Immune System Stimulation (ISS) Alters the Protein Deposition (PD) and Increases Dietary Threonine Requirements of Growing Pigs

In previous studies we have shown that systemic ISS decreased apparent ileal digestibility of dietary amino acids (AA). Furthermore, we have shown that ISS increased the expression of Thr rich mucin 2, a major component of intestinal endogenous AA loss (EAAL), at the transcriptional level. We also observed a 1.5 times increase in plasma Thr flux, a measure of Thr utilization, in ISS pigs, suggesting an increased metabolic demand for Thr during ISS. To our knowledge, the effect of ISS on the dietary requirements for Thr has not been evaluated. Therefore, the current study evaluated the impact of ISS on dietary Thr requirements in growing pigs. A total of 39 gilts (initial BW 32 ±2.1 kg) of commercially relevant genetics were individually housed in metabolism crates, and fed a corn-SBM based diet in which Thr was the first limiting among other AA. After 4-d adaptation to the experimental diets, ISS was induced by two intramuscular injections of increasing amounts of E.coli lipopolysaccharide (ISS+; 25 and 35 µg/kg BW) given 48-h apart. Pigs in the ISS- group received sterile saline. Pigs within each ISS group were assigned to three diets containing 70, 90, and 110 % of SID Thr levels that were estimated based on the potential of each group for PD using NRC Swine, 2012 model for estimation of nutrient requirements. Blood samples were collected 24-h post-ISS and assayed for blood chemistry. Body temperature (BT) was also monitored on a daily basis. N-balance was determined during a 3-day post-ISS period. Results were analyzed in SAS using a completely randomized design and mixed procedures. ISS increased BT (0.7ºC SE 0.15), BUN (10mg/dl SE 3.6), and blood creatinine (0.7 mg/dl SE 0.08) indicating an effective ISS in pigs (P < 0.05). SID Thr intake increased PD in a linear fashion in both ISS groups (P<0.01). Marginal efficiency of SID Thr utilization for PD, represented by the slope, was not affected by ISS; 1 g of additional SID Thr intake supported 28 and 20 SE 3.9 g/d PD in the ISS+ pigs and ISS- pigs, respectively. However, ISS substantially increased the extrapolated maintenance SID Thr requirements, represented by the intercept at zero PD (ISS- vs. ISS+, -11.2 vs. -56.3 SE 14.8; P<0.05). These results suggest that ISS increases the dietary requirements for SID Thr due to an increase in maintenance requirements. The latter may be associated with increased intestinal EAAL during ISS.