Interaction between dietary tryptophan and pyridoxine on tryptophan metabolism, immune responses and growth performance in post-weaning pigs

Several aspects of tryptophan (Trp) metabolism are dependent upon pyridoxine (B6) status. The present experiment aimed to determine if Trp metabolism and the eventual growth responses to dietary supplementation of Trp is modulated by dietary B6. Exploratory measurements related to the modulation of the immune system were also made. Piglets weaned at 15 to 18 days of age (n = 544) were grouped in 32 pens of 17 animals and distributed in a factorial arrangement of treatments with two dietary additions of B6 [0 (B6−) or 5 mg/kg (B6+)] and two dietary additions of synthetic Trp [0 (Trp−) or 0.5 g/kg (Trp+)]. Basal dietary concentrations of B6 varied according to age, between 2.1 and 3.5 mg/kg, and for Trp, between 1.9 and 2.5 g/kg. Growth performance was recorded every week from 2 to 9 weeks of age. Blood samples were taken from two piglets in each pen at 2, 4, 6 and 8 weeks of age. Another piglet in each pen was immunized twice with ovalbumin emulsified with incomplete Freund adjuvant to evaluate the antibody response. No treatment effect was observed on feed intake, body weight gain or feed conversion ratio for the whole experimental period but feed conversion increased more drastically in Trp− than in Trp+ piglets during the last week of experiment (B6 × Trp × age, P<0.05). Pyridoxal, an active B6 vitamer, was 19% lower in red blood cells of B6− than B6+ piglets (B6 × age, P<0.01). Plasma Trp and kynurenine (an intermediate metabolite of Trp oxidation) decreased with age (P<0.01). Plasma nicotinamide, an indicator of niacin status, increased with age and remained approximately 45% higher in Trp+ piglets than in Trp− piglets (Trp × age, P<0.01). Antibody response to ovalbumin was enhanced in Trp− piglets (Trp × age, P<0.01). The lymphocyte proliferative response to Concavalin A was lower (P<0.05) in B6+ than in B6− piglets. The percentage of B lymphocyte population at 4 and 6 weeks of age dropped in B6− animals whereas it remained stable in B6+ piglets (B6 × age, P<0.05). In spite of late and short changes of feed conversion ratio (interaction B6 × Trp × age), the results indicate that, globally, within the present dietary levels, pyridoxine is not a major factor for Trp metabolism and therefore, for Trp requirements of post-weaning piglets. The absence of interaction between B6 and Trp on the present immunological indicators suggest that each nutrient modulates the immune response through independent mechanisms.