Assessing diet, the faecal microbiome, and trypotphan metabolites in the domestic cat

Cats are obligate carnivores and therefore don’t require dietary fibre. However, it is commonly included in pet food. Previous research showed when cats consume a high protein diet lacking dietary fibre (<1% crude fibre on a dry matter basis), their faecal microbiome, faecal output, and defecation rate is altered. Adding dietary fibre (plant or animal derived) to the high protein diet increased faecal output. Tryptophan metabolite serotonin is involved in colonic transit and may be implicated in the changing defecation rate. Serotonin is released in response to changing mechanical pressure, bacterial interaction, and concentration of metabolites in the gut lumen. Therefore, interactions between host, diet and tryptophan metabolites were investigated. 

Cats were fed one of three diets a high protein diet (Raw), with hydrolysed collagen (Raw+HC) at 4% or 6% (as is), or Raw with inulin and cellulose (Raw+PF: 2% of each). The faecal metagenome was analysed by shotgun sequencing. Functional genes were assigned using the KEGG database. Organic acids were analysed using gas chromatography, and tryptophan metabolites using targeted liquid chromatography mass spectrometry. 

Faecal output was lowest in the cats consuming Raw+HC. Bacterial genes involved in tryptophan metabolism were significantly greater in cats consuming the Raw+HC diets compared to Raw+PF. However, circulating concentration of serotonin was not significantly different. These findings suggest the microbiome responded to the changes in tryptophan availability in the colon, but not through the serotonin pathway. Further investigation is required to better understand tryptophan metabolite interactions between the microbiome and host.