Vitamin D: correct a deficiency, don't chase a supplement

Vitamin D is where a real biological necessity got stretched into a cure-all. The nutrient genuinely matters — bone, immune function, muscle — and true deficiency is worth correcting. The overreach was assuming that if some is essential, more must be protective for everyone.

The trials say otherwise. VITAL gave roughly 26,000 mostly-replete adults 2000 IU/day and found no reduction in cancer or cardiovascular events. The NIH’s own review lands in the same place: for people who are already sufficient, supplementing doesn’t move the hard outcomes. That’s the crucial distinction the supplement aisle blurs — replete versus deficient are two different situations.

The sensible version

If you have reason to be low — little sun, higher latitude, darker skin, malabsorption, or a symptomatic picture — test and correct it, and modest daily dosing does that well and cheaply. If your level is fine, adding more doesn’t buy you longevity, and very high doses carry their own small risks (calcium handling). This is a targeted fix wearing the costume of a universal one, which is why it sits low in the supplement tier: real when it applies, inert when it doesn’t.