Vitamin D is the one deficiency almost everyone in the UK is at risk of, and the one most likely to be quietly running in the background while you put the symptoms down to a busy life or a long winter. It is not really a vitamin at all but a hormone your skin makes from sunlight, and because the UK sits too far north to make meaningful amounts between about October and March, deficiency is genuinely common, national surveys suggest roughly one in five adults has low levels, and far more sit in the “insufficient” grey zone.
The frustrating part is how vague and easy to dismiss the symptoms are. This guide covers what low vitamin D actually feels like, who is most at risk, what a vitamin D blood test checks, and the level genuinely worth aiming for.
What vitamin D does
Vitamin D is best known for bone health, it lets your body absorb calcium, and long-term deficiency weakens bones. But its role is much wider. Receptors for vitamin D sit on cells throughout the body, and it plays a part in immune function, muscle strength, mood regulation and the hair growth cycle. That breadth is exactly why a deficiency produces such a scattered, non-specific set of symptoms.
What low vitamin D feels like
Vitamin D deficiency rarely announces itself. Instead it tends to show up as a cluster of low-grade complaints:
- Persistent tiredness and low energy that sleep does not fix. This is one of the commonest presentations.
- Low mood, particularly through the darker months, where low vitamin D overlaps with seasonal low mood.
- Aches in the muscles and bones, a deep, hard-to-place ache, sometimes tenderness in the shins or hips.
- Frequent coughs, colds and infections, a run of illnesses that seems worse than usual.
- Muscle weakness, especially noticeable climbing stairs or getting up from a chair.
- Hair shedding, vitamin D is involved in the follicle cycle, and low levels are a recognised contributor to diffuse hair shedding.
- Slow recovery from exercise or illness.
None of these are specific on their own, which is the whole problem. Together, and especially through winter, they point towards checking your level. If fatigue or brain fog is your main concern, vitamin D sits alongside ferritin and thyroid as one of the first things worth ruling in or out.
Who is most at risk
Some people are far more likely to be deficient:
- Anyone who spends most of the day indoors, office workers, and anyone who covers up or uses sunscreen consistently (both sensible for skin, but they reduce vitamin D synthesis).
- People with darker skin. More melanin means less vitamin D made from the same sun exposure, so risk is considerably higher.
- Older adults, whose skin makes vitamin D less efficiently.
- Anyone through the UK winter, October to March, when sunlight is too weak to produce it regardless of how much time you spend outside.
- People who are overweight, as vitamin D is sequestered in fat tissue and less available.
- People with gut absorption conditions such as coeliac or Crohn’s disease.
The NHS advises that everyone in the UK consider a vitamin D supplement through autumn and winter for this reason. A blood test tells you whether you are actually topped up or running low, and by how much.
What the blood test checks, and what “normal” means
The test measures 25-hydroxyvitamin D (25-OH vitamin D), the stored, circulating form and the accepted marker of your status. This is where reading results in context matters, because the standard thresholds are lower than many people expect:
| 25-OH vitamin D level | What it means |
|---|---|
| Below 25 nmol/L | Deficient |
| 25 to 50 nmol/L | Insufficient |
| Above 50 nmol/L | Sufficient (the minimum, not the target) |
The catch is that “sufficient” at 50 nmol/L is a floor for bone health, not an optimum for how you feel. Plenty of people sitting at 40 to 50 nmol/L, technically only “insufficient” or barely “sufficient”, feel tired and low and improve when their level is brought up. A result reported simply as “normal” will not tell you this, which is why every result in our panels is reviewed by a GMC-registered doctor who reads your number against your symptoms rather than just the lab flag.
A quick note on vitamin D3 (cholecalciferol): this is the form used in supplements and made by your skin. The 25-OH test is what measures your status; D3 is what you take to raise it.
What to do about a low result
The reassuring part is that vitamin D deficiency is one of the easiest things to correct:
- Supplement with vitamin D3 at a dose matched to how low you are, a maintenance dose is very different from a correction dose, which is why testing first is useful.
- Retest after around three months to confirm your level has responded and to settle on the right ongoing dose.
- Do not mega-dose blindly. Vitamin D is fat-soluble and stored, so it is one of the few vitamins where taking far too much for a long time can cause harm. Matching the dose to the deficiency is the sensible approach.
Because vitamin D so often travels with other correctable causes of fatigue, checking it as part of a broader health screen, alongside ferritin, B12 and thyroid, usually gives a far more useful picture than a single marker in isolation.
Getting tested at WMG Health
If you have been tired, low, aching or catching every bug going, particularly through the darker months, a vitamin D test is a simple, high-value check. You can book a same-day appointment at our Harley Street clinic as part of the General Wellness panel or a broader screen, results within four hours, reviewed by a GMC-registered doctor, no GP referral required.
Sources
- NHS: Vitamin D overview and supplementation advice. nhs.uk
- NICE Clinical Knowledge Summary: Vitamin D deficiency in adults. cks.nice.org.uk
- Public Health England / OHID: Vitamin D: national dietary and supplementation guidance. gov.uk
- Royal Osteoporosis Society: Vitamin D and bone health. theros.org.uk
This article is for general information only and does not constitute clinical advice. If you have concerns about your symptoms, book an appointment or speak to your GP.