By definition, a vitamin is an essential nutrient the body can’t make on its own and must get from food. Vitamin D breaks that exact rule: skin produces it on its own, from a cholesterol precursor, when exposed to the sun’s UVB rays.
Once produced, the substance is activated in the liver and kidneys, turning into calcitriol – a form that behaves, functionally, like a hormone, not like a simple nutrient absorbed from food.
That’s why some researchers prefer calling vitamin D a “prohormone,” even though the popular name stuck around for historical reasons, not scientific ones.
Sometimes a name survives out of habit, long after science has figured out what the thing actually is.
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