As the most common type of dementia, Alzheimer's disease (AD) often presents challenges in terms of early identification. In particular, there is a notable lack of reliable and accessible biomarkers. To measure the levels of amino acids and carnitine metabolites in the peripheral blood of AD patients to identify cognitive impairment-associated metabolites, two cohorts were recruited in this cross-sectional study from September 2018 to October 2023. Serum amino acid and carnitine levels were measured using liquid chromatography-mass spectrometry. The test cohort (normal cognition (NC), n = 70; amnesic mild cognitive impairment (aMCI), n = 41; dementia, n = 92) was used to analyze differences in serum amino acid and carnitine levels and to create a metabolite profile. The diagnostic performance of the metabolite profile was first assessed within the test cohort through receiver operating characteristic (ROC) curve analysis and machine learning approaches. Subsequent cognitive impairment subgroup analysis further confirmed its diagnostic efficacy. Finally, the validation cohort (NC, n = 10; aMCI, n = 10; dementia; n = 10; FDPV (FTD, DLB, PDD and VaD), n = 30) was used to determine the diagnostic and differential diagnostic capabilities of the metabolite profile. In the test cohort, serum levels of a total of 36 amino acids and carnitine metabolites were dysregulated. A profile of seven key metabolites successfully identified patients with NC, aMCI, and dementia. Cognitive subgroup (based on the Montreal Cognitive Assessment) analysis revealed that the profile was suitable for screening for mild, moderate, and severe cognitive impairment. The validation cohort further demonstrated the successful application of the metabolic profile for the discrimination of NC, aMCI, and dementia, as well as for the differential diagnosis of dementia and FDPV. In conclusion, extensive alterations in amino acid and carnitine metabolism levels were found in the peripheral blood of dementia patients, and a profile of seven amino acids and carnitine could be used as potential indicators for aMCI and dementia.© 2025 International Society for Neurochemistry.