Vitamin D and UK athletes

Why UK sunlight stops working for half the year

Vitamin D is made in the skin when ultraviolet B radiation hits it, and UVB reaching the UK is seasonal. NHS guidance states that from about late March or early April to the end of September most people can make all the vitamin D they need from sunlight, and that between October and early March we do not make enough from sunlight at all. This is not about how much time you spend outdoors. During those months the necessary wavelength is not arriving in useful quantity regardless of how long you train outside.

Based on NHS and Scientific Advisory Committee on Nutrition guidance for the UK population. Individual synthesis also depends on skin tone, clothing cover, time outdoors and sunscreen use.

Two consequences follow for athletes specifically. Indoor and evening training means less exposure even in the months when synthesis is possible, and a summer result tells you little about where you will sit in February. Testing in late winter, when levels are at their lowest, gives you the most useful answer.

What UK guidance recommends, and what it was designed to do

The Scientific Advisory Committee on Nutrition set the UK reference nutrient intake for vitamin D at 10 micrograms, equivalent to 400 international units, per day for everyone aged four and over. It concluded in its 2016 report that the risk of poor musculoskeletal health rises below a blood 25-hydroxyvitamin D concentration of 25 nanomoles per litre, and set the intake figure as the amount needed for most of the population to stay at or above that concentration when UVB exposure is minimal.

Read that carefully, because it explains the whole disagreement. The 10 microgram recommendation was designed to keep the population above the level at which bones and muscles start to suffer. It was not designed to answer what an athlete in heavy training might need, and SACN never claimed it was. NHS guidance does the job it set out to do, and does it well. It simply answers a different question from the one an athlete is asking.

Government advice also identifies groups who should supplement all year rather than only in winter: people who get little or no sun exposure, people who usually cover most of their skin outdoors, and people with darker skin from African, African-Caribbean or south Asian backgrounds.

Why nobody agrees on what a good vitamin D level is

If you have searched this before and come away confused, that is a reasonable response to the actual state of the evidence. Different expert bodies use different thresholds for the same measurement, and they were derived to answer different questions.

SACN identifies 25 nanomoles per litre as the level below which musculoskeletal health risk increases in the UK population. The 2022 meta-analysis by Harju and colleagues in the European Journal of Nutrition used 50 nanomoles per litre as its definition of insufficiency, describing it as the contemporary and relatively widely accepted cut-off. Sports nutrition practice frequently works to 75 nanomoles per litre as a sufficiency target, which is the figure Owens, Allison and Close use in their Sports Medicine review when discussing where supplementation stops adding anything.

The gap between 25 and 75 is not a scientific scandal, it is three different questions. What level prevents disease. What level counts as insufficient in research. What level a practitioner aims for when there is no downside to being comfortably above the line. IMULAB reports the number, states which threshold each interpretation comes from, and does not pretend a single figure is settled when it is not.

Worth knowing that the older prevalence figures you may encounter used a higher bar again. The 2015 meta-analysis by Farrokhyar and colleagues, which reported vitamin D inadequacy in 56 per cent of athletes and found the rate significantly higher in the UK than most locations, defined inadequacy at 80 nanomoles per litre. Later authors have noted that threshold would be considered high by current standards, which is why the more recent figure of roughly 30 per cent at 50 nanomoles per litre is the more defensible one to quote.

What vitamin D does that matters to athletes

Vitamin D is best known for regulating calcium and supporting bone, and it also acts on muscle and immune tissue. The strength of evidence differs considerably between those roles, and athletes deserve to know which is which.

Why more vitamin D is not better

Vitamin D is fat soluble, which means the body stores what it does not use, and this is the point most supplement marketing skips. NHS guidance is explicit: adults should not take more than 100 micrograms, or 4000 international units, a day, because taking too much over a long period causes calcium to build up in the body, which can weaken bones and damage the kidneys and heart. You cannot reach that state through sunlight. It comes from supplements.

The athlete-specific picture adds a further caution. Owens, Allison and Close report that dosing practice in elite sport ranges from a thousand international units daily up to blanket bolus regimens of a hundred thousand international units weekly, and state that there is no ergogenic effect from doses that push levels far above the sufficiency cut-off. Their own work in elite athletes compared high weekly doses and found the larger one raised a catabolic vitamin D metabolite that stayed elevated even after supplementation stopped, with the possibility that the net effect works against what was intended. Their closing recommendation is against blanket supplementation, mega doses, bolus doses, and one-size-fits-all approaches.

Two practical implications. Correcting a low level is worth doing. Taking 5000 international units daily indefinitely because a supplement brand suggested it, without ever measuring, is not a considered decision.

Which UK athletes have the strongest case for testing

Given the limits above, the useful question is not whether every athlete should test, but who gets a decision-changing answer from it. Risk factors that stack the case are well established in the literature.

