Every year, somewhere around October, the UK quietly stops being able to make vitamin D. Not because anyone is doing anything wrong, but because the sun sits too low in the sky. The NHS puts it plainly: between October and early March, we do not make enough vitamin D from sunlight. For the next five months or so, your body runs on whatever it stored over the summer, plus whatever comes from food and supplements.
For many people that is enough. For many others it is not, and they head into January and February with a level that has drifted steadily downwards without them noticing. This guide is about the season rather than the symptoms: why winter matters, who is most likely to run low, and why a test gives you a useful baseline. If you are already feeling tired, achy or low and wondering whether vitamin D is behind it, our guide to vitamin D deficiency symptoms covers that side in detail.
Why the UK winter is the problem
Your skin makes vitamin D when it is exposed to a particular part of sunlight, ultraviolet B (UVB). From about late March or early April to the end of September, the NHS says most people should be able to make all the vitamin D they need from sunlight. In winter, the sun’s lower angle means far less UVB reaches the ground, and the NHS says that between October and early March we do not make enough vitamin D from sunlight.
Vitamin D is fat-soluble, so the body does hold a reserve. The trouble is that the reserve only lasts so long. In the UK, levels typically peak in September after a summer of sunshine and then decline from October through the winter months until spring. In one study of 125 healthy adults followed for a year, the average was highest in September (71 nmol/L) and lowest in February (46 nmol/L), according to the Scientific Advisory Committee on Nutrition (SACN). That is exactly when many people feel at their flattest.
The NHS also notes that it is difficult to get enough vitamin D from food alone. Oily fish, egg yolks, red meat and fortified foods such as some breakfast cereals and fat spreads all help, but few people eat enough of them to replace what the summer sun provides.
That is why the NHS advises that everyone in the UK should consider taking a daily 10 microgram vitamin D supplement during autumn and winter (10 micrograms is the same as 400 IU). The safe upper limits for supplements, and whether vitamin D helps with colds, are covered further down this page.
Who is most likely to run low
Anyone can end up low by the end of a UK winter, but some people start from a lower point or lose ground faster. The NHS highlights people who:
- Are not often outdoors, for example if they are frail or housebound
- Live in a care home or other institution
- Usually wear clothes that cover most of their skin when outside
- Have dark skin, for example people of African, African-Caribbean or South Asian background, who may not make enough vitamin D from sunlight
For these groups, the NHS advises considering a supplement all year round, not just in winter.
Other risk factors listed in NHS trust guidance based on SACN and Royal Osteoporosis Society advice include:
- Being 65 or over, because the skin makes vitamin D less efficiently with age
- A vegetarian, vegan or fish-free diet, since oily fish is one of the main natural sources
- Conditions that affect absorption, such as coeliac disease, Crohn’s disease or cystic fibrosis
- Chronic liver or kidney disease
- Pregnancy and breastfeeding
- Certain medicines, including some anticonvulsants (epilepsy medicines) and glucocorticoids (steroids)
- Little time outdoors or routine use of high-factor sun protection, which can mean very little sun exposure even in the months when UK sunlight is strong enough
If you recognise yourself in one or more of these, the NHS supplement advice matters all the more, and a test can show you where you actually stand.
Why a test gives you a baseline
You do not need a blood test to follow the NHS advice on a 10 microgram winter supplement. UK clinical guidance does not recommend routine vitamin D testing, so on the NHS a test is generally reserved for people with symptoms that could be due to vitamin D deficiency (for example suspected osteomalacia) or with bone conditions where the result would change treatment. That is a sensible use of NHS resources, but it does leave a lot of people guessing.
A test replaces the guess with a number. It tells you:
- Where you are starting from. Someone at 30 nmol/L going into winter is in a very different position from someone at 80 nmol/L.
- Whether you are actually deficient. A deficiency is treated differently from simple winter maintenance, and the dose should be decided with a clinician rather than picked off a shelf.
- Whether what you are already doing is working. Plenty of people take a supplement faithfully and assume they are covered. A test confirms it.
- Whether vitamin D is part of a wider picture. Winter tiredness, low mood and a run of colds have several possible causes. Low vitamin D is only one possibility, and the evidence linking it to mood and infections is still debated.
