Background: Alzheimer's disease (AD) is a leading cause of disability among older adults, imposing substantial burden on individuals, families, and health systems. As the ageing population grows, identifying accessible metabolic biomarkers to better understand AD progression represents a critical opportunity for innovation in clinical practice. This study explored associations between serum amino acid levels and AD severity in people with mild and moderate AD. Methods: A cross-sectional study was conducted on 60 individuals with mild or moderate AD. Serum levels of 20 amino acids were measured using High-Performance Liquid Chromatography (HPLC). Cognitive function was assessed via the Mini-Mental State Examination (MMSE) and Categorical Verbal Fluency Test (CFT); functional abilities were evaluated using the Barthel Index and IADL scale. Multivariable binary logistic regression examined associations between amino acid tertiles and AD severity. Results: Among 60 participants (mean age 67.9 years), 53.3% had mild and 46.6% moderate AD. Higher serum leucine was associated with lower odds of moderate AD (OR=0.57; 95% CI: 0.47–0.88; P-trend=0.02), as was higher valine (OR=0.59; 95% CI: 0.15–0.75; P-trend=0.01). No significant associations were found for isoleucine or aromatic amino acids. Conclusion: These findings suggest selective involvement of specific branched-chain amino acids in AD severity, pointing toward novel metabolic pathways relevant to aged care assessment and intervention. Longitudinal research is needed to determine whether circulating BCAAs could inform early, low-cost strategies for monitoring disease progression in older adults. Furthermore, given that BCAAs are dietary-modifiable, nutritional strategies may offer a promising and accessible avenue for future intervention.