Increasing dietary tryptophan in conjunction with decreasing other large neutral amino acids increases weight gain and feed intake in weaner pigs regardless of experimental infection with enterotoxigenic Escherichia coli.

Dietary tryptophan (Trp) is a precursor for serotonin, a neuromediator involved in stress responses. Tryptophan competes with other large neutral amino acids (LNAA: tyrosine, isoleucine, leucine, valine, and phenylalanine) to cross the blood-brain barrier; therefore, the regulation of circulating LNAA can influence Trp availability in the cortex and serotonin biosynthesis. The hypothesis examined in this study was that increased supplementation of dietary Trp and a reduction in LNAA for weaned pigs experimentally infected with enterotoxigenic Escherichia coli (ETEC; F4) will increase Trp availability in plasma and reduce indices of the stress response, which will translate to reduced production losses. At 21 ± 3 d of age (mean ± SEM), 96 male pigs (Large White × Landrace) weighing 6.3 ± 0.98 kg (mean ± SEM) were individually penned and allocated to a 4 × 2 factorial arrangement of treatments, with respective factors being 1) four dietary standardized ileal digestible (SID) Trp and LNAA contents, being HTrpHLNAA (Low Trp-High LNAA; 0.24% SID Trp: 5.4% SID LNAA), HTrpHLNAA (Low Trp-Low LNAA; 0.24% SID Trp: 4.6% SID LNAA), HTrpHLNAA (High Trp-High LNAA; 0.34% SID Trp: 5.4% SID LNAA), and HTrpHLNAA (High Trp-Low LNAA; 0.34% SID Trp: 4.6% SID LNAA), and 2) without/with ETEC infection. Pigs were orally infected with 0.8 mL (3.6 × 109 CFU/mL) ETEC at days 7 and 8 after weaning. Pigs fed diets high in Trp irrespective of the level of LNAA (HTrpHLNAA and HTrpLLNAA) had higher plasma Trp concentrations (P < 0.001) and a Trp:LNAA ratio (P < 0.001) before infection and 6 d after infection. Following infection, noninfected pigs had higher plasma Trp (P = 0.03) and a Trp:LNAA ratio (P = 0.004) compared with pigs infected with ETEC. Plasma cortisol levels after infection were higher in ETEC-infected pigs (P = 0.05) and altering dietary Trp and LNAA concentrations did not influence (P > 0.05) plasma cortisol. Pigs fed diet HTrpLLNAA had higher serum serotonin levels 24 h after infection (P = 0.02) compared with pigs fed diets LTrpLLNAA and HTrpHLNAA. Similarly, pigs fed diet HTrpLLNAA had a higher (P = 0.02) average daily gain during the 3-wk study. Overall, average daily feed intake tended to be higher in pigs fed an HTrpLLNAA diet compared with the other diets (P = 0.08). These results suggest that the increased supplementation of dietary Trp with reduced LNAA increased circulating Trp levels that, in turn, likely caused higher serum serotonin levels, irrespective of infection with ETEC, and improved aspects of post-weaning performance.