Now that October has arrived, UK health advice is that sunlight is no longer strong enough for most people to make the vitamin D they need. This article explains what the official guidance says, who is advised to take a supplement all year round, and how much confidence the research deserves.
Why does vitamin D matter in the first place?
Vitamin D helps the body regulate calcium and phosphate, which are needed for healthy bones, teeth and muscles. According to the NHS, deficiency can lead to bone deformities such as rickets in children and bone pain from osteomalacia in adults. Public Health England, in its 2016 advice based on the Scientific Advisory Committee on Nutrition (SACN), put the requirement at an average of 10 micrograms a day to protect bone and muscle health.
That is the established, well-evidenced role. Claims that go well beyond bones and muscles are a different matter, and they are covered further down.
When can sunlight do the job, and when can’t it?
The skin makes vitamin D when it is exposed to summer sunlight outdoors. According to the NHS, from late March or early April to the end of September most people can make enough this way. From October to early March, sunlight is not strong enough for the body to make enough.
Some government pages describe the winter window slightly differently, for example October to late March or early April, so the exact edges are fuzzy. The practical point is the same: autumn and winter are when a supplement is worth considering.
The NHS also notes that you cannot overdose on vitamin D from sunlight alone. That is not a reason to skip sun protection: it still advises covering up or protecting your skin during long periods in the sun to reduce the risk of skin damage and skin cancer.
What is the official advice for autumn and winter?
Public Health England’s advice, published on 21 July 2016, was that everyone should consider taking a daily supplement in autumn and winter, because it is hard to reach 10 micrograms from food alone. The NHS currently gives the same amount for adults and children over 4, including pregnant and breastfeeding women: 10 micrograms a day, which is equal to 400 international units (IU).
Food still contributes. The NHS lists oily fish (such as salmon, sardines, trout, herring and mackerel), red meat, egg yolks and fortified foods such as some fat spreads and breakfast cereals. Cows’ milk is not a useful UK source because it is not fortified. Liver contains vitamin D but should be avoided in pregnancy. For a wider look at everyday eating, see our guide to what a balanced everyday diet looks like.
Who is advised to take it all year round?
For some people, summer sunshine is not a reliable source. The NHS advises a daily 10 microgram supplement throughout the year for:
- people who are rarely outdoors, such as those who are frail or housebound
- people living in an institution such as a care home
- people who usually cover most of their skin when outdoors
- people with dark skin, including those with African, African-Caribbean or South Asian backgrounds, who may not make enough vitamin D from sunlight
Babies and young children
The advice differs by age, according to the NHS:
| Age group | Daily vitamin D advice |
|---|---|
| Babies up to 1 year | 8.5 to 10 micrograms if breastfed, or formula-fed and having less than 500ml of infant formula a day |
| Children aged 1 to 4 | 10 micrograms, all year round |
| Over 4 and adults | 10 micrograms in autumn and winter; all year if in an at-risk group |
Infant formula is already fortified, which is why babies having more than 500ml a day do not need extra. Vitamin D drops for under-5s are sold in most pharmacies and supermarkets, and women and children eligible for the Healthy Start scheme can get free supplements. Parents with questions about a baby’s supplement should ask a health visitor, pharmacist or GP.
How do micrograms and IU line up?
Supplement labels may use either unit, which causes confusion. According to the NHS, 1 microgram equals 40 IU, and a microgram is 1,000 times smaller than a milligram. Illustrative arithmetic:
- 10 micrograms × 40 = 400 IU, the standard recommended amount
- 25 micrograms × 40 = 1,000 IU
- 100 micrograms × 40 = 4,000 IU
Reading a label carefully matters, because a product marked “1,000 IU” contains 25 micrograms, two and a half times the standard 10 microgram amount.
Is more always better? Upper limits and safety
No. The NHS says long-term excess intake can cause a build-up of calcium in the blood (hypercalcaemia), which may weaken bones and damage the kidneys and heart. It sets daily upper limits of:
- 100 micrograms (4,000 IU) for adults, pregnant and breastfeeding women, older adults and children aged 11 to 17
- 50 micrograms (2,000 IU) for children aged 1 to 10
- 25 micrograms (1,000 IU) for infants under 12 months
The standard 10 microgram amount therefore sits at one-tenth of the adult limit (10 ÷ 100). Government advice describes 10 micrograms as a safe level of intake, designed to meet nutritional needs, and does not currently recommend taking more. Anyone told by a doctor to take a different amount should follow that advice, and people with certain medical conditions should check with a doctor or pharmacist before starting a supplement.
What does the research show beyond bones?
Vitamin D is often marketed as a defence against winter colds, low mood and a long list of other problems. The evidence is more cautious.
- Respiratory infections: A 2024 systematic review and meta-analysis of 12 randomised trials in adults aged 50 or over found that supplementation probably does not reduce acute respiratory infections (relative risk 0.99, 95% confidence interval 0.97 to 1.02, moderate certainty). In people with low starting levels, there was a possible benefit that was not statistically significant, and the authors called it inconclusive.
- Mortality: A Cochrane review of 56 randomised trials with 95,286 participants reported a small reduction in deaths overall (relative risk 0.97). Its authors also reported that vitamin D3 combined with calcium increased kidney stones. A small average effect in trials of older adults is not a reason for healthy younger people to expect a health boost from extra doses.
- Bone protection: The 10 microgram advice is about maintaining bone and muscle health, not about preventing any particular disease.
In short, taking vitamin D to avoid deficiency is well supported. Taking it as a general cold remedy is not.
If low mood in winter is your main concern, a supplement is not a substitute for proper support; our explainer on seasonal affective disorder covers what the NHS suggests, and why sleep matters is worth a read alongside it.
Do you need a test, and when should you speak to someone?
Most people following the standard advice do not need a blood test. The NHS describes the main deficiency symptoms as bone deformities in children and bone pain in adults, and those are reasons to be assessed rather than to self-treat.
Speak to a GP or call NHS 111 if you have persistent bone pain, muscle weakness or tenderness, or if a child’s legs appear bowed or they are slow to walk. Also check with a GP, pharmacist or midwife before starting supplements if you are pregnant, have kidney disease or another long-term condition, or take regular medicines. If you are invited to an NHS Health Check, it is a good moment to raise any questions.
A small autumn habit with a clear purpose
For most adults in the UK, the guidance comes down to a simple, inexpensive habit: consider a daily 10 microgram (400 IU) supplement from October to early March, and all year if you are in an at-risk group. The case rests on bone and muscle health and on the difficulty of getting enough from food. It does not rest on promises about colds or immunity. Check the label units, keep to the standard amount, and use food and sensible sun habits in the months when sunlight can help.
This article is general information, not a substitute for advice from a doctor or other qualified health professional. If you are worried about your health, speak to your GP or call NHS 111.
Sources: NHS; GOV.UK; Cochrane; PubMed Central
