This study highlights the possible role of the microbiota-gut-brain-axis (MGBA) in the disease progression, as some metabolites found altered in MCI and/or AD, such as 3-keto-LCA, are produced by the human microbiome. However, the role of BAs in CSF needs to be further investigated, as the link between peripheral and central BAs is poorly understood. Matching samples of CSF, plasma, and feces would be needed to study the influence of microbiota composition.