Intestinal Integrity of Broiler Chickens is Negatively Affected only at the Lowest Level of Dietary Protein Fortified with Synthetic Amino Acids

Despite extensive research on reduced protein (RP) diets in broiler chickens, the consequences of such practices for gut health and barrier function are not well defined. This study was conducted to investigate the effect of reduced protein and the source of protein. The control normal protein level diets had either meat and bone meal (MBM) or an all-vegetable diet (VEG). Accordingly, four diets were formulated; 1) VEG standard protein, 2) standard protein containing MBM, 3) RP (17.5% in growers and 16.5% in finisher) and 4) RP diet (15.6% in grower and 14.6% in finisher). Off-sex Ross 308 birds were assigned to each of the four diets and performance measurements were taken from d 7 to 42 post-hatch. Each diet was in 8 replicates (10 birds per replicate). A challenge study was conducted on an additional 96 broilers (24 birds per diet) placed in individual cages from d 13 to 21. Half of the birds in each dietary treatment were given three repeated injections of dexamethasone (DEX) to induce a leaky gut (Barekatain et al, 2019). On d 21, intestinal permeability was measured using fluorescein isothiocyanate dextran (FITC-d, 4.16 mg/kg BW). Ileal tissues for gene expression assays and caecal content for microbiota analysis were then collected from all birds. Both RP diets decreased weight gain (P< 0001) and increased feed conversion ratio (P< 0001) from d 7 to 42 of age compared with control diets, but there was no difference between VEG and MBM control diets. As shown in Table 1, reducing protein to 15.6% increased (P< 0.05) intestinal permeability independent of the DEX challenge. Gene expression of ileal samples revealed independent downregulation of claudin-3, a barrier forming tight junction protein in birds fed the lowest level of dietary protein (P< 0.05). A significant interaction was observed between diet and DEX (P< 0.05) where reductions of protein in grower diets (17.5% and 15.6%) upregulated claudin-2 only in unchallenged birds. Sequencing of 16S rRNA gene amplicons showed that the overall composition of the caecal microbiota was affected in birds fed 15.6% protein compared with other treatments; they had a significantly lower richness of microbiota in both sham and DEX injected birds. To conclude, reducing protein below 17% may significantly compromise intestinal integrity and richness of caecal microbiota in broiler chickens, evidenced by differential mRNA expression of selected tight junction proteins, higher permeability, and changes in the overall microbiota composition. Inclusion of MBM at 4.08% in the control diet had no effect on the studied parameters compared to the VEG diet.

Table 1- Concentration of FITC-d (µg/ml) in broilers at d 21 of age