Vitamin D has had an unusual trajectory. Observational studies linked low levels to an enormous range of conditions, enthusiasm followed, large randomised trials were conducted, and the results were considerably more modest than expected.
The current position is worth stating clearly because the popular understanding hasn't caught up with the trial evidence.
What it does
Vitamin D is essential for calcium absorption and bone mineralisation. Severe deficiency causes rickets in children and osteomalacia in adults — conditions involving inadequate bone mineralisation with real and serious consequences.
It's produced in the skin on exposure to UVB radiation and obtained in smaller amounts from a limited range of foods — oily fish, egg yolks, and fortified products where fortification exists.
Because sunlight is the main source, status depends heavily on latitude, season, skin pigmentation, sun exposure behaviour, clothing and age.
Who's likely to be deficient
Deficiency is common, particularly in some groups.
People living at higher latitudes during winter months, when UVB is insufficient for synthesis regardless of time spent outdoors.
People with darker skin, since melanin reduces UVB penetration and therefore synthesis for a given exposure.
People who cover extensively for cultural or medical reasons.
Older adults, whose skin synthesises less efficiently and who may spend less time outdoors.
People who are housebound or in institutional care.
People with malabsorption conditions or obesity, where vitamin D is sequestered in adipose tissue.
What the evidence supports
Bone health. The clearest case. Supplementation prevents rickets and osteomalacia in deficient individuals. In older adults, evidence for fracture prevention is somewhat mixed, and combined calcium and vitamin D supplementation has evidence in institutionalised populations.
Correcting deficiency. If levels are low, supplementation raises them. Straightforward.
Beyond bone health, the picture is less encouraging than the observational literature suggested.
Where large trials disappointed
Several major randomised trials with tens of thousands of participants examined vitamin D supplementation with cardiovascular, cancer and mortality outcomes.
The general finding has been that supplementation in generally replete populations did not significantly reduce these major outcomes.
Trials examining fracture prevention in community-dwelling adults have also produced mixed results, with some finding no benefit.
Interpretations vary. One argument is that these trials enrolled people who were largely not deficient, so there was little room for benefit — supplementing somebody who is sufficient shouldn't help. Another is that observational associations were confounded, with low vitamin D being a marker of poor health rather than a cause of it, since ill people go outside less.
Both explanations are plausible and probably both contribute. Either way, the practical conclusion is similar: supplementation is valuable for correcting deficiency and has not been shown to benefit people who are already sufficient.
Testing
Testing measures 25-hydroxyvitamin D in blood.
Thresholds for deficiency and sufficiency differ between organisations, which produces confusion — the same result can be labelled deficient by one body and adequate by another.
Routine population-wide testing is generally not recommended by health bodies, on the grounds that it's expensive and doesn't change management for most people, since those at risk can simply be advised to supplement.
Testing is appropriate where there's a specific clinical reason — symptoms, a malabsorption condition, or before certain treatments.
Dosing and safety
Public health recommendations in many countries suggest a modest daily supplement for the general population during winter months, with higher recommendations for at-risk groups.
Vitamin D is fat-soluble and accumulates. Toxicity is possible with excessive intake, causing hypercalcaemia with symptoms including nausea, weakness, kidney problems and cardiac effects.
Toxicity is rare and essentially always results from very high-dose supplementation rather than from sun exposure or food.
Notably, some trials of very high intermittent doses have found unexpected adverse outcomes, including increased falls in some studies. That's a useful reminder that more is not automatically better and that dosing schedules matter.
The reasonable position
If you're in a group at risk of deficiency — and in higher-latitude countries during winter, that's most people — a modest daily supplement at recommended levels is sensible, inexpensive and safe.
If you're taking it expecting effects on cardiovascular disease, cancer, respiratory infection or general vitality, the large trial evidence doesn't support that expectation.
And if you're taking very high doses on the reasoning that more must be better, that's the one version of this worth reconsidering, particularly without monitoring.
General information only. Discuss supplementation with a qualified healthcare professional, particularly if you have kidney disease, take medication, or are considering high doses.
Sun exposure, weighed honestly
An awkward tension worth naming, since two pieces of health advice point in opposite directions.
UV exposure produces vitamin D and also causes skin damage and cancer risk. Advice to get more sun for vitamin D sits uneasily alongside advice to protect against UV.
The resolution generally offered by dermatology and public health bodies is that supplementation is the safer route. Vitamin D from a tablet carries no UV exposure, costs very little, and delivers a reliable dose independent of latitude, season, skin tone or weather.
Which makes this one of the rare cases where the supplement is genuinely preferable to the natural route, and where advice to seek sun exposure specifically for vitamin D is not well founded.