Abstract
Background
Environmental factors, such as alterations in diet and microbiota, have been linked to inflammatory bowel diseases (IBD). The incidence of IBD is rising, particularly in Canada and other industrialized nations that consume western-style diets high in fat and protein. While most dietary proteins and amino acids are absorbed in the small intestine, substantial amounts can enter the colon for microbial metabolism and to exert effects on intestinal tissue and immune cells. Prospective cohort studies suggest that diets high in protein are associated with an increased risk of IBD. However, the role of excess dietary protein and amino acids in IBD pathogenesis is not clear.
Aims
To study whether and how consumption of diets high in protein or amino acids influences intestinal inflammation, colitis severity, and intestinal microbiota.
Methods
To assess the influence of dietary protein composition on colitis severity, specific pathogen-free C57BL/6 mice were fed isocaloric casein-based purified diets containing low (7%), normal (14%), or high (35%) protein (HPD). Mice were also fed an amino acid-defined diet (AAD) with amino acid and ingredient composition matched to the normal protein diet. Following three weeks of diet consumption ad libitum, mice were continued on the same diet and mucosal injury was induced with 2% dextran sulfate sodium (DSS; 5 days) followed by water (2 days) before sacrifice. Mice were monitored daily for clinical signs of colitis. Susceptibility to colitis was assessed by analysing stool consistency and blood, microscopic scoring (Cooper score), and by immunohistochemistry of colon tissue. Fecal microbiota (16S rRNA Illumina), intestinal permeability (Ussing chambers), proinflammatory gene expression (NanoString and RT-qPCR), and bacterial translocation (plating) were analysed.
Results
Following DSS exposure, mice fed HPD and AAD experienced greater weight loss, bacterial translocation to the spleen, stool blood, and diarrhea compared to mice fed the normal protein control diet. While all DSS-treated mice developed colitis, HPD and AAD fed mice also developed greater histologic damage, intestinal permeability, and innate immune cell infiltration. Cytokine profiling revealed that AAD is associated with significant up-regulation of IL-18 during colitis. Principle coordinates analysis based on Bray-Curtis dissimilarities demonstrates distinct shifts in the fecal microbiota of mice fed HPD and AAD.
Conclusions
These results suggest that excess dietary protein and amino acids are associated with more severe colitis and microbiota alterations in the DSS model. Previous studies demonstrate that IL-18 is up-regulated in IBD patients. Its overexpression may incite inflammation by stimulating cytokine signalling through NFκB and modify microbial community structure by regulating antimicrobial peptides.
Funding Agencies
CIHRFarncombe Family Digestive Health Research Institute, Douglas Family Chair in Gastroenterology Research
Role of the High Affinity Immunoglobulin E Receptor in Bacterial Translocation and Intestinal Inflammation
