Though I rarely pass up an opportunity to remind people that taking vitamins and supplements is often pointless, the key to science is learning how to be a skeptic without being a cynic. The role of vitamin D in preventing or treating multiple sclerosis demonstrates just how difficult that can be.

Multiple sclerosis is a complex disease. The ultimate cause is unclear though very likely immune-mediated and possibly triggered by pathogens like Epstein-Barr virus. It is nearly twice as common in women compared to men, which suggests genetics play a role. It is also more common in people living in northern latitudes and where lack of sunlight and vitamin D deficiency have been hypothesized as possible contributing factors.

With any such association, it is always hard to tease out if vitamin D deficiency is an independent cause of disease or simply a marker of other underlying issues. With multiple sclerosis, genetic analyses suggested that it might in........

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