Vitamin D and supplements: what the evidence says

Why vitamin D matters

Vitamin D helps the body absorb calcium and phosphate, minerals that keep bones, teeth and muscles healthy. Without enough, bones can become soft and weak — causing rickets in children and a condition called osteomalacia in adults, both of which lead to bone pain and deformity. Low levels are also linked to muscle weakness and more falls in older people. Our main natural source is sunlight: when skin is exposed to summer sunshine, it makes vitamin D. We also get smaller amounts from foods such as oily fish, eggs and fortified products. Because the body can store vitamin D made in summer, the aim is to keep levels topped up through the darker months when sunlight is too weak.

Why the UK has a problem

The UK's northern location means that from about October to March, the sunlight is too weak for skin to make vitamin D, whatever the weather. On top of this, modern life keeps many people indoors, and covering the skin or using sunscreen — sensible for skin-cancer prevention — also reduces vitamin D production. Some groups are at higher risk of running low: people with darker skin, who need more sun to make the same amount; those who cover their skin or spend little time outdoors; babies and young children; older adults, especially those in care homes; and people who are housebound. Because dietary sources are limited, many people simply cannot get enough from food and sunlight alone in winter.

What the official advice says

UK public health advice, based on expert review, is clear and practical. Everyone should consider taking a daily vitamin D supplement during the autumn and winter months, when sunlight cannot do the job. Certain groups are advised to take a supplement all year round because they are at higher risk of deficiency: babies and young children, people who are not often outdoors, those who cover their skin, and people with darker skin. Breastfed babies, and formula-fed babies having less than a certain amount of formula, are also advised to have a daily vitamin D supplement. This targeted approach reflects the evidence: vitamin D reliably supports bone and muscle health, and supplements help those who cannot get enough naturally.

What supplements can and cannot do

The strongest evidence for vitamin D is for bone and muscle health, and for preventing deficiency diseases like rickets and osteomalacia. In older or at-risk people, keeping levels adequate — often alongside calcium — supports bone strength. Beyond this, vitamin D has been studied for many other conditions, from infections to heart disease and cancer, and the results are mostly disappointing or unclear. Large trials have generally not shown that taking extra vitamin D prevents these illnesses in people who already have enough. So while correcting a genuine deficiency is worthwhile, the idea that high-dose vitamin D is a general tonic or protects against a wide range of diseases is not supported by good evidence. Treat sweeping health claims with caution.

Taking supplements safely

For most people, a standard daily supplement in line with UK advice is safe and inexpensive, and there is no need for routine blood testing. It is possible, though, to take too much vitamin D over time, which can cause a harmful build-up of calcium in the body, leading to nausea, weakness, and effects on the kidneys and heart — so avoid very high-dose products unless a doctor has specifically advised them. If you have a health condition, take other medicines, or are considering high doses, check with your pharmacist or GP first. Vitamin D deficiency that causes symptoms is diagnosed and treated by a clinician, sometimes with higher doses for a short period. For everyone else, the sensible message is: keep it simple, follow the official advice, and be wary of hype.