Increasing dietary tryptophan and decreasing other large neutral amino acids increases weight gain and feed intake in weaner pigs infected with Escherichia coli

Manipulating endogenous production of neurotransmitters in the peri-weaning period by increasing the ratio of tryptophan (Trp) to other large neutral amino acids (LNAA) in the diet increases serotonin production and dopamine metabolism in the brain (Fernstorm 2013). Tryptophan is a precursor for the synthesis of serotonin, a neuromediator associated with appetite regulation and down-regulation of the hypothalamic–pituitary–adrenal axis (Le Floc’h and Seve 2007). Other LNAA compete with Trp to cross the blood–brain barrier, therefore regulating LNAA in plasma, which can influence Trp availability and thus serotonin biosynthesis (Shen et al. 2012). In this study it was hypothesised that increased supplementation of Trp and/or reduction in LNAA, to increase the Trp:LNAA ratio, in diets for weaned pigs experimentally infected with enterotoxigenic E. coli (ETEC) would improve growth performance and reduce cortisol levels.

A total of 96 male weaned pigs (Large White x Landrace) with the Mucin 4+ allele (affecting resistance to E. coli infection) were individually housed and allocated into treatments based on weaning weight, sow parity and location in the building (eight treatments × 12 pigs = 96 pigs). The study was designed as a 2 × 4 factorial arrangement with respective factors being without/with ETEC infection and four dietary Trp:LNAA (LNAA: tyrosine, valine, phenylalanine, isoleucine and leucine) ratios (Table 1). Pigs in the infection group were inoculated with 0.8 mL of ETEC (serotype O149; K88) solution in two gelatinised capsules, on d 7 and 8 after weaning. Faecal consistency score, diarrhoea index, faecal β-haemolytic E. coli shedding and number of therapeutic antibiotic treatments were recorded. Blood samples were collected on d 6, 9 and 14 from eight pigs per treatment, plasma cortisol was assessed using ELISA (Enzo Life Sciences, NY, USA). Data were analysed by two-way ANOVA using SPSS (v21, IBM, Armonk, NY, USA).

Table 1.  Diet description and analysed standardised ileal digestible Trp:LNAA composition

Diet 4, with the highest Trp:LNAA, had higher ADG (P < 0.05) during d 8 to 14 and 15 to 21 periods (Table 2), and from d 0 to 21 when compared to Diet 3 and Diet 1. During d 8 to 14, pigs in the infection group grew more slowly (P = 0.04) than their non-infected counterparts, and had increased incidence of diarrhoea (60.4% v. 39.6% respectively; P = 0.017). Between d 15 and 21, ADFI was higher in pigs fed Diet 4 compared to Diets 1 and 3 (527 g v. 429 g and 438 g, respectively; P = 0.021). Plasma cortisol at d 9 was higher in ETEC pigs (11.9 v. 16.3 ng/mL respectively; P = 0.05), but there were no dietary differences. Data suggested that increased dietary Trp and reduction in LNAA (Diet 4) for weaned pigs improved ADG and ADFI irrespective of infection with ETEC or not, but did not modulate the stress response, as assessed by cortisol levels.