Vitamin D deficiency symptoms are rarely as obvious as the lists online suggest. Most people with a low level feel nothing at all, and the signs that do appear (aching bones, weak legs, cramps) are easy to put down to age or a bad back. In the UK the question matters every winter, because from October to early March the sun is simply too weak for your skin to make what you need.
In short
| What to know | The benchmark |
|---|---|
| Most common situation | A low level with no symptoms at all |
| Signs that really point to it | Bone pain (lower back, pelvis, legs), muscle weakness, difficulty climbing stairs or getting up from a chair |
| Blood test (25-hydroxyvitamin D) | Below 25 nmol/L: deficient. 25 to 50 nmol/L: may be too low for some people. Above 50 nmol/L: enough for most (NICE CKS, revised August 2026) |
| UK advice for adults | 10 micrograms (400 IU) a day from October to March, all year round if you rarely get sun on your skin (NHS) |
| Upper limit for adults | No more than 100 micrograms (4,000 IU) a day unless your doctor prescribes more |
| Weakly supported signs | Frequent colds, hair loss, weight gain, low mood: possible links, no proof that vitamin D is the cause |
What vitamin D actually does
Vitamin D helps your body absorb calcium and phosphate from food, the two minerals that keep bones, teeth and muscles working. Without enough of it, your gut absorbs less calcium, and your body starts drawing on the reserve held in your bones.
That is why the genuine symptoms of a deficiency are bone and muscle symptoms. In adults, a long-standing, severe deficiency softens the bones: doctors call this osteomalacia. In children, the same process causes rickets, with bones that bend as they grow.
Your skin makes most of your vitamin D when it is exposed to direct sunlight. The NHS points out that from about late March or early April to the end of September, most people in the UK make enough this way. Between October and early March, they do not.
The symptoms that really count
The signs below are the ones the NHS links to a lack of vitamin D and the bone softening it causes. They tend to appear slowly, over months.
- Bone pain, often a deep, dull ache in the lower back, pelvis, hips, ribs or legs. Pressing on the shin or the breastbone can be tender.
- Muscle weakness, especially in the thighs and hips: climbing stairs, getting up from a low chair or out of the bath becomes hard work.
- A change in the way you walk, sometimes a waddling gait, because the hip muscles are weak and the bones ache.
- Muscle aches and cramps, and sometimes pins and needles in the hands and feet when calcium levels drop too.
- Tiredness, which is real but so common that, on its own, it says very little.
None of these signs is specific. Back pain and tired legs have dozens of causes, and only a blood test can say whether vitamin D is involved.
In children
A child who lacks vitamin D may have painful or swollen wrists, legs that bow, slow growth, late teeth or weak muscles, and may be reluctant to walk. These signs need a GP appointment, not a supplement bought on a hunch.
Signs you may read about, with less evidence
Many websites list ten or fourteen “hidden signs”. Some are based on links seen in studies, which is not the same as proof that vitamin D causes them.
- Frequent colds or infections: vitamin D plays a part in the immune system, but taking it does not reliably prevent infections in people who already have enough. The NHS notes there is not enough evidence to take it to prevent or treat COVID-19.
- Low mood or depression: low levels are often found in people who are depressed, but they also go out less, which lowers vitamin D. Which comes first is not settled.
- Hair loss, slow wound healing, weight gain: links exist in studies, nothing that lets you blame vitamin D on your own.
If one of these is bothering you, it deserves a proper look, and vitamin D is only one possibility among many.
Who is most likely to be low
The NHS advises a supplement all year round, not only in winter, for people who make little vitamin D from sunlight:
- people who are rarely outdoors, for example if they are frail, housebound or living in a care home;
- people who usually cover most of their skin when outside;
- people with dark skin, for example of African, African-Caribbean or south Asian background, whose skin makes less vitamin D from the same amount of sun.
Other situations raise the risk: being over 65, being pregnant or breastfeeding, having a condition that affects absorption in the gut (coeliac disease or Crohn’s disease, for instance), kidney or liver disease, and some long-term medicines. If you take regular medication, ask your pharmacist whether it affects vitamin D.
The blood test and what the numbers mean
The test measures 25-hydroxyvitamin D, the form of vitamin D stored in your blood, often written 25(OH)D on the results. UK laboratories give it in nanomoles per litre (nmol/L). If you read a figure in ng/mL on an American website, it is a different unit: multiply by about 2.5 to compare.
The thresholds used by GPs come from NICE’s clinical knowledge summary (revised August 2026):
| Level of 25(OH)D | What it means |
|---|---|
| Below 25 nmol/L | Deficiency: treatment is usually needed |
| 25 to 50 nmol/L | May be too low for some people, depending on symptoms and risk |
| Above 50 nmol/L | Enough for most people |
The reference range printed by your own laboratory is the one that applies to you. Vitamin D testing is not routine in the UK: your GP will usually check it when you have symptoms such as bone pain or muscle weakness, a bone condition, or a strong risk factor, rather than as a general check-up.
When a deficiency is treated, NICE suggests the level may be rechecked 3 to 6 months after starting, rather than every few weeks.
How much vitamin D you need
For children from 1 year and adults, including during pregnancy and breastfeeding, the UK reference is 10 micrograms a day, which is 400 IU (1 microgram equals 40 IU).
Food alone rarely provides it. The best sources are oily fish (salmon, sardines, trout, herring, mackerel), egg yolks, red meat, liver and fortified foods such as some spreads and breakfast cereals. Unlike in some other countries, cows’ milk in the UK is generally not fortified, so it does not count as a good source.
That is why the NHS suggests that everyone consider a daily 10 microgram supplement from October to March. Over-the-counter supplements at this dose are for prevention. A confirmed deficiency is treated with higher doses decided by your GP, over a set period.
Taking too much is possible
Vitamin D is stored in the body, so large doses taken for months can build up. Too much raises the calcium level in the blood (hypercalcaemia), which can weaken bones and damage the kidneys and heart.
The NHS upper limits are 100 micrograms (4,000 IU) a day for adults and children aged 11 to 17, and 50 micrograms (2,000 IU) a day for children aged 1 to 10. Some people should stay below these figures because of a medical condition. You cannot get too much vitamin D from the sun.
Watch out for doubling up: a multivitamin, a fish oil and a separate vitamin D tablet can add up quickly. Check the micrograms on each label.
When to see a GP
Book an appointment if you have:
- bone pain that lasts, especially in the lower back, hips, pelvis or legs;
- weakness that makes stairs, chairs or walking difficult;
- a bone that broke after a minor fall or bump;
- a child with painful wrists, bowed legs, slow growth or reluctance to walk;
- symptoms that persist after three months of taking a supplement as advised.
Call NHS 111 or get urgent help if you have muscle spasms in the hands and feet, tingling around the mouth, or confusion, as these can be signs of a very low calcium level. Seek advice too if you have taken high doses of vitamin D and feel very thirsty, sick, or need to pass urine much more than usual.
If you are on regular treatment, do not start high-dose vitamin D without checking with your GP or pharmacist, and never stop a prescribed medicine to take a supplement instead.
