Vitamin D
Vitamin D is a fat-soluble vitamin needed for calcium absorption and bone health. Its most important source is not food but production in the skin under sunlight.
| Type | Fat-soluble vitamin |
|---|---|
| Main roles |
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| Sources |
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| Deficiency note | A shortfall is associated with weakening bone health and muscle weakness; the level can only be established with a blood test, not judged from symptoms. |
| Caution | Because it is fat-soluble, high doses can accumulate; the dose and duration of a supplement should be set with a doctor alongside a blood test. |
The vitamin of the sun
Vitamin D works differently from the other vitamins in this atlas: its main source is not what we eat but production in the skin driven by the sun's UVB rays. Light that has passed through glass does not start that production. How much is made varies greatly with the season, the time of day, latitude, skin colour, age and sunscreen. Production can fall markedly in the winter months and during long stretches spent indoors.
What does it do in the body?
The best-known task of vitamin D is enabling calcium to be absorbed from the gut, which makes it one of the key players in the health of bones and teeth. It is also known to play a part in muscle function and in regulating the immune system. Food sources are limited: oily fish such as salmon, egg yolk and fortified products are the main examples, and reaching an adequate level from food alone is difficult for most people.
How should the decision to supplement be made?
Vitamin D deficiency is a widespread matter around the world and supplements are often used; but because the vitamin is fat-soluble, taking high doses without oversight can lead to accumulation and to an imbalance in calcium. The sound approach is to find out the current level with a blood test and to settle the dose, the duration and the need together with a doctor. Requirements differ from person to person with age, circumstances such as pregnancy, and habits of sun exposure.
Related terms: Calcium, Salmon, Egg, Magnesium