Vitamin D deficiency UK: are 90 percent really low?

The claim that 90 percent of people in the UK are vitamin D deficient is not supported by the standard UK deficiency threshold. The real story is more nuanced: low vitamin D is common, especially in winter and in higher-risk groups, but UK survey data for 2019 to 2023 found 18% of adults aged 19 to 64 and 12% of adults aged 65 and over below 25 nmol/L across the year.¹ The right response is targeted supplementation and testing where useful, not panic dosing.

Where the 90 percent claim goes wrong

Vitamin D status depends on the cut-off used. The UK public-health concern is often framed around 25-hydroxyvitamin D below 25 nmol/L, because that level increases risk of poor bone and muscle health.¹ Some private clinics and wellness content use higher "optimal" cut-offs, then describe anyone below them as deficient. That changes the headline dramatically.

The National Diet and Nutrition Survey 2019 to 2023 reported low vitamin D status in 10% of children aged 4 to 10, 23% of children aged 11 to 18, 18% of adults aged 19 to 64, and 12% of adults aged 65 and over.¹ Those numbers are important, but they are not 90%.

The same data still supports action. A country can have a serious deficiency problem without everyone being deficient. The risk is uneven: winter, skin tone, clothing, indoor life, geography, diet, body composition, malabsorption and medicines all change the picture.

Who is most at risk in the UK

Vitamin D is made in skin when ultraviolet B light is strong enough. In the UK, that is unreliable through autumn and winter. Sunscreen, clothing, indoor work, shift work, air pollution, northern latitude and avoiding midday sun reduce production further. Diet contributes some vitamin D, but it is difficult for many people to get enough from food alone.

NHS vitamin D guidance says people should consider 10 micrograms daily in autumn and winter, and some people should consider it throughout the year, including those who are not often outdoors, live in care homes, usually wear clothes that cover most of the skin outdoors, or have dark skin.⁴ Babies, young children, pregnancy and breastfeeding also have specific advice, so dosing should match life stage.

When testing is useful

Not everyone needs a vitamin D blood test before taking a standard low-dose supplement. Testing is more useful when symptoms, medical risk or high dosing are involved. Consider asking about testing if you have bone pain, muscle weakness, recurrent low-trauma fractures, osteoporosis risk, malabsorption, kidney disease, liver disease, bariatric surgery, certain medicines, or you are already taking high doses.

The vitamin D testing guide explains how 25-hydroxyvitamin D is interpreted and why nmol/L cut-offs matter. If fatigue is the only symptom, also think wider. Iron deficiency, B12 deficiency, thyroid disease, sleep apnoea, depression, chronic inflammation and diabetes risk can all overlap. The B12 and folate guide and iron guide cover two common mimics.

If you test privately, bring the result to a clinician with the reference range, units, supplement dose and timing. A blood level taken after weeks of high-dose supplementation tells a different story from a baseline winter level.

Units are a common source of confusion. UK laboratories usually report 25-hydroxyvitamin D in nmol/L, while some international material uses ng/mL. They are not interchangeable: 25 nmol/L is about 10 ng/mL. If a website uses a different unit or a higher wellness target, its "deficiency" claim may not match NHS or UK public-health language.

Testing can also uncover the wrong question. If vitamin D is low because you have coeliac disease, inflammatory bowel disease, pancreatic problems, liver disease, kidney disease or bariatric surgery, the long-term answer is not simply a bigger bottle. The absorption or activation problem needs to be recognised.

How to supplement safely

NHS guidance says 10 micrograms is enough for most people as a daily supplement, and adults should not take more than 100 micrograms a day because it could be harmful.⁴ That does not mean 100 micrograms is a target. It is an upper safety limit for most adults.

For confirmed deficiency, clinicians may prescribe or advise higher short-term doses, then maintenance dosing. That should be monitored where risk is higher. If you have kidney disease, high calcium, sarcoidosis, lymphoma, primary hyperparathyroidism, kidney stones or take certain medicines, ask before taking high-dose vitamin D.

SACN's rapid review on vitamin D fortification reflects the public-health challenge: supplementation works for individuals who take it, but food fortification is considered because population intake and status remain uneven.² Public Health England's advice also supported 10 micrograms daily for everyone over one year during autumn and winter.³

Do not combine several products without adding the dose. A multivitamin, bone formula, immune blend and separate vitamin D capsule can quietly stack into a high daily intake. Check whether the label uses micrograms or international units. Ten micrograms is 400 IU. Twenty-five micrograms is 1000 IU. More impressive numbers are not automatically better.

Sunlight is not a simple prescription either. Some sun exposure can help vitamin D, but burning raises skin-cancer risk, and many people cannot rely on sun because of work, clothing, skin type, disability, medicines or medical advice. For most UK adults, a boring daily supplement is more controllable than chasing perfect sun exposure.

Food still helps, just usually not enough on its own. Oily fish, eggs, fortified spreads, fortified breakfast cereals and some dairy alternatives can contribute, but labels vary. If you avoid animal foods, check fortification and consider whether B12, iodine and calcium also need attention.

What vitamin D can and cannot fix

Vitamin D matters for bones and muscles. NHS rickets and osteomalacia guidance describes how vitamin D or calcium deficiency can cause bone problems, including rickets in children and osteomalacia in adults.⁵ NHS osteoporosis prevention guidance also includes vitamin D, calcium, exercise, stopping smoking and limiting alcohol as part of bone health.⁶

But vitamin D is not a universal fatigue, mood, hormone or immune cure. If a low level is present, correcting it is sensible. If the level is adequate, taking more is unlikely to fix unrelated problems. Use the insights section to pressure-test supplement claims before buying high-dose products, and use Start here if symptoms persist despite reasonable supplementation.

It is also worth separating deficiency symptoms from deficiency risk. Many people with low vitamin D have no obvious symptoms. Many people with severe fatigue have normal vitamin D. That is why symptom-based guessing is weak. Risk factors, blood tests and response to treatment give a much clearer answer.

The practical conclusion is simple: vitamin D deficiency is common enough to take seriously, especially in UK winter and higher-risk groups, but not so universal that everyone should mega-dose. Use the right threshold, the right dose and the right follow-up.

This article is educational and does not constitute medical advice, diagnosis, or a treatment recommendation. Medication uses described as “off-label” are not licensed for that purpose in the UK and should only be considered under qualified clinical supervision. Always speak to your GP, pharmacist, or a registered specialist before starting, stopping, or changing any treatment. If you have severe or alarm symptoms - unintentional weight loss, blood in your stool, difficulty swallowing, persistent vomiting, a fever, or severe pain - seek urgent medical care.