The short answer: symptoms of vitamin D deficiency are often absent entirely, and when they do appear they are unspecific: persistent tiredness, aching bones and muscles, low mood, and in more severe cases muscle weakness or bone pain. Because those complaints have dozens of possible causes, the only way to know your level is a blood test. What makes vitamin D unusual is that in the UK, deficiency is common enough that national guidance assumes most of us run low in winter. Here is what to look for and what to do about it.
What are the symptoms of vitamin D deficiency?
- Tiredness and fatigue that rest does not fix.
- Aches in bones and muscles, particularly the lower back, hips and legs.
- Low mood, especially through winter.
- Muscle weakness, which in older adults shows up as more frequent falls.
- More frequent infections, though the evidence linking vitamin D to immunity is mixed.
Severe vitamin D deficiency symptoms look different from the vague early picture. Prolonged, severe deficiency causes bone softening (osteomalacia in adults, rickets in children), which brings genuine bone pain, difficulty walking or climbing stairs, and in children bone deformity. That end of the spectrum is uncommon in the UK but not gone.
You will also see lists of weird symptoms of vitamin D deficiency online, from hair shedding to tingling hands. Most of these associations come from weak observational studies and are not reliable signs of low vitamin D on their own. If a symptom worries you, it deserves a GP conversation rather than a self-diagnosis from a supplement website.
Why the UK is a special case
Your skin makes vitamin D from sunlight, and between roughly October and March UK sunlight is too weak for that to work regardless of how long you spend outside. Food provides some (oily fish, egg yolks, fortified cereals) but not reliably enough to cover the gap. This is why NHS guidance is that most adults should consider a 10 microgram (400 IU) supplement through autumn and winter.
What causes vitamin D deficiency?
Lack of sunlight on skin is the dominant cause, which is why season and lifestyle matter so much. Beyond that, some causes are medical: conditions that impair fat absorption, such as coeliac or Crohn's disease, reduce uptake of this fat-soluble vitamin; kidney and liver disease can impair the conversion of vitamin D to its active form; and some medicines, including certain anticonvulsants, increase its breakdown.
Body weight plays a role too: vitamin D is stored in fat tissue, so people living with obesity tend to have lower circulating levels and may need more to reach the same blood level. The SACN review of vitamin D and health, which underpins the UK advice, found low vitamin D status across a meaningful share of the UK population in winter, not just in obviously high-risk groups.
Occasionally, low vitamin D is itself a symptom of something else. A level that stays stubbornly low despite proper supplementation can point to an undiagnosed absorption problem or kidney or liver disease, which is one more reason persistent deficiency belongs with a GP rather than a bigger supplement dose.
Who is at risk all year
For most people the risk is seasonal, but the NHS advises some groups to supplement all year round because their skin makes little vitamin D regardless of the month:
- People with darker skin, which makes vitamin D more slowly from the same sunlight.
- People who cover most of their skin when outdoors, or who spend little time outside.
- Older adults in care settings, and people who are housebound.
- People with conditions affecting fat absorption, and after some types of weight-loss surgery.
Testing and treatment
The test measures 25-hydroxyvitamin D in blood. NICE guidance treats below 25 nmol/L as deficient, with 25 to 50 nmol/L commonly considered insufficient for some people. Confirmed deficiency is usually treated with a short course of higher-dose vitamin D prescribed by your GP, then a maintenance dose. Improvement in tiredness and aches, where vitamin D was genuinely the cause, typically takes weeks to a few months, and bone pain from severe deficiency can take longer still. If symptoms persist once your level is corrected, the deficiency probably was not the whole story, and it is worth going back to your GP rather than raising the dose.
Because the early picture is so vague, GPs often check for other causes at the same time. Low vitamin B12 and iron deficiency produce overlapping fatigue, and it is possible to be low in more than one at once, particularly with a restricted diet or an absorption problem. A single blood panel can settle all three, which beats guessing with supplements.
It is worth being honest about the other direction, too: vitamin D is not a general tonic, and correcting a deficiency will not transform energy or mood if the level was normal to begin with. Trials of supplementation in people who are not deficient have been largely disappointing. The value is in finding and fixing genuine deficiency, which in the UK is common enough to be worth taking seriously, not in high doses for their own sake.
For the wider picture on how much to take routinely, upper limits and food sources, see our guide to vitamin D in the UK, and our honest review of whether you need supplements at all.
