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Vitamin D

/ˈvɪt.ə.mɪn ˈdiː/ · VIT-uh-min DEE noun

Also: calciferol; D2 (ergocalciferol) and D3 (cholecalciferol)

A fat-soluble vitamin that the skin produces when exposed to ultraviolet B sunlight, and which is also found in a small number of foods. It contributes to the normal absorption and use of calcium and phosphorus and to the maintenance of normal bones.

What it is and how it works

Vitamin D is unusual. Most vitamins come almost entirely from food, but this one is mainly made in the skin. When ultraviolet B (UVB) light reaches the skin, it turns a form of cholesterol into vitamin D3. The liver and kidneys then convert it, in two steps, into the active form the body uses.

Its best-known job concerns calcium. Vitamin D contributes to the normal absorption and use of calcium and phosphorus, and through this to the maintenance of normal bones and teeth. It also contributes to the maintenance of normal muscle function, to the normal function of the immune system and to the process of cell division.

Morning sunlight falling across an unmade bed and a bedside table
Low morning sunlight across a bedroom. In northern winters, the sun is too low for the skin to make much vitamin D.

Sun, seasons and latitude

How much vitamin D the skin can make depends on the angle of the sun. In much of Europe and North America, from roughly October to March the sun stays too low in the sky for useful UVB to reach the ground, whatever the weather. Time of day, cloud, sunscreen, clothing, age and skin tone all influence the amount produced in summer too. Darker skin needs longer exposure to make the same amount.

Sunburn is never necessary. Short, regular periods outdoors in the warmer months, with the usual care to avoid burning, are part of ordinary daily life for most people.

Food sources

Few foods contain much vitamin D naturally:

  • oily fish such as salmon, mackerel, herring and sardines,
  • egg yolks,
  • liver,
  • mushrooms exposed to UV light (vitamin D2),
  • foods fortified by the manufacturer, which may include some breakfast cereals, plant drinks, margarines and, in some countries, milk.

Because food sources are limited, several health authorities in northern countries suggest that some groups consider a vitamin D supplement in autumn and winter. The right approach depends on age, life stage, skin tone and health, and is something to discuss with a doctor, pharmacist or dietitian. Vitamin D is fat-soluble and stored in the body, so more is not automatically better: very high intakes over time can cause harm.

In everyday life

  • Get outside regularly in spring and summer: a walk at lunchtime counts; see circadian rhythm for why daylight matters in other ways too.
  • Oily fish once a week brings vitamin D along with omega-3 fats.
  • Read the label of plant drinks and cereals if you rely on fortified foods.
  • Ask in winter if you are in a higher-risk group: older adults, people who rarely go outdoors, those who cover most of their skin, and people with darker skin living far from the equator.

Common confusions

"Sitting by a sunny window makes vitamin D." Ordinary window glass blocks most UVB.

"A sunny winter day tops you up." At northern latitudes the winter sun is too low, even when it feels bright.

"Vitamin D is just about bones." Bones are the headline, but its recognised roles include muscle function and the immune system.

"You can't have too much." You can, almost always through supplements rather than sun or food. Follow professional advice on any supplement.

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Added Oct 10, 2026 · Filed under Food & nutrients