OBJECTIVE: There is increasing evidence that vitamin D can be protective against the development of MS, but it may also be beneficial for the clinical course of the disease. The objective was to prospectively investigate if 25-hydroxy-vitamin D (25-OH-D) levels are associated with exacerbation risk in MS in a study with frequent serum measurements.
METHODS: This was a prospective longitudinal study in 73 MSers with relapsing-remitting MS. Blood samples for 25-OH-D measurements were taken every 8 weeks. Associations between 25-OH-D levels and exacerbation rates were assessed using Poisson regression (generalized estimating equations) with the individual serum levels as time-dependent variable.
RESULTS: During follow-up (mean 1.7 years), 58 MSers experienced a total of 139 exacerbations. Monthly moving averages of 25-OH-D levels were categorized into low (<50 nmol/L), medium (50-100 nmol/L), and high (>100 nmol/L) levels. Exacerbation risk decreased significantly with higher serum vitamin D levels: respective relative exacerbation rates for the medium and high-level category as compared to the low-level category were 0.7 and 0.5 (p value for trend: p = 0.007). The association between 25-OH-D concentrations and exacerbation rate was log linear without a threshold. With each doubling of the serum 25-OH-D concentration the exacerbation rate decreased by 27% (95% confidence interval 8%-42%, p = 0.008).
CONCLUSIONS: The finding that higher vitamin D levels are associated with decreased exacerbation risk in relapsing-remitting MS suggests a beneficial effect of vitamin D on disease course in MS. However, the possibility of reverse causality cannot be ruled out completely. Randomized intervention studies are therefore needed to investigate the effect of vitamin D supplementation in MS.