From systems to cells: Metabolic mechanisms underlying risk divergence in Alzheimer's disease and amyotrophic lateral sclerosis.

Epidemiological studies show an inverse relationship between metabolic disorders and two major neurodegenerative diseases, Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS). Obesity, type 2 diabetes (T2DM), and reduced physical activity increase AD risk, whereas in ALS cardiometabolic factors, particularly T2DM, show inverse, age-dependent associations with disease risk. This review integrates epidemiological, clinical, and experimental evidence to suggest that cell-type-specific energy metabolism underlies these contrasting risk profiles. Neurons and skeletal muscle differ in metabolic organization, substrate use, and redox capacity. Neurons rely mainly on glucose and lactate and have limited fatty acid oxidation, making them vulnerable to lipid overload, insulin resistance, and oxidative stress, hallmarks of AD. In contrast, skeletal muscle is metabolically flexible, efficiently oxidizes fatty acids, and has strong antioxidant defenses, which may protect against ALS. These cell-type-specific metabolic profiles are proposed to causally shape disease susceptibility: neuronal lipid overload and impaired redox homeostasis promote amyloid and tau pathology in AD, whereas preserved muscle fatty acid oxidation and antioxidant capacity support neuromuscular junction stability and delay motor neuron degeneration in ALS. Hypermetabolism, hypothalamic dysfunction, glial-neuronal coupling and lactate shuttling may further shape disease susceptibility. Overall, these patterns likely reflect distinct cellular responses to metabolic stress.