Seasonally Normalized Vitamin D Status and Long-Term Mortality and Multisystem Disease Risk in the UK Biobank

Background: Vitamin D deficiency has been associated with mortality and chronic disease. In a recent study of Israeli Leumit Health Services (LHS) and US TriNetX populations, severe deficiency was reproducibly associated with mortality and multisystem morbidity, while longitudinal supplementation reconstructed from pharmacy dispensing was associated with lower risk. Methods: We sought to reproduce these associations and address a wide range of potential confounders in the deeply phenotyped UK Biobank. Baseline circulating 25-hydroxyvitamin D [25(OH)D] was standardized to a common late-winter nadir using an eight-harmonic model, and participants were categorized according to nadir-equivalent 25(OH)D as <5, 5-10, 10-15, 15-20, or [&ge;]20 ng/mL. Primary survival analyses used one-year delayed entry and complete-case multivariable adjustment. Results: Analyses included 412,020 participants and 31,197 deaths. Mortality hazard ratios were 2.03 (95% CI 1.90-2.18), 1.34 (1.29-1.39), 1.08 (1.04-1.11), respectively, for <5, 5-10 and 10-15, versus [&ge;]20 ng/mL. Associations previously observed in LHS and TriNetX were reproduced, notably for diabetes, myocardial infarction, cerebrovascular disease, dementia, diabetic retinopathy, and end-stage renal disease, whereas skin malignancies showed the opposite gradient, consistent with greater ultraviolet exposure. DAG-informed causal analyses using generalized overlap weighting achieved acceptable balance across measured covariates; weighted 10-year mortality was 9.7% with very severe deficiency versus 4.8% with [&ge;]20 ng/mL, supporting a causal interpretation. Estimates remained stable with landmark lags extending to 5 years after measurement. Conclusions: Seasonally normalized 25(OH)D identifies a reproducible state of systemic vulnerability while reducing exposure misclassification caused by calendar timing of measurement. An interactive implementation of the normalization model is available: https://vitamind.medil.ai.