Steering protein fermentation by adding fibres in pig diets

Protein fermentation is associated with diarrhoea in pigs through the presence of potentially toxic metabolites, including ammonia, branched−chain fatty acids, biogenic amines, and indolic and phenolic compounds. One approach to reduce these metabolites is the dietary inclusion of fibres. In this experiment, 128 boars (mean BW ± SEM; 24 ± 1.5 kg) were assigned to diets without added fermentable fibres (NFF), with rapidly fermentable fibres (RFF), or with slowly fermentable fibres (SFF), using either bovine collagen (BC) or zein (ZE) as the only, low-digestible protein source. In addition, two control groups received easily digestible whey protein isolate (WPI) either with NFF or SFF. After 2 weeks, digesta contents from the ileum, cecum, proximal- and distal colon were analysed to assess protein digestibility, digesta mean retention time, and the concentrations and flows of protein-derived metabolites. Metabolite flow was used as a proxy for the extent of protein fermentation, reflecting the net appearance of metabolites, while the concentrations provided insight into the direct exposure of the intestinal epithelium to these metabolites. Metabolite flow was generally linked to the precursor amino acid profiles of the protein sources. Apparent ileal protein digestibility did not differ significantly between BC and ZE, except when RFF were added (Fibre × Protein < 0.05), where it was lower in ZE-fed pigs (P < 0.05). Both fibre treatments, but particularly SFF, reduced the concentrations of most protein-derived metabolites. The effects of RFF and SFF on colonic metabolite flows varied depending on metabolite type and protein source. In the proximal colon of ZE-fed pigs, dietary fibres reduced the flows of branched−chain fatty acids (BCFA; RFF: 26%, SFF: 33%), indole (RFF: 70%, SFF: 59%), and p-cresol (SFF: 28%), while no effects were observed in BC-fed pigs (Fibre × Protein < 0.05). No fibre effects on the flows of ammonia, biogenic amines, valeric acid, phenol, and skatole were observed in either BC- or ZE-fed pigs. In WPI-fed pigs, SFF significantly increased the flows of BCFA (47%), p-cresol (75%), and biogenic amines (120%; all P < 0.05) in the proximal colon. It can be concluded that protein fermentation primarily occurs in the proximal colon, starting in the ileum, with metabolite production generally linked to amino acid precursor composition. While dietary fibres reduced the concentrations of most protein-derived metabolites, the ability of different fibres to steer the extent of protein fermentation is strongly dependent on the source of proteins being fermented.