Vitamin D

Vitamin D is a fat-soluble nutrient that the body can make itself when skin is exposed to sunlight, and that also comes in smaller amounts from certain foods. Because of this dual origin, it behaves partly like a vitamin and partly like a hormone. Its best-established job is helping the body absorb calcium and phosphate, which are essential for strong bones and teeth, but it also plays a part in muscle function and in the immune system.

Vitamin D deficiency is common in many parts of the world, particularly among people with limited sun exposure, darker skin, older age or certain health conditions. At the same time, enthusiasm for high-dose supplements has outpaced the evidence for benefits in people who are not deficient, making it a good example of how a genuinely important nutrient can also be over-sold.

How the body makes and uses it

When ultraviolet B (UVB) light from the sun reaches the skin, it triggers the production of a precursor that the body converts into a form of vitamin D. This form then travels to the liver and kidneys, where it is turned into the active hormone that regulates calcium and phosphate. Sunscreen, clothing, glass, cloud cover, latitude and season all reduce how much UVB reaches the skin, and in winter many people in higher latitudes make little or none.

Because vitamin D is stored in body fat, the body can draw on reserves during darker months. Very dark skin, which contains more of the pigment melanin, needs longer sun exposure to make the same amount, a factor that helps explain higher deficiency rates among people with darker skin living far from the equator.

Food sources

Naturally rich foods are few. Oily fish such as salmon, mackerel and sardines are among the best sources, along with egg yolks and some mushrooms exposed to ultraviolet light. Many countries fortify foods such as milk, margarine, breakfast cereals or orange juice, and fortified products are a major dietary source in those places. For people who avoid these foods or get little sun, supplements are often the practical route.

Why it matters: bones and beyond

The critical role in calcium absorption means severe deficiency causes rickets in children, in which bones soften and deform, and osteomalacia in adults, characterised by bone pain and muscle weakness. Long-term low levels contribute to osteoporosis and fractures, especially in older adults.

Beyond bone health, vitamin D receptors are found in many tissues, including immune cells. Laboratory work shows the vitamin influencing immune responses, and observational studies have linked low levels to a range of conditions. However, correlation is not causation: people who are ill or housebound often have both low vitamin D and poorer health for other reasons. Large randomised trials testing whether supplements prevent infections, cancer or heart disease have generally shown little or no benefit in populations that were not deficient, which has tempered earlier hopes.

Deficiency and who is at risk

Risk factors for deficiency include living at high latitude, spending most time indoors, covering the skin for cultural or religious reasons, having darker skin, being older, and taking certain medications. People with conditions affecting fat absorption, such as some gut or liver diseases, and those who have had bariatric surgery, may absorb vitamin D poorly. Infants who are exclusively breastfed are usually advised to receive a supplement, because breast milk contains little vitamin D unless the mother is supplemented.

Symptoms of deficiency are often vague or absent and may only become clear through blood tests or when bone problems appear. Clinicians measure a circulating form of the vitamin to assess status, and thresholds for what counts as adequate continue to be debated.

Getting the balance right

Public health guidance typically suggests short, regular periods of sun exposure for most people, taking care to avoid burning, which is a skin cancer risk. The exact amount of sun needed depends on skin type, location and season; there is no single figure that fits everyone.

Where supplements are used, moderate daily doses are generally preferred, and official recommended intakes are usually expressed in international units or micrograms. Because vitamin D is fat-soluble, it can accumulate, and very high doses over long periods can cause harm, including raised blood calcium that may damage the kidneys and weaken bones. This is why "more is better" is a poor rule for this nutrient, and why guidance usually stops well short of megadoses.

Common misconceptions

One widespread belief is that vitamin D supplementation improves almost everything. The evidence supports treating genuine deficiency and protecting bone health, but not routine high-dose use for the general population to prevent unrelated diseases. A second misconception is that sunlight through a window will produce the vitamin; ordinary glass blocks the relevant UVB rays, so it will not. A third is that a tan is a reliable sign of adequate levels — tanning indicates UV exposure but does not guarantee optimal vitamin D status.

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