Serum Vitamin D Levels as a Potential Predictor of Disability Progression and Therapy Escalation in Early Multiple Sclerosis: A 5-Year Retrospective Cohort Study

Background Serum 25-hydroxyvitamin D (25-OH-D) deficiency is considered a modifiable risk factor for the development of multiple sclerosis (MS). However, its impact on the long-term clinical course and disability progression in early MS remains unclear. Methods We conducted a retrospective observational cohort study at the MS Centre of the University Hospital Ostrava, including patients with clinically isolated syndrome (CIS) or relapsing–remitting MS (RRMS). Mean serum 25-OH-D concentrations over a five-year follow-up were analyzed in relation to clinical outcomes, including therapy escalation and a composite progression severity score (PSS) based on EDSS change and treatment escalation. Multivariable logistic regression models were used to assess independent associations, adjusting for age, sex, and baseline EDSS. Results A total of 106 patients (73 women, 69%) were included, with a mean age at treatment initiation of 32.8 ± 9.32 years. Higher mean serum 25-OH-D levels were associated with lower odds of therapy escalation (OR = 0.923 per 1 nmol/L; 95% CI 0.886–0.962; p < 0.001) and a more favorable disease course according to the PSS (OR = 0.892 per 1 nmol/L; 95% CI 0.848–0.937; p < 0.001). Patients with a favorable prognosis had significantly higher mean serum 25-OH-D concentrations compared to those with unfavorable outcomes (70.43 ± 13.99 vs. 56.07 ± 11.25 nmol/L; p < 0.001). Conclusion Higher mean serum 25-OH-D levels were associated with a more favorable disease course and lower likelihood of therapy escalation in early MS. These findings highlight the potential role of serum 25-OH-D as a marker of disease activity; however, given the observational design and the use of a study-specific composite outcome, the results should be interpreted with caution and confirmed in prospective studies.