Vitamin D

What is Vitamin D?

Vitamin D is a fat-soluble vitamin that behaves more like a hormone. Your skin makes vitamin D3 (cholecalciferol) when it is exposed to UVB light from the sun, and that is where most people get most of theirs. Food supplies a little: oily fish such as salmon, sardines, trout, herring and mackerel, red meat, liver, egg yolks, and fortified foods such as some fat spreads and breakfast cereals. Supplements contain either D3 or vitamin D2 (ergocalciferol), which comes from plants and fungi.

In the UK, the sun is strong enough to make vitamin D only from about late March or early April to the end of September. For the rest of the year, most people make little or none — which is why the NHS advice is seasonal. Because vitamin D is fat-soluble, the body stores it: summer levels carry some way into winter, and too much from supplements can build up.

How it works

The liver converts vitamin D into 25-hydroxyvitamin D, the form a blood test measures, and the kidneys convert that into calcitriol, the active hormone. Calcitriol regulates the amount of calcium and phosphate in the body, mainly by increasing how much the gut absorbs. Bones, teeth and muscles need both.

Without enough vitamin D, children develop rickets and adults develop osteomalacia: soft, painful bones and weak muscles. Receptors for vitamin D are also found in many other tissues, including immune cells, and that is the basis for the wider claims made for it. But a receptor being present does not mean that extra vitamin D does more there, and the trials testing those claims in people who were not deficient have mostly come back negative.

What the research actually shows

Evidence strength varies by what is being claimed, so each use is graded separately against the published criteria rather than averaged into one reassuring letter.

Who should take it

In the UK, the NHS advises everyone — including pregnant and breastfeeding women — to consider a daily supplement of 10 micrograms (400 IU) from October to March. It advises taking it all year if you are not often outdoors, live in a care home, usually wear clothes that cover most of your skin outdoors, or have dark skin. Breastfed babies should have 8.5 to 10 micrograms a day from birth, and children aged 1 to 4 years 10 micrograms a day all year.

The Endocrine Society's 2024 guideline suggests a daily vitamin D supplement for children and teenagers, adults over 75, people who are pregnant, and people with high-risk prediabetes. It suggests against taking more than the recommended amounts to prevent disease in healthy adults under 75 — and against routine blood tests for vitamin D, including in people with dark skin or obesity.

That last point reverses a common habit. For most people, the useful step is the small seasonal dose, not a blood test. Testing is for someone with symptoms, a condition that affects absorption, or a doctor's reason to check.

Effective dosage

The NHS amount is 10 micrograms (400 IU) a day for adults and children over one. In the US, the recommended amount from food and supplements together is 15 micrograms (600 IU) a day up to age 70 and 20 micrograms (800 IU) over 70. The upper limit for adults is 100 micrograms (4,000 IU) a day; the NHS sets lower limits for children, at 50 micrograms for ages 1 to 10 and 25 micrograms for babies.

Take it daily rather than in large occasional doses. The Endocrine Society prefers daily dosing for people over 50, the respiratory-infection benefit appeared only with daily or weekly dosing, and an annual 500,000 IU dose increased falls and fractures in older women. Vitamin D is fat-soluble, so taking it with a meal is a sensible habit.

D3 or D2? In a 2012 meta-analysis, D3 raised blood levels more than D2 when given as large, infrequent doses; with daily dosing the difference was not significant. For a daily supplement either works. Vegans can use D2, or D3 made from lichen.

Where products diverge from the research. In this category the usual problem is too much, not too little. Most single-ingredient supplements are 25 micrograms (1,000 IU) or 50 micrograms (2,000 IU) — two and a half to five times the NHS amount, but still within the upper limit — while "high-strength" 4,000 and 5,000 IU products sit at or above it. None of the extra has been shown to benefit healthy adults.

Units cause the rest. The same product may say 25 mcg on one line and 1,000 IU on another, and because the two differ by a factor of forty, comparing a microgram figure with an IU figure can be badly wrong. Add up everything you take: a multivitamin, a separate vitamin D and fortified foods can together pass the limit.

Safety, side effects and interactions

At normal doses vitamin D is very safe, and you cannot overdose on it from sunlight. Taking too much from supplements over a long period can raise the amount of calcium in the blood (hypercalcaemia), which the NHS warns can weaken the bones and damage the kidneys and the heart. In the Cochrane review, supplements slightly increased mild hypercalcaemia, and vitamin D with calcium slightly increased kidney problems.

Ask a doctor or pharmacist before taking vitamin D if you have kidney disease or a condition that affects calcium levels, or if you take medicines that interact with it — including some diuretics, steroids such as prednisone, the weight-loss medicine orlistat, and some statins.

None of this is medical advice. See the medical disclaimer for what that means before you act on anything here.