Vitamin D: Dose, Deficiency and Winter

HEALTH · GLOBAL

Vitamin D: Dose, Deficiency and Winter

Why it matters, the recommended amounts, upper limits and who needs more

Vitamin D is the one supplement with a broad public health recommendation behind it in northern countries, and the reason is straightforward.

Why supplementation is advised

Vitamin D is made in skin exposed to ultraviolet B. Between roughly October and March at UK latitudes, sunlight is too weak for the skin to make any, whatever the weather does.

Health services therefore recommend a daily supplement of around 10 micrograms, which is 400 international units, for everyone through autumn and winter.

Who should take it year-round

People who are rarely outdoors, anyone in institutional care, people who cover their skin, and people with darker skin, who produce less from the same exposure.

Breastfed babies should be given a daily supplement from birth, and young children generally, following your health service's guidance.

Upper limits

More is not better. Long-term intake above 100 micrograms daily in adults risks raising blood calcium, which damages kidneys and bones. Very high doses sold as therapeutic should be used only on medical advice.

Check the total across everything you take, since multivitamins, fortified foods and cod liver oil all contribute.

Testing

Routine testing is not generally recommended for people without symptoms or risk factors. Where someone has bone pain, muscle weakness or a condition affecting absorption, a doctor may test and advise a higher dose.

D2 and D3

D3 raises blood levels more effectively than D2. D3 is usually from lanolin or fish, and vegan D3 from lichen is widely available.

Food sources

Oily fish, egg yolks, red meat and fortified foods contribute, and it is difficult to get enough from diet alone, which is why supplementation is advised in winter rather than a change of diet.

Mushrooms exposed to ultraviolet light contain D2, which is less effective than D3.

Children

Health services recommend a daily supplement for babies and young children in many countries, including breastfed babies from birth. Follow the specific guidance rather than adult products.

Why it matters

Vitamin D regulates calcium and phosphate, which keeps bones, teeth and muscles healthy. Severe deficiency causes rickets in children and osteomalacia in adults, both of which still occur.

Claims about it preventing colds, cancer or heart disease are far less well supported than the bone health case, and large trials in people who are not deficient have generally been disappointing.

Absorption

It is fat soluble, so take it with a meal containing some fat. A daily dose is as effective as a weekly one for most people, and easier to remember.

Sun exposure

Short periods of unprotected sun in summer make vitamin D, and burning gives no additional benefit and causes lasting damage.

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