INTRODUCTION Alzheimer's disease (AD), a neurodegenerative disease, involves early alterations in the gut microbiota. Bile acids (BAs), which are metabolites produced by the microbiota, may impact brain function through the gut-brain axis. METHODS By reference to multimodal datasets from the Chinese Preclinical Alzheimer's Disease Study (n = 1397) and the Alzheimer's Disease Neuroimaging Initiative (n = 1275), we analyzed differences in BA levels and their associations with AD biomarkers. RESULTS Lithocholic acid (LCA) -family BAs are associated with the amyloid-beta status. Longitudinal changes in BA levels correlated with amyloid and tau pathologies. LCA and the deoxycholic acid (DCA) family exhibited predictive value with respect to AD pathology. Imaging transcriptomic analyses suggested that BAs modulated amyloid pathology through multiple mechanisms. DISCUSSION DCA- and LCA-family BAs were proposed as molecular bridges that connect age signatures with AD pathology. They represent a new avenue for the development of biomarkers and therapeutic interventions.