Vitamin D is made in skin exposed to sunlight and obtained in smaller amounts from food. The body converts it into a hormone that regulates calcium and bone, and receptors for it exist in most tissues, which is where the hope that it does far more than bones comes from.

Low blood levels are common, especially at high latitudes, in winter, in people with darker skin and in people who stay indoors. Low levels are also associated, in observational studies, with almost every bad outcome there is. That pattern is the heart of the vitamin D story, and of its problem: being ill, old or housebound lowers vitamin D, so a low level is as much a marker of poor health as a cause of it.

The 2010s settled part of the argument. Several trials large enough to answer the question gave vitamin D to tens of thousands of adults and watched for years. This entry is mostly about what they found.

The vitamin is converted in the liver and then the kidney into calcitriol, the active hormone, which raises calcium absorption in the gut and works with parathyroid hormone to keep blood calcium in range. In real deficiency the consequences are skeletal: rickets in children, softened bone and muscle weakness in adults.

Calcitriol also binds receptors in immune cells, muscle, blood vessels and elsewhere, and in cell studies it changes the expression of hundreds of genes. This is the mechanistic basis for the claims about infection, cancer, heart disease and mood. A receptor being present is not the same as supplementation changing an outcome, and the gap between the two is what the big trials tested.

It raises levels and treats deficiency. This is not in dispute. Supplementation reliably raises the blood marker, and in genuine deficiency it reverses the bone disease that deficiency causes.

For everyone else, the large trials were mostly null. VITAL randomized nearly 26,000 American adults to vitamin D or placebo for a median of five years and found no reduction in invasive cancer or major cardiovascular events. D-Health gave over 21,000 Australians aged 60 to 84 a monthly dose for up to five years: major cardiovascular events were slightly lower in the vitamin D group, but the confidence interval included no effect and the absolute difference was small. DO-HEALTH, in 2,157 Europeans over 70, found no benefit of vitamin D, omega-3 or an exercise program, alone or combined, on blood pressure, physical performance, cognition, fractures or infections over three years.

Mortality and fractures. A 2019 BMJ meta-analysis of 52 trials in over 75,000 people found no association between supplementation and all-cause or cardiovascular mortality, and a 16% reduction in cancer death that the authors flagged as needing confirmation. A 2017 JAMA meta-analysis of 33 trials in over 51,000 community-dwelling older adults found no association between vitamin D, calcium or both and hip or any other fracture.

The honest summary: in people who are not deficient, supplementing has not been shown to prevent the diseases it is sold to prevent. The trials were large enough that a big effect would have been seen.

At the doses used in the large trials, vitamin D was safe: VITAL specifically reported no excess of high blood calcium or other adverse events. The risk is at the top end. Vitamin D is fat-soluble and stored, and very high intakes over months raise blood calcium, with kidney stones, kidney damage and heart rhythm problems the known consequences. Cases of toxicity in the literature come from manufacturing errors, mislabelled products and people taking many times the usual amount on their own initiative.

Interactions matter for a few groups: people with sarcoidosis or other granulomatous disease, who convert it to the active form without the usual control, and people with kidney disease or on certain heart medicines. None of this is a reason for a deficient person not to correct a deficiency; it is a reason for the dose to be someone's decision rather than a guess.

Vitamin D exists as both a licensed medicine and a food supplement, and the line between them is the dose. Prescription-strength products for treating deficiency are approved medicines in the United States, the United Kingdom and the European Union. Lower-dose products are sold as supplements without any review of their claims.

Several countries fortify staple foods with it, and most publish a reference intake and an upper limit from their nutrition authorities. Those figures are the closest thing a supplement has to a label, and this site links to them rather than copying numbers it cannot keep current.