Also known as: Cholecalciferol, D3, Sunshine vitamin, Vegan D3 (lichen)
VitaminsStrong evidence
A fat-soluble hormone precursor made in skin from sunlight, and one of the few supplements with a specific UK public health recommendation. At British latitudes production stops entirely through the winter months.
Typical dose
10.0–25.0 mcg per day
With a meal containing fat; daily rather than intermittently
What it is
Vitamin D3, or cholecalciferol, is technically a prohormone rather than a vitamin. It is synthesised in skin when UVB radiation strikes 7-dehydrocholesterol, then converted in the liver and kidneys into the active hormone.
At UK latitudes, roughly 50 to 60 degrees north, the sun sits too low from around October to March for UVB to penetrate the atmosphere sufficiently. Production effectively ceases for those months regardless of time spent outdoors, which is the reason UK guidance recommends supplementation through autumn and winter for the whole population.
D3 is generally preferred to D2 for raising blood levels. It is usually sourced from lanolin; lichen-derived D3 is the vegan alternative. Being fat-soluble, it accumulates in tissue, which matters for both efficacy and toxicity.
How it works
Vitamin D is converted first to 25-hydroxyvitamin D in the liver, the form measured in blood tests, then to the active 1,25-dihydroxyvitamin D in the kidneys. The active form binds the vitamin D receptor, which is present in most tissues in the body.
Its classical role is regulating calcium and phosphate. It increases intestinal calcium absorption substantially, and without adequate vitamin D much dietary calcium is simply not absorbed, which is why the two are so often paired.
Because the vitamin D receptor is so widely distributed, including in immune and muscle cells, it influences gene expression well beyond bone metabolism. This breadth explains the volume of research linking vitamin D status to many outcomes, though association and causation are frequently conflated in that literature.
What the evidence shows
Bone health is the firmly established area. Vitamin D is essential for calcium absorption and bone mineralisation, and low status impairs both. Supplementation in populations with low status clearly helps.
Respiratory research has produced a reasonably consistent finding: meta-analyses suggest supplementation reduces the likelihood of acute respiratory infection, with benefit concentrated in those with low status to begin with, and with regular daily dosing rather than large intermittent doses.
Beyond that, large trials examining a range of long-term health outcomes in populations that already had adequate status have mostly been negative. The pattern is consistent and important: correcting a shortfall helps, while supplementing people who are already sufficient generally does not.
Dosing
UK guidance recommends 10 micrograms, or 400 IU, daily for everyone during autumn and winter, and year round for people with limited sun exposure or darker skin.
Supplemental products commonly supply 25 micrograms, or 1,000 IU, which is a reasonable general dose. Higher doses, 50 to 100 micrograms, are used to correct documented deficiency and are best guided by a blood test rather than guesswork.
The tolerable upper intake for adults is 100 micrograms, or 4,000 IU, daily. Take with a meal containing fat, which meaningfully improves absorption. Daily dosing outperforms large weekly or monthly doses in the infection literature.
25-hydroxyvitamin D has a half-life of 2-3 weeks; status changes over 8-12 weeks
Timing
With a meal containing fat; daily rather than intermittently
Competition status
Permitted — not on the WADA Prohibited List
Category
Vitamins
Safety and side effects
Safe at recommended intakes, with a long record of use. Because it is fat-soluble and accumulates, too much over a long period is possible in a way it is not for water-soluble vitamins.
Excess vitamin D raises blood calcium, producing nausea, vomiting, weakness, excessive thirst and confusion, and if prolonged it can damage the kidneys. Cases generally involve intakes far above the upper limit, often from dosing errors with concentrated drops.
Anyone taking high doses long term should have blood levels checked rather than assuming more is better.
Who should avoid it
Do not exceed 100 micrograms daily without medical supervision and blood monitoring. Speak to your doctor before use if you take any regular medication, if your calcium levels have ever been high, or if you have been told your kidneys are not working normally. Some people are more sensitive to vitamin D than others, so if you are under medical care, check first.
This entry is provided for information only. Food supplements should not be used as a substitute for a varied and balanced diet and a healthy lifestyle. Consult a qualified healthcare professional before use if you are pregnant, breastfeeding, taking medication or under medical supervision.
