Parkinson Disease, Thiamine-Dependent Metabolic Reserve, and the Convergent Metabolic Bottleneck: A Falsifiable Subgroup Hypothesis

Background: Parkinson disease (PD) exhibits biological heterogeneity and region-dependent metabolic abnormalities, including changes in thiamine pyrophosphate (TPP)-dependent enzyme systems and intrinsic mitochondrial respiratory-chain function. Evidence: Human PD studies report reduced α-ketoglutarate dehydrogenase complex (OGDHC) activity, less consistent pyruvate dehydrogenase complex findings, and reduced transketolase protein in selected regions. Complex-I activity deficits can persist when immediate assay substrates are supplied and cannot be attributed solely to deficient upstream NADH generation. These findings do not establish neuronal or mitochondrial TPP insufficiency. Thiamine measurements in blood or bulk cerebrospinal fluid (CSF) likewise do not directly determine intracellular cofactor engagement, and non-low values neither prove nor exclude a specific cerebral functional abnormality. Hypothesis: We propose a Convergent Metabolic Bottleneck in which distinct lesions may constrain oxidative metabolic output and narrow effective reserve. In a prospectively definable PD subgroup, thiamine-dependent metabolic limitation could be one contributing process, independently or alongside intrinsic Complex-I dysfunction. Interventions and tests: Small, uncontrolled human reports describe clinical changes during parenteral or intramuscular (IM) thiamine administration but do not establish efficacy, neuroprotection, or route superiority. We distinguish the general reserve model from thiamine-specific dysfunction, an IM-superiority prediction, and a product-specific thiamine tetrahydrofurfuryl disulfide (TTFD)-containing Thiamax response probe. Direct TPP/enzyme-saturation studies, controlled rescue experiments, longitudinal measurements, prospectively defined biomarkers, randomized interventions with verified target engagement, and direct route comparisons can support or falsify these propositions. Conclusion: The proposed thiamine-dependent PD subgroup remains unproven; it must be weakened or rejected when adequately conducted, prespecified tests disconfirm its necessary predictions.