A recent French study has confirmed what many in functional and integrative medicine have long suspected: high-dose vitamin D supplementation significantly reduces disease activity in early multiple sclerosis (MS).

The D-Lay MS trial, the largest of its kind, demonstrated that patients with clinically isolated syndrome (CIS)—the earliest detectable stage of MS—experienced a statistically significant delay in disease progression when treated with vitamin D. Disease activity, defined by relapse or MRI findings, occurred in only 60.3% of the vitamin D group compared to 74.1% of the placebo group. That’s a 14% absolute risk reduction and a median delay in disease activity of over 200 days, with no increase in adverse events.

This is not a fringe finding—it is a meaningful clinical outcome with major implications for early MS management.

Vitamin D and MS: A Long-Standing Hypothesis, Now Validated

Vitamin D has long been implicated in the pathogenesis of MS. Low serum vitamin D levels are associated with both increased risk of MS and higher disease activity. Despite this, previous trials had produced mixed results—often due to heterogeneous patient populations, inconsistent dosing, or study design limitations.

The D-Lay trial overcame these issues by:

Focusing on CIS patients (the earliest, most modifiable phase of MS)

Administering a high dose (100,000 IU every two weeks) of cholecalciferol

Using well-defined, MRI-confirmed criteria for disease activity

The result? Statistically significant reductions in all MRI-based markers of MS activity and a well-tolerated safety profile, even at high doses.

What Functional Practitioners Have Known All Along

While the conventional field has awaited large trials to validate vitamin D, functional medicine practitioners have been early adopters of this strategy—based on its:

Benign side effect profile

Widespread deficiency across populations

Plausible mechanistic role in neuroinflammation, immune regulation, and myelin protection

In practice, we’ve seen patients with fatigue, autoimmune tendencies, and cognitive complaints improve under careful vitamin D repletion. Now, we have rigorous data showing its utility in one of the most feared neuroimmune disorders.

Implications for Clinical Practice

This study reinforces that early intervention matters—not just with pharmaceuticals, but with targeted nutritional strategies. It also raises questions about why vitamin D screening and correction isn’t standard of care in all patients at risk of neuroinflammatory disease. How much more effective is corrective or even supraphysiological supplementation of vitamin D prior to the onset of distressing symptoms?

The findings highlight the importance of:

Routine vitamin D testing in patients with neurological symptoms or autoimmune risk

Therapeutic supplementation when levels are <100 nmol/L (and especially <30 nmol/L)

Integrating nutritional and lifestyle strategies alongside conventional care

Education Is Key to Implementation

Despite the positive outcomes, many clinicians remain hesitant to prescribe high-dose vitamin D—often due to outdated safety concerns or a lack of education in nutritional therapeutics. This is where accredited education and practical guidance can close the gap.

Understanding how to assess, dose, monitor, and educate patients on nutrient-based interventions is a critical skill for all healthcare professionals—and essential for bringing accessible, low-risk therapies into broader clinical use.

The D-Lay MS trial isn’t just a win for vitamin D—it’s a win for evidence-based preventive medicine. It reminds us that safe, affordable, and widely accessible treatments like vitamin D can play a central role in managing complex diseases—especially when applied early and appropriately.

As the conversation around MS evolves, vitamin D should no longer be optional—it should be fundamental.

Thouvenot E, Laplaud D, Lebrun-Frenay C, et al. High-Dose Vitamin D in Clinically Isolated Syndrome Typical of Multiple Sclerosis: The D-Lay MS Randomized Clinical Trial. JAMA. Published online March 10, 2025. doi:10.1001/jama.2025.1604