The participants (ages 18 to 55) had clinically isolated syndrome (CIS, a neurological episode that is often an early sign of multiple sclerosis), and 89% of them met the current diagnostic criteria for the relapsing-remitting type of multiple sclerosis.

The participants took a placebo or 100,000 IU of vitamin D every 2 weeks.

The primary outcome was disease activity, a combination of disease relapse (the development of new symptoms or the reoccurrence of old symptoms) and lesion development (i.e., new, enlarging, or active lesions detected in the brain or spinal cord with MRI).

Vitamin D supplementation reduced disease activity by 34%, which delayed the median time to a disease activity event by 208 days (224 days vs. 432 days). This benefit was larger in the participants with vitamin D deficiency at baseline (<12 ng/mL).

Vitamin D supplementation also reduced lesion development by 29%. Disease relapse was also 32% lower with vitamin D compared to the placebo, but this difference was not statistically significant (p=0.15), possibly due to a low relapse rate in both groups.

The study results

Outcome measure	Effect of vitamin D
Disease activity*	↓34% (main sample) ↓67% (vitamin D deficient subgroup)
Disease relapse	Not statistically significant
Lesion development	↓29%
New lesions	↓39%
Contrast enhancing (inflammatory) lesions	↓53%

*Defined as disease relapse or lesion development

Vitamin D supplementation did not result in any apparent benefits to any clinical symptoms (e.g., disability level, anxiety, depression), which remained unchanged in both the vitamin D and placebo group.

Supplemental vitamin D prevents lesion development in multiple sclerosis Original paper