INTERACTIVE EFFECT OF DIETARY FIBRE AND IMMUNE CHALLENGE ON THREONINE REQUIREMENT AND INTESTINAL BARRIER FUNCTION IN GROWING PIGS

High dietary fibre (DF) and immune system stimulation (ISS) are thought to limit amino acid availability for protein deposition (PD) and growth in pigs. Fibre and threonine (Thr) may also play an important role in intestinal barrier function. Therefore, this thesis evaluated the independent and combined effects of high DF and immune challenge [Salmonella typhimurium and systemic E. coli lipopolysaccharide (LPS)] on the Thr requirement for PD and growth in pigs, and on the interactive effects of DF, Thr supply and immune challenge on intestinal barrier function. A nitrogen-balance study estimated 0.68% and 0.78% standardized ileal digestible (SID) Thr required to maximize PD in pigs fed low fibre (LF) and high fibre (HF) diets, respectively when systemic ISS was not present. When systemic ISS was present, SID Thr requirement for PD was estimated at 0.76% and 0.72% for pigs fed the LF and HF diets, respectively. Therefore, HF and ISS independently, but not additively, increased the Thr requirement to maximize PD. A subsequent growth performance study using the same HF diet estimated Thr required to maximize average daily gain (ADG) at 0.76% and 0.80% SID Thr using the linear and curvilinear breakpoint model respectively. In a third study, supplementing Thr to meet the requirement for HF and systemic ISS, resulted in a numerically lower ADG in the HF-fed and Salmonella-challenged pigs, compared to the LF-fed and Salmonella-challenged pigs. This suggested that Thr supply to meet HF and ISS was not sufficient to maintain ADG during an enteric immune challenge and therefore, indicates an additive effect of HF and enteric immune challenge on Thr requirement. Finally, systemic ISS increased lactulose recovery in LF fed pigs but not in HF fed pigs, suggesting that feeding HF had a protective effect against loss of intestinal barrier integrity. This effect appears to be partly associated with mucus secretion in the gut, as HF increased fecal mucin output and ileal intestinal goblet cell numbers and tended to increase MUC2 gene expression in the ileum. The non-additive effect of systemic ISS and HF on PD is consistent with the LPS induced loss of barrier function in the LF fed pigs which contributed to increased Thr requirement for PD. Indeed, no loss of barrier function was observed when systemic ISS and HF were combined, hence no further increase in Thr requirement was observed. In contrast, we postulate that an enteric immune challenge and HF diet resulted in a higher magnitude of impact on gut mucosal protein dynamics that exceeded the gut mucosal protein response to the effect of HF alone, resulting in increased Thr utilization to support mucosal protein synthesis and thereby increasing dietary Thr requirement for growth. In summary, results indicate that immune challenge and high DF will increase Thr requirement for growth, but DF will have beneficial effects on improving intestinal barrier function in pigs