That last point matters. If you are tired all the time this winter, checking vitamin D on its own can miss something else that is easy to correct, such as low ferritin, low vitamin B12 or an underactive thyroid.
When to test
There is no wrong time to check, but the timing changes what the result tells you.
- Early autumn (September to October). This shows your level at or near its summer peak. If it is already low now, it is very likely to be lower by February, and that is useful to know before winter starts.
- Late winter (February to March). This shows your level at or near its lowest point. It is the best test of whether your winter routine, with or without a supplement, is actually keeping you topped up.
- After a deficiency has been treated. Whether and when to retest is a clinical decision. NHS trust guidance says routine monitoring is usually unnecessary, but it may be appropriate if you had symptoms, have a condition that affects absorption, or there is doubt about whether the supplement is being taken.
If you take a vitamin D supplement already, there is no need to stop it before the test. The result will reflect your supplemented level, which is exactly what you want to know. Just tell us what you take and how much. The one exception is a multivitamin or B-complex: if your panel includes B12 or folate, as the General Wellness and Hair Loss Essentials panels do, hold those for 24 to 72 hours before the test, because recent B12 or folate can mask a real deficiency.
What the result means
The test measures 25-hydroxyvitamin D (25-OH vitamin D), the main form of vitamin D circulating in your blood and the usual indicator of your vitamin D status. UK clinical guidance from the Royal Osteoporosis Society, reproduced in NHS trust guidelines, sets out the thresholds, which relate to bone health, like this:
| 25-OH vitamin D | Classification |
|---|---|
| Below 25 nmol/L | Deficient |
| 25 to 50 nmol/L | May be inadequate for some people |
| Above 50 nmol/L | Adequate: sufficient for bone health in most people |
A result reported only as “in range” does not tell you where within that range you sit, or how that fits with how you feel. SACN notes that 25 nmol/L is not a clinical threshold diagnostic of disease, but a level below which the risk of poor bone and muscle health increases. Every result is reviewed by a GMC-registered doctor, who reads the number against your symptoms and risk factors rather than just the lab flag. We explain the numbers, and what “sufficient” does and does not mean, in more depth in our vitamin D deficiency guide.
You may also see a separate vitamin D3 (cholecalciferol) test offered. That is a specialist measurement for particular clinical questions. For most people, 25-OH vitamin D is the right test.
Vitamin D is one of several winter checks
Winter brings a familiar cluster of complaints: tiredness, low mood, feeling the cold and catching every bug going. It is tempting to put all of it down to the season, and sometimes that is right. But several of these symptoms overlap with problems a blood test can pick up, which is why vitamin D is usually best checked alongside a few other markers rather than on its own.
If you are catching more colds than usual, see Why Do I Keep Catching Every Cold Going Round?. If your mood dips every winter, Winter Blues or Something Else? looks at when low mood is worth a blood test. And our guide to feeling tired in winter brings the whole winter picture together.
How much vitamin D is safe to take
More is not better. Vitamin D is stored in the body, and the NHS warns that taking too much over a long period can cause too much calcium to build up (hypercalcaemia), which can weaken the bones and damage the kidneys and the heart. The NHS gives these upper limits for daily intake:
| Group | NHS upper limit |
|---|---|
| Adults, including pregnant and breastfeeding women and older people | 100 micrograms (4,000 IU) a day |
| Children aged 11 to 17 | 100 micrograms (4,000 IU) a day |
| Children aged 1 to 10 | 50 micrograms (2,000 IU) a day |
| Infants under 12 months | 25 micrograms (1,000 IU) a day |
Two practical points follow. The upper limit is a ceiling, not a target: the NHS recommendation for most people is 10 micrograms, a small fraction of it. And count everything you take, because vitamin D also turns up in multivitamins, cod liver oil and fortified products, so several at once can push your total higher than you intended. If your doctor has recommended a different amount, follow their advice, and if you are in doubt the NHS says to consult your doctor. If you feel unwell while taking a high-dose supplement, stop it and seek medical advice.
