The Associations of Serum Tryptophan and Tyrosine With Alzheimer's Disease

Objective A growing body of research suggests a strong link between metabolic imbalance and Alzheimer's disease (AD). In recent years, the development of metabolomics makes it possible to study the characteristic changes of peripheral metabolism in AD. Serum levels of tryptophan and tyrosine were associated with mild cognitive impairment (MCI) and AD. The purpose of this study is to further characterize tryptophan and tyrosine levels in MCI and AD.Methods We stratified 765 participants from the Alzheimer's Disease Neuroimaging Initiative-1(ADNI-1) cohort into cognitively normal (CN;n=207), stable mild cognitive impairment (sMCI;n=201), progressive mild cognitive impairment (pMCI;n=171), and dementia due to AD (n=186). Serum tryptophan and tyrosine were analyzed for diagnostic value of MCI and AD. To evaluate the relationships between serum tryptophan and tyrosine and cerebrospinal fluid (CSF) biomarkers, brain structure (magnetic resonance imaging, MRI), cerebral glucose metabolism (18F-fluorodeoxyglucose-positron emission tomography, FDG-PET), and cognitive declines, through different cognitive subgroups.Results Serum tryptophan was decreased in patients with AD, pMCI or sMCI compared with CN. Serum tyrosine was decreased in patients with AD or pMCI compared with CN. Serum tryptophan has diagnostic value for pMCI and AD.Serum tyrosine has diagnostic value for AD.Conclusion Serum tryptophan and tyrosine contribute to the early diagnosis of AD. The detection of tryptophan and tyrosine can contribute to the pathogenesis of AD. Serum tryptophan and tyrosine were not significantly associated with core AD markers, cognitive function, brain structure and brain metabolism, so serum tryptophan and tyrosine may not be good peripheral AD biomarkers.