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Understanding vitamin D
Audience: patients-public, students-learners ยท Review state: published
What it does
Vitamin D helps the body absorb calcium, making it essential for strong bones and teeth. It also plays a part in muscle function, nerve signaling, and immune defense.
Too little vitamin D over time weakens bones: rickets in children, and osteomalacia, soft and painful bones, in adults, and it contributes to osteoporosis risk in older people.
Where it comes from
Unlike most vitamins, the main source is not food but sunlight: the skin makes vitamin D when exposed to ultraviolet light. Food sources are fewer - oily fish such as salmon and sardines, egg yolks, liver, and fortified foods like some milks, cereals, and spreads.
How much sunlight a person needs varies with skin tone, season, latitude, and how much skin is exposed.
Who runs low
Deficiency is common in people who get little sun: those who are housebound, cover their skin, live at high latitudes through winter, or have darker skin, which needs more sun to make the same amount. Older adults, breastfed babies, and people with conditions affecting fat absorption are also at risk.
A blood test shows vitamin D status when deficiency is suspected.
Supplements and safety
Many health authorities recommend a daily supplement for at-risk groups, and some recommend one for everyone through autumn and winter. Typical maintenance doses are modest; more is not better.
Vitamin D is fat-soluble and stored in the body, so very high doses can build up and cause harm, raising blood calcium and affecting the kidneys and heart. Supplements should follow recommended doses or a clinician's advice, especially alongside other medicines.
Limitations
General educational overview, not medical advice. Supplement needs and doses are individual, especially in pregnancy, kidney disease, or with other medicines.