Immune system stimulation reduces the efficiency of tryptophan utilization for body protein deposition in growing pigs

The effect of immune system stimulation (ISS) on N retention and Trp utilization in pigs fed Trp-limiting diets was evaluated using 36 growing pigs (20.0 +/- 1.1 kg BW; 3 blocks of 12 barrows). Pigs were randomly assigned to 1 of 5 diets (Diet 1, 2, 4, and 5, n = 7; Diet 3, n = 8) and fed restrictively at 800 g/d. Diets 1 to 4 were generated by blending Diet 1 with a protein-free supplement and were calculated to contain varying amounts of standardized ileal digestible (SID) Trp (1.31, 1.05, 0.80, and 0.55 g/kg). To confirm that Trp was the first-limiting AA in Diets 1 to 4, an additional diet was used (Diet 5), which was equivalent to Diet 4 and contained 0.34 g/kg of added Trp. After a 5-d adaptation period, pigs were injected every 2 d with increasing amounts of E. coli lipopolysaccharide to induce ISS (initial dose 20 mu g/kg BW, increasing 15% each subsequent injection). Whole body N balance was measured in 3 periods: before immune stimulation (pre-ISS) and during ISS in 2 subsequent periods (ISS-1, 3 d; ISS-2, 4 d). Regression analysis was used to estimate the marginal efficiency of Trp utilization for whole body protein deposition (PD; N retention x 6.25). Plasma concentrations of acute-phase proteins and white blood cell counts increased (P < 0.001) and plasma albumin decreased (P < 0.001) during ISS. Nitrogen retention increased (P < 0.001) as Trp intake increased. Nitrogen retention was numerically greater but not statistically different between Diet 5 (added Trp diet) and Diet 4. Whole body N retention was less (P < 0.05) during ISS due primarily to an increase (P < 0.05) in urinary N excretion. There was a linear response (P < 0.05) in N retention, urinary N, and total excreted N to increasing Trp intake. Protein deposition increased by 88.2 +/- 5.2, 82.5 +/- 5.1, and 92.5 +/- 3.4 g/d for each additional g/d of SID Trp intake during pre-ISS, ISS-1, and ISS-2, respectively, but the intercept was not different (-32.3 g/d). The slope of the response of PD to increasing Trp intake (based on the common intercept) was less during ISS-1 compared with pre-ISS (P = 0.01) or ISS-2 (P = 0.002) but not different between pre-ISS and ISS-2. Immune system stimulation reduced N retention in pigs fed limiting dietary Trp. The efficiency of Trp utilization for protein deposition was also reduced during ISS, indicating that the Trp requirement for PD is increased similar to 7% during an inflammatory state.