Vitamin D deficiency is one of the most common nutritional deficiencies in the UK. The country's latitude means that for roughly six months of the year — October to March — the sun is too low in the sky to trigger meaningful vitamin D synthesis in the skin. Public Health England recommends that everyone in the UK consider supplementing during this period.
UK reference ranges for vitamin D (25-OH-D)
Vitamin D is measured as 25-hydroxyvitamin D (25-OH-D) in the blood. UK labs report in nmol/L:
| Level (nmol/L) | Classification | What it means |
|---|---|---|
| Below 25 | Severe deficiency | Risk of rickets (children), osteomalacia (adults), significant health impact |
| 25–49 | Deficiency | NHS threshold for deficiency; supplementation typically recommended |
| 50–74 | Insufficient | Below optimal; many experts recommend supplementing to above 75 |
| 75–200 | Sufficient | Generally considered adequate; optimal range debated |
| Above 250 | Potentially toxic | Rare from diet or sun; possible with very high-dose supplementation |
Symptoms of vitamin D deficiency
Vitamin D deficiency can be entirely asymptomatic, particularly when mild. When symptoms do occur, they are often non-specific and easy to attribute to other causes:
- Fatigue and low energy — one of the most commonly reported symptoms
- Bone pain and muscle aches — particularly in the back, hips, and legs
- Muscle weakness — difficulty climbing stairs or rising from a chair
- Low mood and depression — vitamin D receptors are present in brain tissue; deficiency is associated with seasonal affective disorder
- Frequent infections — vitamin D plays a role in immune function
- Hair loss — some evidence links severe deficiency to alopecia areata
- Impaired wound healing
Who is most at risk in the UK?
Certain groups are at significantly higher risk of vitamin D deficiency:
- People with darker skin tones (melanin reduces UV absorption)
- Those who cover most of their skin for cultural or medical reasons
- People who spend little time outdoors
- Adults over 65 (skin becomes less efficient at synthesising vitamin D)
- People with obesity (vitamin D is fat-soluble and can be sequestered in adipose tissue)
- Those with malabsorption conditions (Crohn's, coeliac disease)
- Exclusively breastfed infants
NHS supplementation guidance
The NHS recommends 10 micrograms (400 IU) of vitamin D daily for adults and children over 4 during autumn and winter. For those at higher risk, or those confirmed deficient by blood test, a GP may prescribe higher doses — typically 800–3,000 IU daily, or a loading dose for severe deficiency.
Vitamin D3 (cholecalciferol) is generally preferred over D2 (ergocalciferol) as it is more effective at raising blood levels. Taking it with a meal containing fat improves absorption, as vitamin D is fat-soluble.
Monitoring after supplementation
If you are supplementing at higher doses, it is worth retesting after three to four months to confirm your levels have responded. Vitamin D toxicity is rare but possible with very high doses over prolonged periods — symptoms include nausea, weakness, and hypercalcaemia.
What to ask your GP
- What is my vitamin D level, and does it explain my symptoms?
- What dose do you recommend, and for how long?
- Should I retest after supplementing, and when?
- Is there any reason I might not be absorbing vitamin D normally?
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