Vitamin D in the UK: how much you need and when a blood test helps
Short answer: from around October to March, UK sunlight is too weak to make vitamin D, so the NHS advises everyone to consider a daily supplement of 10 micrograms. You do not need a blood test to start that. A test is most useful if you are at higher risk or have symptoms such as bone pain or muscle weakness.
Why vitamin D matters
Vitamin D helps your body absorb calcium, so it supports healthy bones, teeth and muscles. Severe, long-term deficiency can cause rickets in children and softening of the bones (osteomalacia) in adults.
Who is at higher risk of low vitamin D?
People who spend little time outdoors, or who are housebound
People who cover most of their skin when outside
People with darker skin, which makes less vitamin D from the same sunlight
Older adults
People with conditions that affect absorption, such as coeliac disease, or with some kidney or liver conditions
People in these groups are advised to consider a supplement all year round, not just in winter.
How a vitamin D result is read
The vitamin D blood test measures a form called 25-hydroxyvitamin D. In the UK, a level below 25 nmol/L is generally classed as deficient, and 50 nmol/L or above is generally regarded as enough. Results between 25 and 50 nmol/L may be too low for some people, including people with darker skin, while around 50 nmol/L or above is usually considered adequate. A clinician will advise based on your symptoms and risk.
Can you take too much?
Yes. Taking more than 100 micrograms (4,000 IU) a day over a long period can raise blood calcium to harmful levels. High-dose products are best used only on a clinician's advice.
When a blood test is worth doing
Bone pain, muscle weakness or frequent aches that are not explained
You are in a higher-risk group and want to know your level
You have a condition that affects absorption
You have been treated for low vitamin D and a clinician wants to check the response
Frequently asked questions
Do I need to fast for a vitamin D test?
No. A normal meal does not change the result.
How long before a vitamin D level improves?
It takes weeks to months of regular supplements. If a clinician advises retesting, it is usually after about three months of treatment.
Does sunscreen stop me making vitamin D?
In practice, short sun exposure without burning is enough for most people in summer. Do not avoid sunscreen to make vitamin D, as the risk of skin damage outweighs the benefit.
Related reading
Clinically reviewed by Zak Toon, ACP, on 15 October 2026.
This article is general information based on UK guidance from NHS and NICE. It is not personal medical advice. If you are worried about your health or have symptoms, speak to your GP or call 111.