Nutrition

Vitamin D: How Much You Need, Who Is Short, and What Trials Show

Nutrition · Published 2026-10-24

Vitamin D is the one vitamin that UK health authorities advise almost everyone to consider taking for part of the year. Yet the largest trials have found that extra vitamin D does little for people who are not deficient. Both statements are true, and the difference between them explains most of the confusion about this nutrient.

What vitamin D does

Vitamin D helps the body absorb calcium and phosphate from food, which keeps bones, teeth and muscles working normally. Severe, long-lasting deficiency causes rickets in children (soft, deformed bones) and osteomalacia in adults (bone pain and muscle weakness). Vitamin D receptors are found in many tissues, which is why vitamin D has been studied for heart disease, cancer, infections, depression and diabetes. Interest in those areas does not mean supplements prevent those diseases, as the trials below show.

Unlike most vitamins, the body makes its own vitamin D: the skin produces it when exposed to ultraviolet B (UVB) light. Few foods contain much of it naturally. The main ones are oily fish (salmon, sardines, herring, mackerel), egg yolks, liver and red meat, plus fortified foods such as some breakfast cereals, fat spreads and plant drinks, and milk in countries that fortify it.

Reference intakes

Reference values differ between bodies because they are built on different assumptions about sun exposure and on different target blood levels. They are population figures, not personal prescriptions.

One microgram (mcg) of vitamin D equals 40 international units (IU). For infants, the US adequate intake is 10 mcg (400 IU) a day; the NHS advises 8.5 to 10 mcg a day for breastfed babies from birth, while babies drinking more than 500 ml of infant formula a day should not be given an extra supplement because formula is already fortified. Upper limits for children are lower than for adults (in the US, 25 mcg for babies up to six months, rising with age to 100 mcg from age nine).

The UK autumn-and-winter advice

In the UK, sunlight between about late March or April and the end of September lets most people make enough vitamin D. From October to early March the sun is too low for the skin to make it. The NHS therefore advises that everyone consider a daily 10 mcg supplement in autumn and winter, and that people at higher risk consider one all year round. That higher-risk group includes people who are rarely outdoors (for example, care home residents or the housebound), people who usually cover most of their skin outdoors, and people with dark skin, such as those of African, African-Caribbean or South Asian origin, whose skin makes less vitamin D from the same sunlight.

This advice targets a public-health problem: national surveys show that a sizeable share of the UK population has low blood levels in late winter. It is a preventive measure for bone and muscle health, not a claim that vitamin D prevents other diseases.

Blood levels and testing

Vitamin D status is measured as serum 25-hydroxyvitamin D, written 25(OH)D. The US National Academies used these thresholds in their 2011 report:

• below 30 nmol/L (12 ng/mL): risk of deficiency;
• 30 to 50 nmol/L (12–20 ng/mL): possibly inadequate for bone health in some people;
• 50 nmol/L (20 ng/mL) or more: adequate for bone health in almost everyone;
• above 125 nmol/L (50 ng/mL): linked to potential adverse effects.

Some laboratories and societies use higher targets, which is one reason different sources disagree about how common "deficiency" is. The Endocrine Society's 2024 guideline recommends against routine 25(OH)D testing in healthy adults under 75, because trials have not shown that testing and treating to a target level improves outcomes in that group. Testing remains useful for people with symptoms, bone disease, malabsorption or other medical reasons, which a clinician decides.

What the trials found

Observational studies long showed that people with low vitamin D had more heart disease, cancer and fractures. Randomised trials were designed to test whether supplements change that, and the large ones in generally healthy adults have mostly been negative:

• VITAL (United States, 25,871 adults, median 5.3 years, 2,000 IU a day): no reduction in invasive cancer or in major cardiovascular events compared with placebo (Manson et al., NEJM 2019).
• VITAL fractures substudy: in generally healthy midlife and older adults who were not selected for deficiency or osteoporosis, vitamin D did not lower the risk of fractures (LeBoff et al., NEJM 2022).
• Endocrine Society 2024: based on a review of trials, the guideline suggests empiric vitamin D for some groups (children and adolescents, adults over 75, pregnant people and adults with high-risk prediabetes) and does not suggest it for healthy adults under 75 beyond the usual reference intakes.

The picture that emerges is consistent: correcting a real deficiency matters, and some groups benefit from a modest daily amount, but topping up people who already have adequate levels has not delivered the broad benefits once hoped for.

When vitamin D causes harm

Vitamin D toxicity is almost always caused by supplements, not by sunlight or food. Very high intakes over weeks or months raise blood calcium (hypercalcaemia), which can cause nausea, vomiting, confusion, excessive thirst, kidney stones and, in severe cases, kidney damage and heart rhythm problems. Cases have been reported after people took products mislabelled with far more vitamin D than stated, or took high-dose products daily for long periods. This is why the 100 mcg (4,000 IU) adult upper limit matters, and why high-dose prescriptions should only be taken under medical supervision.

Vitamin D can interact with some medicines, including thiazide diuretics (raising the risk of high calcium), some anti-epileptic drugs and steroids (which can lower vitamin D levels). Anyone on long-term medication should check with a pharmacist.

In short

• Adult reference intakes are 10 to 20 mcg a day depending on the authority and age; the adult upper limit is 100 mcg.
• In northern latitudes, many people cannot make vitamin D in winter, which is why the NHS advises a 10 mcg daily supplement from October to March.
• Higher-risk groups (dark skin, little sun exposure, older age, infants) may need it all year.
• Large trials in healthy adults have not shown that extra vitamin D prevents cancer, heart disease or fractures.

This page is general information, not medical advice. Ask a doctor, pharmacist or registered dietitian before taking a supplement, especially if you are pregnant, take medication or have kidney disease or another long-term condition.

Sources

• NIH Office of Dietary Supplements, Vitamin D fact sheet for health professionals.
• NHS, Vitamin D.
• EFSA, Dietary reference values finder.
• Manson JE et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med 2019;380:33–44.
• LeBoff MS et al. Supplemental vitamin D and incident fractures in midlife and older adults. N Engl J Med 2022;387:299–309.
• Demay MB et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab 2024.

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