Vitamin D

For about a decade, vitamin D was the closest thing nutrition had to a miracle. Low levels were linked to nearly everything bad: heart disease, cancer, infections, autoimmune conditions, depression, and early death. The logic seemed irresistible. If deficiency tracks with all of that, surely topping people up would prevent it.

So the trials were run, some of them very large, and the result was a quiet anticlimax. Giving vitamin D to the general population did not meaningfully lower the rates of cancer or cardiovascular disease. The number went up, and the outcomes mostly did not change. That gap, between a marker that predicts disease and a pill that fails to prevent it, is the most important thing to understand about vitamin D.

It does not mean vitamin D is unimportant. A real deficiency genuinely harms the bones and probably more, and correcting it is worthwhile. It means the popular story had the direction of causation partly backwards. Read this number with that in mind and it becomes genuinely useful. Read it the way the supplement aisle wants you to, and you will chase a number that was never the point.

Vitamin D is less a vitamin than a hormone the body can make for itself. When ultraviolet light from the sun strikes the skin, it converts a cholesterol-derived molecule into vitamin D3. The liver then adds a chemical group to make 25-hydroxyvitamin D, the stable storage form that circulates for weeks and is what a blood test measures. Finally, mostly in the kidney, it is converted to the active hormone, which is tightly regulated and so not useful to measure directly ^[1].

The active hormone is genuinely powerful. Nearly every tissue in the body carries a receptor for it, which is why its influence reaches well beyond the bones into immune function, muscle, and cell growth ^[1]. That ubiquity is exactly what fueled the expectation that fixing a low level would fix a long list of diseases. The activation steps also depend on magnesium, which is part of why a magnesium shortfall can blunt vitamin D's effect even when the level looks adequate.

What lowers the measured level is a familiar list, and it explains the marker problem. Limited sun, from northern latitudes, winter, indoor work, clothing, sunscreen, or more melanin in darker skin, reduces the skin's production. Age reduces it further. And excess body fat pulls vitamin D out of circulation, so obesity lowers the blood level without necessarily lowering the total in the body. Several of these are themselves markers of how someone lives, which is the crux of the interpretation problem.

The association is real and strong. In a pooled analysis of more than 26,000 older adults, those in the lowest vitamin D group had 57% higher all-cause mortality than those in the highest ^[2]. Findings like this, repeated across many outcomes, are what built vitamin D's reputation.

The trials are where the reputation met resistance. A systematic review spanning hundreds of cohort studies and randomized trials found the same pattern again and again: consistent observational associations, but supplementation trials that largely failed to deliver the predicted benefits, leading the authors to conclude that a low vitamin D is more a marker of ill health than a cause of it ^[3]. The largest single trial, VITAL, randomized nearly 26,000 people to vitamin D or placebo and found no reduction in cancer or major cardiovascular events, though cancer mortality did edge downward over longer follow-up ^[4].

But the honest reading is not "vitamin D does nothing," and one detail of VITAL shows why: most of its participants were already replete, averaging about 31 ng/mL, so the trial tested whether extra helps the already-sufficient, not whether correcting a true deficiency helps. The reasonable synthesis is this. Frank deficiency harms bone and is worth correcting, the broad cure-all claims did not survive testing, and the sweet spot is moderate. Get out of deficiency, aim for roughly 30 to 50 ng/mL, and do not chase ever-higher numbers, both because the outcome benefit is not there and because sustained megadosing can, rarely, cause genuine toxicity through high calcium.