Vitamin D: Food, Sunlight and Supplements Explained

Vitamin D gets more attention from UK health bodies than almost any other nutrient, and for good reason: it's one of the few vitamins that a large share of the population is officially advised to supplement, regardless of how well they eat. This article explains what vitamin D actually does, why the UK's climate and latitude make it a special case, how to spot a deficiency, and what current NHS guidance recommends.

What vitamin D actually does

Vitamin D's best-known role is helping the body absorb calcium and phosphate from food, which are essential for healthy bones, teeth and muscles. Without enough vitamin D, the body can't regulate calcium properly, which over time can lead to bones becoming soft, weak or misshapen.

In children, severe, prolonged deficiency can cause rickets, a condition where bones don't harden properly, leading to bone pain, poor growth and, in some cases, visible bowing of the legs. In adults, the equivalent condition is called osteomalacia, which causes bone pain and muscle weakness. Vitamin D also plays a role in immune function and muscle function more broadly, which is why deficiency is also linked to general fatigue and aches, even before it reaches the point of affecting bone structure.

Where vitamin D comes from

Unlike most vitamins, the body can make its own vitamin D, but only under the right conditions. When skin is exposed to enough ultraviolet B (UVB) light from the sun, it triggers a chemical reaction that produces vitamin D. For most people in the UK, sunlight exposure between late March and the end of September is enough to produce and store sufficient vitamin D for the rest of the year, provided some skin is regularly exposed outdoors without sunscreen for short periods.

Food provides a smaller, secondary source. Oily fish such as salmon, mackerel, sardines and herring are among the richest natural sources. Smaller amounts are found in red meat, egg yolks and liver. Some foods are fortified with vitamin D, including most UK breakfast cereals, some fat spreads, and infant formula. However, it's genuinely difficult to get enough vitamin D from UK food sources alone, which is exactly why supplementation is so widely recommended here.

Why the UK is a special case

Vitamin D production depends on the angle of the sun, which changes with latitude and season. Between approximately October and early March, the sun in the UK sits too low in the sky for its UVB rays to reach the ground at a strength capable of triggering vitamin D production in skin, no matter how much time is spent outside. This is sometimes referred to as the "vitamin D winter", and it affects the whole of the UK.

Because of this, UK guidance says adults and children aged 4 years and over should consider 10 micrograms (400 IU) of vitamin D a day during autumn and winter. Some people at higher risk of low vitamin D are advised to consider it throughout the year. Guidance for babies and younger children is age-specific.

• People who are not often outdoors, for example those who are frail, housebound, or in a care home.
• People who usually cover most of their skin when outdoors, for cultural or religious reasons.
• People with dark skin, for example those of African, African-Caribbean or south Asian family background, whose skin needs more sun exposure to produce the same amount of vitamin D.
• Babies from birth to one year, particularly if breastfed (infant formula is already fortified, so this generally applies once formula intake drops below 500ml a day).

Recognising low vitamin D

Low vitamin D may cause no obvious symptoms, and symptoms such as tiredness or aches can have many other causes. A questionnaire or symptom list cannot diagnose a deficiency. If symptoms persist or you are worried about your risk, speak to a healthcare professional, who can decide whether assessment or a blood test is appropriate.

The NHS doesn't routinely test vitamin D levels in everyone, largely because the winter supplement advice already applies broadly. A blood test may be offered if a GP suspects deficiency based on symptoms, risk factors, or an underlying condition affecting absorption, such as coeliac disease or certain gut or kidney conditions.

Choosing and taking a supplement sensibly

Vitamin D supplements usually contain vitamin D2 (ergocalciferol) or D3 (colecalciferol). Both can raise vitamin D levels; D3 may raise them more effectively in some circumstances. For a routine supplement, follow the dose and form recommended by the NHS or a healthcare professional rather than assuming a higher-priced form is necessary.

Standard UK guidance is built around a daily dose of 10 micrograms (400 international units, or IU), which is considered enough for the general population's needs. Higher-dose products exist and are sometimes prescribed for confirmed deficiency, but these should generally be used under medical guidance rather than chosen off the shelf, since correcting a genuine deficiency safely usually needs a specific, monitored dose.

Dose matters. NHS guidance says adults should not take more than 100 micrograms (4,000 IU) of vitamin D a day unless advised by a clinician; lower limits apply to children. Check all products you use so that you do not accidentally double up. A clinician may prescribe a higher short-term dose to treat confirmed deficiency.

Common questions

Can you rely on extra sun instead of a supplement? Do not deliberately stay in strong sun long enough to burn. The time needed to make vitamin D varies with skin tone, season and other factors, and there is no single safe exposure time that works for everyone. Follow sun-safety guidance.

Do you need a blood test before starting a standard-dose supplement? No. The 10 microgram daily recommendation is considered safe and appropriate for the general population without needing a test first. Testing is more relevant if a GP suspects a more significant, confirmed deficiency that might need a higher, monitored dose.

Is it possible to get enough vitamin D from diet alone in the UK? For most people, no, not reliably, particularly during autumn and winter. Even a diet rich in oily fish and fortified foods tends to fall short of the recommended amount on its own, which is exactly why the supplement advice applies to virtually the whole population rather than just those with a poor diet.

Should children take vitamin D? UK advice depends on age and feeding. Breastfed babies should receive 8.5 to 10 micrograms daily from birth. Formula-fed babies do not need an extra supplement if they have more than 500ml of infant formula a day. Children aged 1 to 4 years should receive 10 micrograms daily; children aged 4 years and over should consider 10 micrograms during autumn and winter. Ask a pharmacist or health visitor if unsure.

A note for readers outside the UK

Vitamin D recommendations differ outside the UK, partly because sunlight availability varies so much by latitude, and partly because food fortification rules differ. In the United States, the National Institutes of Health sets a Recommended Dietary Allowance of 15 micrograms (600 IU) a day for most adults up to age 70, rising to 20 micrograms (800 IU) for adults over 70, with a safe upper intake level also set at 100 micrograms (4,000 IU) a day, similar to the UK figure. One practical difference is that milk in the US is very commonly fortified with vitamin D by default, which isn't the case for most milk sold in the UK. If you're in the US, or anywhere outside the UK, it's worth checking current guidance from the NIH, the CDC, or your own healthcare provider, since sun exposure, food fortification and official recommended amounts can all vary meaningfully by country and even by region within a country.

The bottom line

Vitamin D is one of the clearest cases where a supplement is genuinely useful for most people, not just those with a diagnosed gap, purely because of how UK daylight works for roughly half the year. A modest, standard daily dose during autumn and winter is considered low-risk and worthwhile for the general adult population, with a small number of groups advised to take it year-round. As with any supplement, more isn't automatically better, and sticking close to the recommended amount, rather than a high-dose product, is the sensible default unless a GP has advised otherwise.