If a test shows you are deficient, the dose to correct it depends on how low you are, your age, your weight, any medical conditions and what else you take. That is a clinical decision rather than a label-reading exercise, and it is best made with a GP or our clinical team.
Will vitamin D stop me catching colds?
This is one of the most common reasons people start taking vitamin D, so it is worth being honest. Low levels are common in winter, when colds are also common. But the evidence that supplements prevent infections is not strong. SACN’s 2016 review concluded that evidence on vitamin D and infection is inconsistent and mainly observational, and that randomised trials do not generally show a beneficial effect of supplementation on infectious disease risk. Its UK recommendations were based on bone and muscle health, not on infections. So take the NHS-advised dose for your bones and muscles, and do not expect it to shield you from every cold.
If you seem to catch everything going, vitamin D is one of several things worth checking, alongside iron, B12 and zinc. We look at the full picture in Why Do I Keep Catching Every Cold Going Round?
When to see your GP
Low vitamin D is common and usually straightforward to correct. See your GP rather than relying on a private test if you have:
- Persistent bone pain, or pain and tenderness that is getting worse
- Muscle weakness that is making it hard to climb stairs or get up from a chair
- A bone that broke after a minor fall or knock
- Kidney disease, liver disease or another long-term condition, as these can affect how your body handles vitamin D and how much you can safely take
Getting tested at WMG Health
Vitamin D is included in the General Wellness panel (£279), alongside a full blood count, iron studies, B12 and folate, thyroid function, inflammation markers, zinc, liver and kidney function, cholesterol and HbA1c, which makes it a practical all-in-one winter check. If hair shedding is part of the picture, the Hair Loss Essentials panel (£249) also includes vitamin D. If you would rather check vitamin D on its own, or add it to other markers, you can build a custom panel.
Blood is drawn at our clinic at 134 Harley Street. Results are released as each marker is reviewed by a GMC-registered doctor, and we confirm the expected timeframe when you book. No GP referral is needed. See our health screening page for the full range of options.
Common questions
Does everyone in the UK need a vitamin D test in winter? No. The NHS advice to consider a 10 microgram supplement in autumn and winter applies whether or not you test. A test is most useful if you have risk factors, symptoms, or simply want to know where you stand.
When is my vitamin D likely to be at its lowest? In the UK, levels are typically lowest towards the end of winter, around February and March, and highest at the end of summer, around September. Testing in early autumn shows your starting point; testing in late winter shows your low point.
Do I need to fast for a vitamin D test? Not for vitamin D itself, but the General Wellness panel as a whole needs a fast of at least 12 hours beforehand, with water only, and not much longer than 12 hours. Do not take iron tablets for 24 hours or a B12 supplement beforehand, and if you take a multivitamin, B-complex or folate supplement, hold it for 24 to 72 hours before a General Wellness test.
How much vitamin D is it safe to take? The NHS advises that adults should not take more than 100 micrograms (4,000 IU) a day, and less for children. The standard advice for most people in autumn and winter is 10 micrograms (400 IU) a day.
Will vitamin D stop me catching colds? The evidence that supplements prevent infections is inconsistent, according to SACN. Take the NHS-advised dose for bone and muscle health, and do not rely on it to prevent colds.
Should I stop my vitamin D supplement before testing? No. Keep taking your vitamin D as normal. The result will show your level on your current routine, which is what a clinician needs to judge whether it is working.
Will a winter holiday in the sun top me up? Sun exposure abroad can raise your level, but it varies a great deal from person to person and does not replace the NHS winter advice. If you want to know the effect, test afterwards.
Is a vitamin D test the same as a vitamin D3 test? No. The standard test is 25-OH vitamin D, which measures your overall status. A vitamin D3 test is a separate specialist measurement that most people do not need.
Sources
- NHS: Vitamin D. nhs.uk
- Royal Osteoporosis Society: Vitamin D and bone health: a practical clinical guideline for patient management (2020), as reproduced in NHS trust guidance (Royal United Hospitals Bath, Vitamin D deficiency in adults, 2024). theros.org.uk
- Scientific Advisory Committee on Nutrition (SACN): Vitamin D and health (2016). gov.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.