Caecal protein fermentation in broilers: a review

Abstract

SUMMARY: Protein fermentation (PF) is the degradation of protein by microbiota in the gastro-intestinal tract. It results from high intake of indigestible protein and/or increased endogenous losses, and it may be the cause of gut health issues. This is important as the use of less digestible protein sources for poultry is expected to increase as a consequence of the food-feed discussion. Here we review the relations between dietary protein, caecal PF and gut health and identify critical knowledge gaps. Finally, we suggest methods for the investigation of caecal PF. The majority of the microorganisms have never been cultured, however, through cultivation-independent molecular approaches, many new taxa have been identified. Researchers have identified taxa that are enriched in healthy/ unhealthy birds. The mechanisms underlying these associations remain unclear. PF results in the production of potentially detrimental metabolites. This generally results in a higher pH, further encouraging PF. Studies on the effects of PF (metabolites) on gut health in poultry are limiting. For the in vivo evaluation of PF an increase in protein flow into the caeca is required, which can result from an increased level of dietary indigestible protein. Heat damage reduces protein digestibility and can therefore be used to create a within ingredient contrast for in vivo studies. A remaining challenge is that the relation between indigestible protein level and subsequent PF is not straightforward, as fractional separation of digesta occurs in poultry, allowing part of the digesta to bypass the caeca. To further study the extent to which microorganisms will ferment the protein fraction flowing into the caeca, in vitro studies can be applied. However, their application depends on the ability to separate the fraction of pre-digested feed that is likely to enter the caeca. Altogether, an increase in PF will affect microbiota composition, metabolite production, and potentially gut health.