39 and 1.85 g/kg improved growth performance, nitrogen retention and utilization of nitrogen absorbed, while the increase from 0.77 to 1.39 g/kg enhanced 5-HT concentration in all six analyzed brain structures. Supplementation with A did not affect blood Trp level in animals fed on Trp-deficient diet while it decreased it considerably on diet supplemented with Trp to 1.85 g/kg. Feeding A affected 5-HT metabolism: decreased its level in five of six brain structures in pigs fed on Trp-deficient diet, changed 5-HT metabolite 5-hydroxy-indolo-3-acetic acid (5-HIAA) and 5-HIAA/5-HT ratio, and modified response of 5-HT to Trp supplementation.It is concluded that feeding A both with Trp-deficient and Trp-supplemented diets has positive net effect on growth and protein deposition in pigs while it seems to affect negatively 5-HT synthesis and, probably, accelerates its turnover. The mechanism of this action is not known, and it was not proved that it is related to destructive effect of acids on crystalline Trp.