Vitamin Dnmol/L
Vitamin D status
Also known as: 25-OH Vitamin D, 25(OH)D
Typical optimal range
50–200 nmol/L
UK laboratory results for 25(OH)D are reported in nmol/L, with most NHS and clinical guidance considering levels around 50–200 nmol/L as adequate or optimal, though reference ranges can differ slightly between laboratories. These figures are indicative — the range printed on your own lab report is the definitive one for your results.
NICE guideline PH56 ('Vitamin D: increasing supplement use among at-risk groups') and NHS guidance on vitamin D both recommend daily supplementation of 10 mcg (400 IU) for the general UK population during autumn and winter, with higher doses for those at greater risk of deficiency.
What is Vitamin D?
Vitamin D (measured in blood as 25-hydroxyvitamin D, or 25(OH)D) is a fat-soluble vitamin that the body produces mainly when skin is exposed to UVB sunlight, with smaller amounts coming from food and supplements. It plays a central role in calcium absorption and bone health, immune regulation, muscle function, and mood. Measuring 25(OH)D in the blood is the accepted way to assess the body's overall vitamin D stores and identify deficiency or insufficiency before symptoms develop.
What a high Vitamin D means
A raised vitamin D result is uncommon unless someone is taking high-dose supplements, and a very high reading is a prompt to review supplementation rather than a sign of illness. Toxicity is rare but worth taking seriously if levels are markedly elevated.
- High-dose supplementation. By far the most common cause of elevated 25(OH)D is taking large doses of vitamin D supplements over time. The NHS advises that daily doses above 4,000 IU (100 mcg) for adults carry a risk of harm, and very high levels (above roughly 250–300 nmol/L) can raise blood calcium to unsafe levels.
- Excessive fortified foods or fish oils. Taking multiple products each containing vitamin D — such as multivitamins, fish oil capsules, and fortified foods — can add up to unexpectedly high intakes, especially alongside a therapeutic supplement.
True toxicity from sunlight alone is not possible, as the skin regulates production. Very high levels are almost always supplement-related; if you are not supplementing, a high result warrants a repeat test and GP review.
What a low Vitamin D means
A low vitamin D result is extremely common in the UK, particularly in winter, and is the clinically important direction for this marker — it is linked to bone loss, fatigue, low mood, muscle weakness, and reduced immune function.
- Insufficient sunlight exposure. The UK's northern latitude means that between approximately October and March, UVB radiation is too weak for the skin to make meaningful vitamin D. People who cover most of their skin, spend most of their time indoors, or have darker skin (which requires longer sun exposure to produce equivalent vitamin D) are particularly at risk year-round.
- Low dietary intake. Very few foods naturally contain significant vitamin D — oily fish, eggs, and red meat provide modest amounts. Vegans and people who avoid these foods are at higher risk of low levels, especially without supplementation.
- Malabsorption or gut conditions. Conditions such as coeliac disease, Crohn's disease, and short bowel syndrome can impair fat-soluble vitamin absorption including vitamin D, as can bariatric surgery. People with liver or kidney disease may also have impaired conversion to the active form.
Obesity is associated with lower circulating 25(OH)D because vitamin D is sequestered in fat tissue; older adults are also at higher risk as skin efficiency in making vitamin D declines with age.
Should I be worried?
A single low vitamin D result is common and very treatable, so there is no need to be alarmed — but it is worth discussing with your GP, particularly if you have symptoms such as bone pain, muscle weakness, fatigue, or frequent infections. Your GP may want to repeat the test, check calcium, phosphate, and parathyroid hormone (PTH) alongside it, and consider whether an underlying condition affecting absorption needs investigation. If you are found to be deficient, they may recommend a loading dose followed by a maintenance supplement. For many people, following the NHS's routine advice to supplement with 10 mcg (400 IU) daily through autumn and winter is sufficient for prevention. A markedly high result in someone taking supplements is also worth a GP conversation to avoid the risk of hypercalcaemia.
What actually helps
Several well-supported lifestyle and dietary measures can help bring low vitamin D levels into a healthy range.
- Safe sun exposure. Exposing the arms and legs (or equivalent area) to sunlight for around 15–30 minutes around midday between April and September — without sunscreen on the exposed areas for that time — is the most efficient way the body makes vitamin D. People with darker skin may need longer. This is not possible in UK winter months.
- Vitamin D3 supplementation. The NHS recommends 10 mcg (400 IU) of vitamin D3 (cholecalciferol) daily for the general population in autumn and winter; higher doses of 1,000–4,000 IU are commonly used to correct deficiency under GP guidance. D3 is generally considered slightly more effective than D2 at raising blood levels.
- Consider vitamin K2 alongside D3. Some evidence suggests that vitamin K2 may help direct calcium to bones rather than soft tissues when taking vitamin D supplements, though the evidence is not yet definitive enough for formal UK guideline recommendations. It is low-risk and widely discussed in nutritional medicine circles.
- Weight management. Because vitamin D is stored in fat tissue, higher body fat is associated with lower circulating levels. Gradual, sustainable weight loss in those with obesity can modestly improve 25(OH)D, though supplementation is still usually needed.
There is good supporting evidence that including these vitamin D-containing foods regularly helps maintain status, particularly alongside supplementation.
Oily fish (salmon, mackerel, sardines, herring)
Among the richest natural dietary sources of vitamin D3; a portion of salmon can provide 300–600 IU, contributing meaningfully to daily intake.
Eggs (especially the yolk)
Contain modest amounts of vitamin D; free-range and outdoor-reared eggs tend to have higher levels due to the hens' own sun exposure.
Fortified foods (plant-based milks, breakfast cereals, some margarines)
Many UK-sold fortified products provide a useful contribution to daily vitamin D intake, particularly for people who avoid fish and meat.
Red meat and liver
Provide small amounts of vitamin D along with other fat-soluble nutrients; liver is particularly rich but should not be eaten daily due to its very high vitamin A content.
Mushrooms exposed to UV light
Certain mushrooms placed gill-side up in sunlight generate vitamin D2; UV-treated mushrooms are available in some UK supermarkets and are the most practical plant-based dietary source.
Track your own Vitamin D
Upload a lab PDF and Better Days trends Vitamin D over time, flags it when it drifts, and suggests foods that help.
More in Vitamins/Minerals
Sources & further reading
These are reputable UK health resources for further reading. This page is written to be consistent with their guidance but is not a substitute for them.
For information only — not medical advice, and not individually reviewed by a clinician. Reference ranges vary by lab and individual. Discuss any results with a qualified clinician before making changes.