Educational information, not medical advice. See our Medical Disclaimer. Last updated: October 29, 2026.
Vitamin D is one of the most discussed supplements, and with good reason: many people in northern countries do not make enough of it during autumn and winter. It is also one of the most oversold. This guide explains what vitamin D actually does, who is most likely to run low, what official bodies recommend, and where the evidence is weaker than the marketing suggests.
What vitamin D does
Vitamin D helps your body absorb calcium and phosphate, which are essential for building and maintaining bones, teeth and muscles. Severe, long-term deficiency causes rickets in children, where bones soften and bend, and osteomalacia in adults, which causes bone pain and muscle weakness.
Where it comes from
- Sunlight. Skin makes vitamin D when exposed to UVB rays. In autumn and winter at higher latitudes, the sun is often too weak for this.
- Food. Oily fish (salmon, sardines, mackerel), egg yolks, some mushrooms and fortified foods such as certain cereals, milks and spreads.
- Supplements. Sold as vitamin D2 or D3. Some research suggests D3 may raise blood levels more effectively. Vegan D3 made from lichen is available.
How much you need: what official bodies recommend
Amounts are given in micrograms (mcg) or international units (IU). One microgram equals 40 IU.
| Source | Recommendation |
|---|---|
| US National Institutes of Health (NIH) | 15 mcg (600 IU) daily for ages 1 to 70; 20 mcg (800 IU) for adults over 70; 10 mcg (400 IU) for infants |
| UK NHS | 10 mcg (400 IU) daily for children from age 1 and adults; everyone should consider a supplement in autumn and winter |
| Upper limit, adults | 100 mcg (4,000 IU) per day from all sources, per both NIH and NHS |
The difference between the two recommendations comes from different assumptions about sun exposure. Both sit well below the upper limit.
Who is at risk of low levels
- People who spend little time outdoors, including housebound or frail people and those in care homes.
- People who cover most of their skin when outside.
- People with dark skin, which makes vitamin D more slowly from sunlight.
- Older adults, whose skin makes less vitamin D.
- People with obesity or conditions that reduce fat absorption, such as coeliac disease, Crohn’s disease or after some weight-loss surgery.
- Breastfed infants, who typically need a daily supplement.
- Anyone living far from the equator in autumn and winter.
Deficiency symptoms and testing
Mild deficiency often has no clear symptoms. When symptoms do appear they can include bone pain, muscle weakness or aches, and tiredness, but these are non-specific and have many other causes. Do not assume tiredness is low vitamin D.
If you are concerned, a doctor can order a blood test for 25-hydroxyvitamin D. Treating a confirmed deficiency may involve higher doses for a limited time under medical supervision, which is different from everyday supplementation.
What the evidence does and does not show
Vitamin D is clearly important for bone health and for preventing rickets and osteomalacia. Claims beyond that are more mixed. Large randomized trials in generally healthy adults, such as the VITAL trial of 2,000 IU daily, did not find that vitamin D supplements reduced cancer or cardiovascular disease overall. Evidence that supplements prevent colds, depression or falls in people who are not deficient is inconsistent. Supplements make the most difference for people who are actually low.
Can you take too much?
Yes. Excess vitamin D, almost always from high-dose supplements rather than sun or food, can cause a buildup of calcium in the blood (hypercalcemia), with nausea, vomiting, weakness, frequent urination and, over time, kidney problems. Stay within the labeled dose and do not exceed 100 mcg (4,000 IU) per day unless a doctor has told you to. Tell your doctor about supplements you take, since vitamin D can interact with some medicines such as certain steroids and anti-seizure drugs.
Vitamin D is fat-soluble, so taking it with a meal that contains some fat helps absorption.
- Vitamin D supports bones and muscles by helping you absorb calcium.
- Official daily recommendations are 10 to 20 mcg (400 to 800 IU) depending on age and country.
- Autumn and winter supplements make sense for many people in northern countries.
- Do not exceed 100 mcg (4,000 IU) per day without medical advice.
- Evidence for benefits beyond bone health is inconsistent in people who are not deficient.
Frequently asked questions
Should I take vitamin D in winter?
The NHS advises everyone to consider a daily 10 mcg supplement in autumn and winter, since sunlight is too weak at that time of year in the UK. If you live elsewhere, ask a pharmacist or doctor what applies in your region.
Is vitamin D3 better than D2?
Some research suggests D3 raises blood levels more effectively, and it is the form most often sold. Both can correct a deficiency under medical guidance.
Can I get enough from food alone?
It is difficult. Few foods are rich in vitamin D, so sunlight and fortified foods usually contribute most, with supplements filling the gap when sun exposure is limited.
Does sunscreen cause vitamin D deficiency?
In everyday use, the evidence does not show that sunscreen leads to deficiency, since most people apply less than the tested amount. Do not skip sun protection to get vitamin D; see our sunscreen guide.
Sources
- NIH Office of Dietary Supplements: Vitamin D fact sheet for consumers
- NHS: Vitamin D
- UK Scientific Advisory Committee on Nutrition (SACN): Vitamin D and Health report
Written by the nohas.es Editorial Team from the official sources listed above. Read our Editorial Policy and Fact Checking Policy. Found an error? Tell us.
