The prognostic potential of circulating BDNF levels and its polymorphisms in age-related cognitive impairment and neurodegeneration.

Brain-derived neurotrophic factor (BDNF) is essential for neuronal survival, synaptic plasticity, and cognitive function. Age-related decline in BDNF signaling has been implicated in the pathogenesis of Alzheimer's disease, Parkinson's disease, and mild cognitive impairment. However, the prognostic value of circulating BDNF and its genetic variants remains controversial due to inconsistent findings across studies. This review synthesizes current evidence on peripheral BDNF levels and the Val66Met polymorphism as potential biomarkers of age-related cognitive decline and neurodegeneration. We critically analyze biological mechanisms linking BDNF to neurodegeneration, including its interaction with amyloid-beta and tau pathology. We further examine factors underlying discrepant results: demographic characteristics, comorbidities, lifestyle factors, pharmacological interventions, and methodological variability. Despite these challenges, BDNF remains a promising diagnostic factor. Nevertheless, standardizing preanalytical protocols and accounting for patient heterogeneity are essential to unlock its diagnostic potential.