What is solid
Genuine deficiency causes real disease — osteomalacia in adults, rickets in children — and correcting it matters. Deficiency is common, particularly in people with darker skin at higher latitudes, people who cover up or stay indoors, and older adults. Measuring 25-hydroxyvitamin D is cheap and the correction is cheap.
What the big trials found
Large randomised trials supplementing broad populations who were largely not deficient did not show the reductions in cancer, cardiovascular events or all-cause mortality that observational data had suggested. The most likely explanation is that low vitamin D is often a marker of being unwell, indoors or inactive rather than the cause of those outcomes. This is one of the clearest examples in nutrition of a correlation that did not survive being tested.
The practical position
Measure it, correct it if it is low, and do not expect correction to do things the trials say it does not do. Very high doses are not better and can cause harm through hypercalcaemia. If you take a supplement, a modest daily dose with a recheck beats an enormous intermittent one.
Key points
- Deficiency is real, common and worth correcting.
- Supplementing non-deficient populations did not deliver the observational promises.
- Modest daily dosing with a recheck. High doses are not better.
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