The worthlessness of vitamin D is mildly exaggerated

For a while there, many people thought vitamin D was magical—that it could improve bones, the heart, infections, cancer, heart disease, longevity, even mental health. But among people I respect, opinion is now overwhelmingly that taking vitamin D does nothing unless you're severely deficient. The central argument is that while vitamin D levels are correlated with ~all positive health outcomes, when you actually test vitamin D supplements against placebo in randomized trials, nothing ever happens.That's what I used to think, too. But I've come to think the skeptics have over-corrected. Yes, randomized trials have shown the magical correlations are not causal. But if you start with non-insane expectations, the trials look like weak but positive evidence. And if you consider what we know about biology and evolution, I think the balance of evidence tips pretty clearly in the direction that people with low-ish levels would be wise to supplement.Am I certain that vitamin D is beneficial for people with low-ish levels? Absolutely not! But I claim that's the best bet given the limits of our knowledge.The classical view: Boring bone vitaminMost vitamins are "ingredients" that the body uses to do stuff. Vitamin D is more like a "signal" that the body uses to communicate with itself about what to do.[1]The classical "endocrine" story of vitamin D is that your body uses it to tell your guts to take in more calcium from food. If you don't get enough vitamin D, then you have calcium problems.That's all you really need to know about the classical view. But if you enjoy gawking at biology's complexity, I recommend this diagram and the following three paragraphs:Ready for science? OK: Almost all the cells in your body make provitamin D.[2]Usually, this is all converted to cholesterol, but your skin cells leave some sitting around. When UVB light hits those skin cells, provitamin D is transformed (physically by the light itself) into previtamin D and then (by heat) into vitamin D. Th