You don’t have to look far to find claims that taking vitamin D supplements is great for your health. It’s supposed to be good for everything from preventing cancer and dementia to avoiding infections and heart disease.

Unfortunately, many supposed benefits of vitamin D supplements remain unproven. Yet, millions of people take vitamin D regularly, thinking it will help prevent a wide range of illnesses, including certain autoimmune conditions. But does it? A new randomized, controlled study published in TheBMJ looks closely at that question.

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