in their gut microbiome, with bacteroidetes and firmicutes being the dominant phyla. diets higher in processed foods and unnatural chemicals can negatively alter the ratios of these species, while diets high in whole foods can positively alter the ratios. additional health factors that may skew the composition of the gut microbiota are antibiotics and probiotics. antibiotics have severe impacts on gut microbiota, ridding of both good and bad bacteria. without proper rehabilitation, it can be easy for harmful bacteria to become dominant. probiotics may help to mitigate this by supplying healthy bacteria into the gut and replenishing the richness and diversity of the gut microbiota. there are many strains of probiotics that can be administered depending on the needs of a specific individual. bile acids and cognitive function microbial derived secondary bile acids produced in the gut may influence cognitive function. altered bile acid profiles occur in cases of mild cognitive impairment and alzheimer’s disease with an increase in cytotoxic secondary bile acids and a decrease in primary bile acids. these findings suggest a role of the gut microbiome in the progression to alzheimer’s disease. in contrast to the cytotoxic effect of secondary bile acids, the bile acid tauroursodeoxycholic acid may be beneficial in the treatment of neurodegenerative diseases.