Vitamin D deficiency: signs, testing and how to top up safely
Quick answer
Between October and March, sunlight in the UK is too weak for your skin to make vitamin D, so the NHS advises most adults to consider a daily 10 microgram supplement through autumn and winter. Deficiency often causes few or vague symptoms — persistent tiredness, aching bones and muscles, low mood — and a simple blood test can confirm your level.
What vitamin D does and where it comes from
Vitamin D helps your body absorb calcium and phosphate, the minerals that keep bones, teeth and muscles healthy. It also plays a part in normal immune function. Unlike most vitamins, food is not the main source: your skin makes vitamin D when it is exposed to sunlight of the right strength, which is why levels naturally rise through the summer and fall through the winter.
Some foods contribute a modest amount — oily fish such as salmon, mackerel and sardines, red meat, liver, egg yolks, and foods fortified with vitamin D such as some breakfast cereals and fat spreads. For most people in the UK, though, diet alone does not provide enough on its own, which is why national advice focuses on sunlight in summer and supplements in the darker months.
Why autumn and winter matter in the UK
From around late March to the end of September, most people in the UK can make all the vitamin D they need from short periods of daily sun exposure on bare skin, alongside what they get from food. The picture changes in the colder months. Between roughly October and March, the sun in the UK sits too low in the sky for the ultraviolet B light your skin needs, so it cannot make vitamin D in any meaningful amount — however long you spend outdoors.
Because of this, NHS advice is that most adults and children over four should consider taking a daily supplement containing 10 micrograms (400 international units) of vitamin D during autumn and winter. Your body stores some vitamin D in fat over the summer, but for many people that reserve runs down as winter goes on, which is why late winter and early spring are when low levels are most common.
Signs of low vitamin D — and why they are easy to miss
Mild to moderate vitamin D deficiency often causes no obvious symptoms at all, and when symptoms do appear they tend to be vague and easy to put down to a busy life. The most common are persistent tiredness that does not improve with rest, general aches in the bones and muscles, muscle weakness, and low mood, particularly through the winter.
More severe, long-standing deficiency can cause bone pain and a condition called osteomalacia in adults, where bones soften and become more prone to fracture. In children, severe deficiency can cause rickets, which affects how bones grow. These are now uncommon in the UK, but they are the reason deficiency is taken seriously rather than dismissed as a minor issue.
Because the symptoms overlap with so many other conditions — thyroid problems, anaemia, poor sleep, stress — a symptom checklist alone cannot tell you whether vitamin D is the cause. That is where testing comes in.
Who is most at risk of deficiency
Some people are at higher risk of low vitamin D all year round, not just in winter, and NHS advice is that they should consider a daily supplement throughout the year. This includes people who are rarely outdoors — for example those who are housebound, frail or in residential care — and people who usually cover most of their skin when outside.
People with darker skin, including those of African, African-Caribbean and South Asian backgrounds, are also at higher risk, because melanin reduces how efficiently skin makes vitamin D from the sunlight available in the UK. Babies who are exclusively or partly breastfed need a daily supplement from birth, as breast milk contains little vitamin D; formula is already fortified. Some medical conditions that affect fat absorption, such as coeliac or Crohn's disease, and some medicines can also lower vitamin D levels — a clinician can advise on your individual situation.
Testing your vitamin D level
Vitamin D is measured with a blood test that checks 25-hydroxyvitamin D, the storage form of the vitamin in your blood. In broad terms, UK laboratories consider a level below 25 nanomoles per litre deficient, levels between 25 and 50 may be insufficient for some people, and a level of 50 or above is sufficient for almost everyone.
The NHS does not routinely screen people without symptoms, and testing is generally reserved for people with symptoms of deficiency, a higher-risk profile, or conditions affecting bones or absorption. If you would like to know your level — for example because you have unexplained tiredness or aches, or you fall into one of the at-risk groups — a private blood test can measure it, often as part of a wider panel that also looks at things like iron, thyroid function and B12, which cause similar symptoms.
Knowing your number matters because it changes what you do next: a sufficient level means looking elsewhere for the cause of your symptoms, mild insufficiency is usually managed with a standard daily supplement, and true deficiency may need a higher, clinician-supervised dose for a period to restore your stores.
Topping up safely — doses, timing and what to expect
For most adults, the standard advice is simple: a daily supplement containing 10 micrograms (400 IU) of vitamin D through autumn and winter, or all year round if you are in an at-risk group. Supplements are inexpensive, widely available from pharmacies, and come as tablets, capsules, sprays and drops — the form matters much less than taking it consistently. Taking it with a meal that contains some fat can help absorption.
On the shelf you will see two forms: vitamin D3 (colecalciferol), the form your skin makes and the one found in most supplements, and vitamin D2 (ergocalciferol), which comes from plant sources. Both raise your blood level, and D3 is the form most commonly recommended; vegan-friendly D3 made from lichen is now widely available too. Whichever you choose, check the label for the dose in micrograms or international units rather than judging by price or brand.
If a blood test confirms genuine deficiency, a clinician may recommend a higher dose for a limited time to restore your levels, followed by a maintenance dose. Higher-dose regimens should always be guided by a clinician and, ideally, a follow-up test.
More is not better. Vitamin D builds up in the body, and taking too much over a long period can raise calcium levels and harm the kidneys, bones and heart. Adults should not exceed 100 micrograms (4,000 IU) a day unless a clinician has specifically advised it. Do not expect to feel different overnight either — levels rise gradually, and it typically takes weeks to a few months of consistent supplementation for stores to recover.
When to speak to a clinician
See a pharmacist or GP if you have persistent tiredness, bone or muscle pain, or muscle weakness that is not improving — particularly if you are in one of the at-risk groups, or your symptoms carry on despite a consistent daily supplement. These symptoms have many possible causes, and a blood test is a quick way to narrow them down rather than guessing.
Seek advice promptly if you have bone pain with tenderness, difficulty walking or climbing stairs, or a child whose growth or bone development is causing concern — these can be signs of more significant deficiency that needs proper assessment and treatment rather than an off-the-shelf supplement.
Frequently asked questions
Can I get enough vitamin D from food alone?
For most people, no. Oily fish, red meat, egg yolks and fortified foods all contribute, but diet alone rarely provides enough in the UK. Between April and September, sunlight on bare skin does most of the work; from October to March, NHS advice is that most adults should consider a 10 microgram daily supplement.
Do I need a blood test before taking a supplement?
No. The standard 10 microgram daily dose is considered safe for the general population, so you can take it through autumn and winter without testing. A test becomes useful if you have ongoing symptoms, you are in a higher-risk group, or you want to know whether a higher, clinician-guided dose is needed.
How long does it take to correct a low vitamin D level?
Levels rise gradually with consistent daily supplementation, and it typically takes several weeks to a few months to restore your stores. If a test showed genuine deficiency, a clinician may use a higher short-term dose to speed this up, usually followed by a repeat test.
Can you take too much vitamin D?
Yes. Vitamin D is stored in the body, and persistently high intakes can raise calcium levels and damage the kidneys and bones. Adults should not take more than 100 micrograms (4,000 IU) a day unless specifically advised by a clinician. The standard 10 microgram dose is well within safe limits.
Does using sunscreen cause vitamin D deficiency?
Used normally, sunscreen is not thought to cause deficiency, and protecting your skin remains important. In the UK the bigger factor is the season: from October to March the sunlight is simply too weak for your skin to make vitamin D, whatever you do, which is why winter supplementation is advised.