Vitamin D in a UK winter, and when a test helps

The sun does come out in a British January. It just is not doing the job you might assume it is. From roughly early October to the end of February the sun never climbs high enough over the UK for much of its ultraviolet B to reach the ground, and UVB is the specific part of sunlight that lets skin make vitamin D. A bright, cloudless, bitterly cold morning in December will do almost nothing for you. A grey and unpromising afternoon in June will do a fair amount.

That is the whole mechanism, and it explains why the advice is seasonal rather than weather-dependent. The NHS states it plainly: between October and early March we do not make enough vitamin D from sunlight. For that stretch, its guidance is that everyone should consider a daily supplement of 10 micrograms.

Ten micrograms is 400 IU. One microgram equals 40 IU, which is worth committing to memory before you stand in a pharmacy aisle comparing labels, because the same dose printed two different ways looks like two wildly different products.

Some people are advised to take it all year

The autumn and winter advice is for the general population. A separate group is advised to supplement year round, and the reasoning is about exposure rather than season:

• people who are not often outdoors, including anyone who is frail or housebound
• people living in a care home or similar setting
• people who usually wear clothes that cover most of their skin when outside
• people with darker skin, who the NHS notes may also not make enough vitamin D from sunlight
• children aged one to four, and babies under one, with separate smaller doses

If you are in one of those groups, a summer of good weather does not settle the question the way it might for someone who spends July outdoors in short sleeves.

Supplementing and testing answer different questions

This is the part that gets muddled, and it is worth being clear about. The 10 microgram recommendation is a population-level figure. It exists to keep most people above the level associated with musculoskeletal problems. It is not calibrated to get any particular person to any particular number, and it does not tell you where you were before you started.

So the two things are not substitutes. Taking the recommended dose is sensible general advice. Testing is a measurement. If your interest is simply "should I be taking something over winter", the answer is already published and free, and a test adds little. If your interest is "how low do I actually get, and is what I am taking making a difference", that is a question only a measurement answers.

A vitamin D blood test measures 25-hydroxyvitamin D, usually written 25(OH)D. It is the storage form, which is why it reflects your status over recent weeks rather than what you did yesterday. In the UK it is reported in nanomoles per litre. A lot of the material you will find online is American and reports nanograms per millilitre instead, so a number that looks alarming next to a UK result may simply be in different units.

UK guidance commonly treats below 25 nmol/L as deficiency and 25 to 50 as insufficiency, with 50 and above generally sufficient for most people. Treat those as orientation rather than a verdict. Laboratories set their own reference intervals and do not all set them identically, so the range printed beside your own result is the one that applies to your sample.

Symptoms are a poor way in

Low vitamin D is associated with tiredness, aching bones and muscles, and low mood. The difficulty is that every one of those has a long list of other explanations, several of them more common. Tiredness in particular is close to useless as a signal on its own: low iron stores, thyroid problems, poor sleep, and a dozen ordinary things all produce it.

Which is an argument for measuring rather than guessing, but also an argument against reading too much into a single number. A vitamin D result that comes back sufficient does not mean nothing is wrong. It means one specific thing has been ruled out.

More is not better

Vitamin D is fat soluble, so it accumulates in a way that water-soluble vitamins do not. The NHS sets an upper limit of 100 micrograms, or 4,000 IU, a day for adults and children over 11, with 50 micrograms for children aged one to ten and 25 micrograms for babies under one. Taking more than that over a long period can raise calcium levels, which is a genuine problem rather than a theoretical one.

High-dose supplements are widely sold and it is easy to end up taking several times the recommended amount without intending to, particularly if you are also taking a multivitamin or a cod liver oil product. Reading the labels is worth the two minutes.

Timing a test

If you want to know how low you get, test late in the winter. Your level tracks the season, so a result from late August tells you about a summer you have already had. January or February is when the trough is.

If you are testing to see whether a supplement is doing anything, leave it a reasonable stretch after starting rather than checking a fortnight in, and try to repeat the test at roughly the same point in the year. Comparing a February result against an August one tells you about the season more than about anything you have changed.

And if you are testing because you feel persistently tired rather than because you are curious about vitamin D specifically, a single marker is a narrow way to approach a broad symptom. It may still be the right place to start in February. It is just worth knowing that it is one question, not the whole set.

Vitamin D, B12 and folate deficiencies overlap in who they affect and in how they feel, and they are usually run together for that reason, which is why the Vitamin D, B12 and Folate Blood Test exists as one order rather than three. If tiredness is the real question rather than vitamin D, our piece on the six blood markers behind constant fatigue works through the rest of the usual suspects, iron stores and thyroid among them.

This article is educational only and cannot diagnose you. Discuss results, especially anything outside your laboratory's reference interval, or that does not match how you feel, with a GP or other appropriately qualified clinician.