Also known as: cholecalciferol, vitamin D3, ergocalciferol, the sunshine vitamin
Correcting a real deficiency matters. Topping up someone who is already replete has failed trial after expensive trial.
Grade A for correcting deficiency and its skeletal consequences. The large outcome trials — VITAL, VIDA, D-Health — found no reduction in cancer, cardiovascular events, or fractures in already-replete adults, which is what drops the overall grade.
Vitamin D is the supplement where the gap between mechanism and outcome is widest, and the last decade of research has been a long lesson in why that gap matters.
The mechanistic case is genuinely spectacular. It is not really a vitamin — it is a prohormone, and its receptor turns up in immune cells, muscle, brain, and vascular tissue. Observational studies linked low levels to nearly every bad outcome in medicine: cancer, heart disease, depression, infection, mortality. For years that looked like a very large public health opportunity.
Then the trials arrived. VITAL randomised over 25,000 adults to 2,000 IU daily for five years and found no reduction in cancer incidence or cardiovascular events. A follow-on analysis found no reduction in fractures either. D-Health, in more than 21,000 older Australians, likewise found no mortality benefit. The pattern has repeated often enough to be the finding rather than a disappointment.
The reconciliation is that low vitamin D was largely a marker rather than a cause. People who are outdoors, active, and healthy have higher levels; people who are ill, obese, or housebound have lower ones. The correlation was real; the arrow mostly pointed the other way.
What survives is important and narrower. Genuine deficiency causes rickets in children and osteomalacia in adults, and correcting it is real medicine. Supplementation reliably reduces fracture risk in institutionalised older adults, usually alongside calcium. The respiratory-infection literature shows a small benefit concentrated in the deficient. And deficiency is common — in northern latitudes, in winter, in people with darker skin, in anyone who works indoors.
So the sensible approach is unglamorous: test rather than guess. If you are below roughly 50 nmol/L, correcting it is worthwhile. If you are already comfortably replete, the trials say more will not help, and above 4,000 IU daily long-term you start trading a nonexistent benefit for a real risk of hypercalcaemia. Sun exposure remains the design intent, and the sun exposure calculator will estimate what your latitude and skin type actually deliver.
Vitamin D from sun or diet is hydroxylated in the liver to 25(OH)D — the form measured on a blood test — then in the kidney to the active hormone calcitriol. Calcitriol regulates calcium absorption and binds receptors found in most tissues in the body, which is the basis for the very long list of claimed effects.
1,000-2,000 IU/day is generally below the adult tolerable upper intake level of 4,000 IU/day, but individual needs vary. Toxicity can cause hypercalcaemia — nausea, constipation, confusion, kidney stones, or calcification — and risk is higher with some medical conditions. Correction of diagnosed deficiency should be guided by a clinician and repeat blood testing; high-dose annual bolus regimens increased falls and fractures in trials and should be avoided.
This guide is educational, not medical advice. Talk to a physician or pharmacist before starting any supplement, especially alongside prescription medication or a medical condition.
Evidence review last updated