Vitamin D: Benefits, Deficiency, Supplements and How Much You Need

Vitamin D is a fat-soluble nutrient that also behaves like a hormone within the body. Its two main forms are vitamin D3, also called colecalciferol, and vitamin D2, also called ergocalciferol.

Vitamin D3 is produced naturally in the skin when it is exposed to ultraviolet B radiation from sunlight. Vitamin D2 is produced in fungi and yeast, while both forms may be used in fortified foods and supplements.

The body converts vitamin D through several stages before it becomes biologically active. A blood test usually measures 25-hydroxyvitamin D, written as 25(OH)D, because this is the main marker used to estimate vitamin D status.

Vitamin D helps regulate the absorption and use of calcium and phosphate. These minerals are vital for building and maintaining healthy bones, maintaining normal muscle function, supporting healthy teeth and allowing normal skeletal growth in babies and children.

Without enough vitamin D, the body cannot use calcium efficiently, even when the diet contains plenty of calcium.

Severe deficiency in children can cause rickets. This affects the mineralisation of growing bone and may cause bowed legs, delayed growth, bone deformity, muscle weakness, delayed walking and, if blood calcium becomes very low, seizures.

The risk of rickets appears to rise when serum 25(OH)D is below approximately 25 nmol/L, although this is not a diagnostic cut-off and calcium deficiency may also contribute.

In adults, severe vitamin D deficiency can cause osteomalacia, which means newly formed bone does not mineralise properly. This may lead to widespread or localised bone pain, tenderness over the bones, muscle weakness, difficulty climbing stairs or getting out of a chair, a waddling gait and an increased risk of falls.

Osteomalacia is associated with very low vitamin D levels, although no single blood level can reliably diagnose the condition by itself.

Vitamin D is important for preventing deficiency-related bone disease, but this does not mean vitamin D tablets alone prevent every type of fracture.

Studies in adults aged 50 and over have been mixed. Overall, vitamin D alone did not clearly reduce fracture risk, while vitamin D combined with calcium appeared more effective in some groups.

Evidence concerning muscle strength and falls is also mixed, although it suggests possible benefit in older adults and in people with low baseline vitamin D. Importantly, some trials using very large annual or monthly doses reported an increase in falls. This is one reason routine daily doses are preferred to unsupervised “mega-doses”.

Vitamin D has been linked in observational studies with many conditions, including cancer, cardiovascular disease, high blood pressure, infection, autoimmune disease, depression, cognitive decline and dementia.

However, an association does not prove that low vitamin D caused the condition. People who are unwell may spend less time outdoors, eat differently or become less active, all of which can lower vitamin D levels.

Randomised trials examining non-musculoskeletal outcomes have been inconsistent, and the evidence was judged insufficient to use these proposed benefits when setting UK vitamin D recommendations. Vitamin D should therefore be taken to maintain adequate status and protect musculoskeletal health — not as a proven treatment for cancer, depression, heart disease or dementia.

Vitamin D comes from three main sources: sunlight acting on the skin, food, and supplements.

For most people, sunlight is the main natural source of vitamin D. In the UK, the skin can usually make vitamin D during the spring and summer, broadly from April to September. During autumn and winter, sunlight generally does not contain enough ultraviolet B radiation for reliable vitamin D production.

How much vitamin D a person produces depends on the time of year, the time of day, latitude, cloud cover and air pollution, how much skin is uncovered, skin pigmentation, age and time spent outdoors.

Darker skin contains more melanin, which reduces the amount of ultraviolet B reaching the vitamin D-producing layers of the skin. People with darker skin can still produce vitamin D, but may require greater ultraviolet exposure than people with lighter skin.

There is no single safe exposure time that applies to everyone. Short periods of summer sunlight may be sufficient, and exposure should always stop before the skin reddens or burns. Sunbeds are not a recommended source of vitamin D.

The safest practical approach is to spend short periods outdoors regularly during spring and summer, expose some skin where culturally and personally appropriate, never allow the skin to burn, use shade, clothing and sunscreen for longer exposure, and avoid sunbeds as a way of raising vitamin D levels.

Vitamin D advice should never be interpreted as a reason to sunbathe or increase the risk of skin cancer.

Only a small number of foods contain meaningful amounts of vitamin D naturally. Sources include oily fish such as salmon, sardines, mackerel, herring, trout and kippers; egg yolk; red meat; liver and offal; some mushrooms; fortified breakfast cereals; fortified fat spreads; some dairy alternatives; and infant formula.

The amount varies substantially between products, so check nutrition labels where possible. Even a balanced diet may not provide enough vitamin D on its own, because there are few naturally rich food sources. Dietary sources become particularly important when ultraviolet B exposure is limited.

The UK Reference Nutrient Intake is 10 micrograms daily, equivalent to 400 IU, for people aged four and over. This was designed to protect musculoskeletal health across the population. In practice:

During autumn and winter, everyone should consider taking 10 micrograms daily. Throughout the year, people at higher risk of deficiency should consider taking 10 micrograms daily all year round.

From birth to under one year, a safe intake is 8.5–10 micrograms daily. From age one to under four years, 10 micrograms daily is recommended.

