Vitamin D, and why almost everyone needs it in winter

Vitamin D is unusual among nutrients: most of it comes from sunlight rather than food.

Which means the plant-based question is less relevant than the latitude question.

How it is made

Ultraviolet B radiation striking the skin converts a precursor into vitamin D, which is then processed by the liver and kidneys into the active form.

UVB does not reach the surface in sufficient quantity when the sun is low in the sky. Above roughly 35 to 40 degrees latitude, synthesis is negligible for several months of the year.

Which means that for a substantial share of the world's population, dietary and supplemental sources are the only ones available in winter.

Other factors reduce synthesis: darker skin, which requires longer exposure to produce the same amount; older age; sunscreen; clothing coverage; air pollution; and spending time indoors.

Why it matters

Vitamin D is required for calcium absorption and bone mineralisation. Severe deficiency causes rickets in children and osteomalacia in adults — conditions that have re-emerged in some populations.

It also has roles in muscle function and immune function, and low levels are associated with a wide range of conditions.

Worth a caveat: association is not causation, and large trials of supplementation for outcomes beyond bone health have frequently disappointed. The strong evidence is for bone and muscle; the broader claims are less settled than popular coverage suggests.

Deficiency prevalence

High. Population surveys in many countries find a substantial proportion of people with low levels, particularly in winter and particularly in people with darker skin at higher latitudes.

This is not a plant-based issue. It affects everyone.

The dietary sources

Limited, and mostly animal-derived.

Oily fish, egg yolks and liver are the main natural food sources. Fortified foods contribute in countries where fortification is common.

The plant sources:

UV-exposed mushrooms. Mushrooms produce vitamin D2 when exposed to ultraviolet light. Some are sold deliberately UV-treated and contain substantial amounts.

You can also do this at home — placing mushrooms gill-side up in direct sunlight for a period increases their vitamin D content measurably.

Fortified plant milks, cereals and spreads, depending on the country.

These are not sufficient on their own for most people in winter.

D2 and D3

Two forms.

D2 is plant or fungal derived and always suitable for a plant-based diet.

D3 is more commonly used and is traditionally derived from lanolin from sheep's wool. Vegan D3 from lichen is now widely available.

Evidence generally suggests D3 is somewhat more effective at raising and maintaining blood levels than D2. Both work.

For anyone plant-based, lichen-derived D3 is the better option where available.

Doses

Recommendations vary by country and generally sit around 400 to 800 international units daily for adults, with some bodies recommending more for those at risk.

Many countries now advise supplementation for the whole population during autumn and winter, and year-round for people at higher risk — those with darker skin, those who cover up, those who are housebound, and infants.

Upper limits exist. Vitamin D is fat-soluble and stored, and toxicity from excessive supplementation is possible, causing high blood calcium with serious consequences.

Very high doses taken without medical supervision are the mechanism, and cases have been reported. There is no benefit to high doses in people who are not deficient.

Testing

A blood test measuring 25-hydroxyvitamin D is the standard. It is not routinely offered everywhere and it is worth asking for if you have risk factors or symptoms.

Symptoms of deficiency are non-specific: fatigue, bone or muscle aches, muscle weakness, and in children, bone deformity and delayed development.

Many people with low levels have no symptoms at all, which is why testing rather than waiting is reasonable for those at risk.

Sun exposure

Where UVB is available — summer, middle of the day, at appropriate latitudes — short periods of exposure produce substantial amounts.

The amount needed varies enormously with skin tone, latitude, time of day and area exposed. General guidance suggests short exposures, well short of burning.

The tension with skin cancer prevention is real. The general position from health bodies is that brief exposure short of burning is compatible with sun protection advice, and that deliberate prolonged exposure for vitamin D is not recommended given the risks.

Supplementation avoids the trade-off entirely, which is why it is generally the recommended route.

The practical version

If you live above roughly 35 degrees latitude, supplement in autumn and winter at minimum.

If you have darker skin, cover up for cultural or religious reasons, are housebound, or are pregnant, supplement year-round.

Use lichen-derived D3 if you want a plant-based option, or D2 if not available.

Take it with a meal containing fat, since absorption is better.

Do not take very high doses without medical supervision.

General information rather than individual advice. Infants, pregnant women and people with kidney disease or conditions affecting calcium should discuss supplementation with a doctor. Do not take high-dose vitamin D without medical supervision.