Vitamin D has a compelling case behind it. The body needs it, sunlight helps produce it, and receptors for it are found throughout the body.

That led researchers to hope supplements might help prevent problems ranging from cancer and heart disease to fractures and winter colds. But three large trials, following tens of thousands of people over several years, found no detectable benefits across those outcomes.

That does not mean vitamin D is unnecessary. Some people may still benefit from supplements, particularly when sunlight or dietary intake is limited.

So where does getting enough vitamin D end—and when does taking extra stop helping?