Mendelian randomisation supports vitamin D as a causal factor in whether someone develops MS. Three randomised trials failed to show it changes the disease once established. Both findings are real, and almost every article you will read picks one.
There is a claim about vitamin D and multiple sclerosis that has been circulating for twenty years. Depending on who is making it, it is either that vitamin D is a suppressed answer to MS, or that the whole idea was hype that quietly collapsed.
Both are wrong, and they are wrong in the same way. The evidence on vitamin D and MS is genuinely strong. It just answers a different question from the one most people are asking.
MS is not evenly distributed across the world. It becomes more common the further you travel from the equator. That gradient has been recognised for decades, and it invited an obvious explanation: less sunlight, less vitamin D synthesised in the skin, more MS.
Supporting observations accumulated. Lower circulating 25-hydroxyvitamin D — the standard marker of vitamin D status — was associated with a higher risk of developing MS. Among people who already had MS, lower levels were associated with more relapses and more disease activity on imaging. The Nurses' Health Study found that women with higher daily vitamin D intake had a lower risk of developing the disease.
By any ordinary reading, that looks like a case. And this is exactly the point where most health writing stops.
Two problems sit underneath every one of those findings.
The first is confounding. People with low vitamin D differ from people with high vitamin D in dozens of ways — how much time they spend outdoors, how much they move, their body composition, their general health, their socioeconomic circumstances. Any of those could be doing the work that vitamin D appears to be doing.
The second is reverse causation, and in MS it is a particularly serious objection. Someone in the early stages of a disabling neurological condition — before diagnosis, sometimes years before — may go outside less. Less sunlight means less vitamin D. So the low vitamin D could be an early consequence of MS rather than a cause of it. Because the true onset of MS is unknown, it is almost impossible to rule this out by observation alone.
Twenty years of association studies could not separate these possibilities. Something else was needed.
The method that broke the deadlock is worth understanding, because it applies far beyond this question.
Certain common gene variants influence how much circulating vitamin D a person carries. These variants are allocated essentially at random at conception, they are fixed for life, and they are not influenced by lifestyle, wealth, or illness. That makes them something close to a natural randomised trial that has been running since birth.
If low vitamin D genuinely causes MS, then people who inherited variants predisposing them to lower levels should have more MS. If the association was confounding or reverse causation, they should not — because their genes were set long before any of that could operate.
In 2015, Mokry and colleagues published exactly this analysis in PLoS Medicine, using variants identified in the SUNLIGHT consortium’s genome-wide study of nearly 34,000 people. Genetically lowered vitamin D was associated with increased MS risk.
A larger analysis followed in 2022 in Neurology Neuroimmunology & Neuroinflammation, drawing on genetic data for vitamin D levels in more than 400,000 people. It supported the causal effect on disease risk, and found that vitamin D levels were also associated with relapse hazard after onset.
This is not weak evidence. Mendelian randomisation is one of the more powerful tools available for establishing causation without a randomised trial, and it points in a consistent direction.
If low vitamin D causes MS, supplementing people who have MS ought to help them. That is the intuitive next step, and it is where the story breaks.
Three randomised controlled trials have tested vitamin D supplementation in established relapsing-remitting MS: SOLAR, CHOLINE, and VIDAMS. None demonstrated the benefit the hypothesis predicted.
VIDAMS is the clearest of them. It compared high-dose vitamin D3 at 5,000 IU daily against low-dose at 600 IU daily, both added to glatiramer acetate, and looked at whether participants experienced a confirmed relapse. High-dose supplementation did not reduce disease activity compared with low-dose.
There are fair criticisms of these trials. Some were limited in size or duration. An individual patient-data meta-analysis pooling all three has been planned to get more statistical power. It is possible that a longer, larger, or earlier intervention would show something.
But as things stand, the honest position is this: the evidence supports vitamin D mattering for whether someone develops MS. It does not support vitamin D changing the course of MS once established.
Because it is inconvenient in both directions.
If you sell supplements, the causal finding is the headline and the failed trials are a footnote. If you are debunking functional medicine, the failed trials are the headline and the Mendelian randomisation work does not get mentioned. Each side has a version of the evidence that supports the position they arrived with.
The complete picture supports neither. It supports a specific, narrow, genuinely interesting claim: vitamin D status looks like a real modifiable risk factor for a disease that mostly begins in young adulthood — which means the relevant window is probably long before anybody has symptoms, and possibly in childhood and adolescence.
That is a public health finding more than a clinical one. It is not a treatment.
Vitamin D deficiency is worth correcting because vitamin D deficiency is worth correcting. It affects bone, muscle and immune function in everyone, and people with reduced mobility or limited time outdoors are at higher risk of being deficient — which describes a substantial number of people with MS.
That is a good reason to know your level and to address it if it is low. It is not the same as expecting it to alter your disease.
Your MS is managed by your neurologist. Disease-modifying therapy is the thing with the evidence behind it, and nothing here is a reason to take it less seriously.
This is the part I want to be plain about, because it is where practitioners like me most often overreach.
I cannot tell you anything about your MS from a functional test. A DUTCH profile, an organic acids test, a stool panel or an inflammatory marker will not illuminate your neurological condition, predict your course, or tell you how you will respond to treatment. Anyone suggesting otherwise is selling you something.
What I can do is work with the person who has the condition rather than the condition the person has — which is the whole basis of functional diagnostic nutrition and, in this instance, is also the only honest offer available.
People with MS commonly carry fatigue, disturbed sleep, digestive symptoms, low mood, deconditioning from reduced activity, and nutritional gaps that nobody has looked at. Every one of those is real, worth addressing, and affects quality of life. None of them requires me to have a view on your demyelination.
That is a smaller claim than the one usually made. It is also one I can stand behind.
The planned individual patient-data meta-analysis of SOLAR, CHOLINE and VIDAMS may find an effect that the individual trials were underpowered to detect. Trials of earlier or higher-dose intervention, or in people at high risk rather than established disease, may show something different.
If that happens, I will say so here. Publishing the correction is the point.
This article is general information and not medical advice. If you have MS, decisions about your treatment belong with your neurology team.
Sources
Mokry LE, et al. Vitamin D and risk of multiple sclerosis: a Mendelian randomization study. PLoS Medicine 2015;12(8):e1001866.
Vandebergh M, Dubois B, Goris A. Effects of vitamin D and body mass index on disease risk and relapse hazard in multiple sclerosis: a Mendelian randomization study. Neurology Neuroimmunology & Neuroinflammation 2022;9:e1165.
Mowry EM, et al. High-dose vitamin D3 supplementation in relapsing-remitting multiple sclerosis: a randomised clinical trial (VIDAMS). eClinicalMedicine 2023.
Fatigue, sleep, digestion and deconditioning are real, addressable, and do not require a view on your neurology.
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