Babies drinking at least 500 ml of infant formula each day generally do not need an additional supplement because formula is already fortified. Parents should follow current guidance from their health visitor, midwife, pharmacist or GP, especially for premature babies or children with medical conditions.

Pregnant and breastfeeding women are included within the 10 microgram daily recommendation. Pregnant women should avoid supplements containing vitamin A in the form of retinol, because excessive retinol may harm the developing baby. Products formulated specifically for pregnancy are usually more appropriate.

Risk is increased when vitamin D production, intake, absorption or metabolism is reduced. Higher-risk groups include people with darker skin; people who rarely go outdoors; people who are housebound; care-home residents; people who cover most of their skin outdoors; babies and young children; pregnant and breastfeeding women; older adults; people with very limited dietary sources; people with malabsorption disorders; people with liver or kidney disease; people taking certain medicines that affect vitamin D metabolism; and people who have previously had vitamin D deficiency.

Obesity, strict vegetarian or vegan diets and medicines such as some antiepileptics, HIV treatments, rifampicin and cholestyramine are also relevant risk factors. Having one of these risk factors does not prove that you are deficient, but it may make year-round supplementation or medical assessment more appropriate.

Many people with low vitamin D have no symptoms. When symptoms occur, they may be vague and can have many other causes. Possible symptoms include tiredness, general aches and pains, bone pain or tenderness, muscle pain, weakness around the hips and shoulders, difficulty climbing stairs, difficulty rising from a chair and frequent falls.

Severe deficiency can contribute to osteomalacia in adults or rickets in children. Because tiredness and generalised pain are common and non-specific, symptoms alone cannot confirm vitamin D deficiency.

Most healthy people do not need routine vitamin D testing before taking the standard preventive dose. A clinician may consider a blood test if you have symptoms suggesting osteomalacia, persistent or focal bone pain, marked muscle weakness, unexplained low calcium, bone deformity, a malabsorption disorder, significant kidney or liver disease, osteoporosis in a context where the result would change treatment, previous severe deficiency, or a poor response to vitamin D treatment.

The usual blood test measures serum 25(OH)D.

A level of 25 nmol/L is generally taken as the point below which the risk of poor musculoskeletal health rises at a population level, while recognising that it is not diagnostic of disease on its own.

For most adults, the preventive dose is 10 micrograms daily, equivalent to 400 IU daily.

Some maintenance products provide 20–25 micrograms, equivalent to 800–1,000 IU. These may be appropriate in selected people, particularly after previous deficiency, but should be discussed with a pharmacist or clinician when other medical conditions or medicines are involved.

Confirmed deficiency may require a temporary loading course containing a much higher total dose, followed by long-term maintenance. The exact regimen depends on the blood result, age, symptoms, pregnancy, kidney function, calcium levels, malabsorption, other medical conditions and local prescribing guidance.

Do not self-treat suspected deficiency with very high doses purchased online. Loading treatment should be prescribed or supervised by a qualified healthcare professional.

Yes. Excess vitamin D can cause the body to absorb too much calcium, leading to hypercalcaemia. Possible symptoms include nausea or vomiting, constipation, abdominal pain, excessive thirst, passing urine frequently, weakness, confusion, kidney stones, kidney damage and, in severe cases, abnormal heart rhythms.

As a general rule, do not take more than 4,000 IU daily unless advised by a specialist.

Check the labels of all products you take. Vitamin D may be present in standalone vitamin D tablets, multivitamins, calcium and vitamin D combinations, cod liver oil, pregnancy supplements and bone-health products. Taking several products together can unintentionally produce a much higher dose than expected.

Speak to a pharmacist or doctor before supplementing if you have high blood calcium, kidney stones, severe kidney disease, sarcoidosis or another granulomatous disorder, hyperparathyroidism, certain rare metabolic disorders, or you take medicines that may interact with vitamin D or calcium.

Both forms can raise vitamin D levels. Vitamin D3 is the form produced in human skin and is widely used in UK supplements. Vitamin D2 may be suitable for people seeking a non-animal-derived product, although vegan vitamin D3 made from lichen is also available.

For routine supplementation, the most important practical points are to choose a reputable product, check the dose carefully, avoid duplicate supplements and take it consistently. Because vitamin D is fat-soluble, taking it with a meal may help absorption, particularly if the meal contains some fat.

Not everyone taking vitamin D needs a calcium supplement. Calcium should ideally come from food, including milk, yoghurt and cheese; fortified plant-based alternatives; calcium-set tofu; tinned fish with edible bones; some green vegetables; and nuts and seeds.

Calcium tablets may be recommended for some people with osteoporosis, low dietary calcium intake or specific medical conditions. However, unnecessary calcium supplementation can cause constipation and may not be appropriate for people with kidney stones or high calcium levels. Vitamin D and calcium should therefore be considered together, but supplements should be tailored to the individual.

Speak to a GP or other healthcare professional if you have persistent or focal bone pain, significant muscle weakness, repeated falls, difficulty walking or climbing stairs, bone deformity, a child with bowed legs, delayed growth or delayed walking, seizures or symptoms of very low calcium, known malabsorption, advanced kidney or liver disease, a history of kidney stones or high calcium, or symptoms after taking high-dose vitamin D.

Urgent assessment is appropriate for seizures, severe confusion, marked weakness, dehydration or suspected severe hypercalcaemia.