Steering Protein Fermentation in Pigs

Protein fermentation in pigs has been associated with diarrhea through the presence of potentially toxic metabolites, including ammonia, branched chain fatty acids, biogenic amines, and indolic and phenolic compounds. Most of the current evidence regarding the impact of these metabolites is, however, derived from in vitro and in situ experiments, which do not accurately reflect the physiological intestinal environment and complex interactions within the animal. In vivo research in this area remains limited. The primary aims of this thesis were to investigate the impact of protein fermentation metabolites on intestinal health in vivo, and to expand our knowledge on various strategies for modulating protein fermentation. The findings of this thesis indicate that although certain protein fermentation metabolites may negatively affect fecal consistency, their overall impact on intestinal health appears limited. Amongst all metabolites measured, only ammonia showed indications that it may adversely affect intestinal barrier function. It was shown that colonic ammonia flow was positively correlated with colonic permeability to 4 kDa FITC-dextran and 40 kDa TRITC-dextran. To study the effects of protein-derived metabolites on intestinal barrier function, the everted gut sac method was used. Typically, intact (non-stripped) intestinal segments are used in this method, however, this may underestimate permeability due to the medial positioning of blood vessels relative to the seromuscular layer and serosa. Removing these layers risks physical damage. In this thesis, the use of intact versus stripped intestinal segments were compared to study intestinal permeability to 4 kDa FITC-dextran and 40 kDa TRITC-dextran, and the absorption of glucose, lysine, and methionine. Based on the results, stripping intestinal segments was recommended. Furthermore, this thesis explored various dietary interventions to modulate protein fermentation. First, methods for quantifying total (basal + specific) intestinal endogenous protein losses were investigated, aiming for improving the accuracy of estimates of availability of dietary amino acids. The most promising approach was calculating the ratio of dietary protein to endogenous protein based on differential amino acid profiles in digesta, endogenous protein, and bacteria. However, qPCR-based methods and DAPA also showed potential, provided these techniques are further refined. Second, the effects of dietary rapidly versus slowly fermentable fibers on protein fermentation were examined. It was found that protein fermentation primarily occurs in the proximal colon, with dietary fibers potentially shifting protein fermentation to more distal compartments or reducing its extent. Dietary fibers reduced the concentrations of most protein-derived metabolites, reflecting direct exposure of the intestinal epithelium to these metabolites, but their ability to reduce the flow of protein-derived metabolites, reflecting the extent of protein fermentation, was strongly dependent on the protein source being fed. Finally, it was observed that housing pigs under low sanitary conditions decreased ileal and total tract protein digestibility with 7 and 9%-units, increased protein fermentation, and decreased fecal consistency. These findings indicate that the impact of dietary interventions on the effects of protein fermentation on intestinal health may be influenced by housing conditions. However, there were no indications that housing under low sanitary conditions aggravated the effects of protein fermentation on intestinal health.