• Indoor and evening trainersFarrokhyar and colleagues found indoor sport significantly raised the risk of inadequacy. If your training happens in a gym, pool or indoor arena, or after dark, you miss the synthesis window even in summer.
• Athletes with darker skinGovernment advice recommends year-round supplementation for people from African, African-Caribbean and south Asian backgrounds because less vitamin D is synthesised for the same exposure. Interpretation of the result needs care, for the reasons set out in the limits above.
• Non weight bearing sportsSwimmers, cyclists and rowers do not get the bone loading stimulus that appears to protect athletes in impact sports, and are at higher risk of low bone density when vitamin D is low.
• Repeated winter illnessIf your training keeps getting broken up by respiratory infections between October and March, this is the marker where the trial evidence most directly supports acting on a low result.
• A history of stress fracture or low energy availabilityBoth warrant a broader clinical assessment, and vitamin D forms part of that picture rather than the whole of it.
• Anyone already taking high dose supplementsIf you are taking well above the government recommendation, measuring is how you find out whether you need to be, and whether you have overshot.

When to test vitamin D and how often

Test at the point where the answer is most useful, which for UK athletes is late winter. A sample taken in February or March captures your annual low, after months without skin synthesis, and tells you whether your current arrangement is holding. A summer sample flatters the result and is only worth taking if you are checking a high dose regimen for safety.

Vitamin D moves slowly, which makes it a poor candidate for frequent retesting. The 25-hydroxyvitamin D metabolite has a half-life measured in weeks rather than days, so allow at least eight to twelve weeks after starting or changing supplementation before retesting, and expect an annual or twice yearly cadence to be sufficient for most people rather than anything more often.

Unlike ferritin, vitamin D is not meaningfully distorted by a hard training session the day before, so timing relative to training matters much less here. See the ferritin page for the marker where that does matter a great deal.

Testing vitamin D with IMULAB

IMULAB is a UK performance blood testing company for athletes and high performers. Vitamin D results are reported against the relevant thresholds with the source of each one stated, alongside the seasonal context that determines what the number means, and with the limits of the measurement set out rather than hidden.

Samples can be collected four ways: at a Superdrug health clinic, at home anywhere in the UK through the mobile nurse network, by finger-prick at home, or by your own healthcare professional if you would rather use someone you already see. Some panels require a venous draw rather than a finger-prick sample, and the booking page states which applies before you order. See the collection options.

Reports say what the result means, what the evidence does and does not support doing about it, and what warrants a conversation with your GP. Vitamin D is usually read alongside other markers rather than alone, and the pillar page on blood tests for athletes covers the wider panel.

Frequently asked questions

• What vitamin D level should a UK athlete have?

  There is no single agreed figure, and different bodies use different thresholds because they were derived to answer different questions. The Scientific Advisory Committee on Nutrition identifies 25 nanomoles per litre as the level below which musculoskeletal health risk increases. Recent athlete research commonly defines insufficiency as below 50 nanomoles per litre. Sports nutrition practice often works to a sufficiency target of 75 nanomoles per litre. What the evidence does not support is aiming for a number far above sufficiency, because no performance benefit has been demonstrated from doing so.

• Do UK athletes need vitamin D supplements in winter?

  NHS guidance is that everyone in the UK should consider taking a daily supplement containing 10 micrograms of vitamin D during autumn and winter, because between October and early March UK sunlight is not strong enough for the body to make it. Athletes are not exempt from that advice and several factors, including indoor or evening training, can make their exposure lower than average. Whether an individual needs more than the standard recommendation is a question for testing rather than assumption.

• Does vitamin D improve athletic performance?

  The evidence does not support that claim as it is usually made. Correcting genuine deficiency is worthwhile, but a meta-analysis of randomised controlled trials in athletes found no significant overall effect of vitamin D3 supplementation on muscle strength, and reviewers of the athlete literature state there is no ergogenic effect from doses that raise levels far above the sufficiency threshold. Vitamin D is best understood as a biological requirement to meet rather than a performance aid to load up on.

• Can I take too much vitamin D?

  Yes. NHS guidance states that adults should not take more than 100 micrograms, or 4000 international units, a day, because excessive intake over a long period causes calcium to build up in the body, which can weaken bones and damage the kidneys and heart. You cannot overdose through sunlight, only through supplements. Reviewers of the athlete literature specifically advise against blanket supplementation, mega doses and large bolus doses.

• When is the best time of year to test vitamin D in the UK?

  Late winter, meaning February or March, gives the most useful answer for UK athletes. That is when levels are at their annual low, after months in which the skin has made none, so it shows whether your current diet and supplementation are holding. A summer result reflects the months when sunlight is doing the work and tells you little about how you will fare in winter.

• How long does it take to raise vitamin D levels?

  Allow at least eight to twelve weeks before retesting after starting or changing supplementation. The 25-hydroxyvitamin D metabolite measured in a blood test has a half-life measured in weeks rather than days, so levels move slowly and an earlier retest is unlikely to give a meaningful picture of the change.

• Does vitamin D prevent injury or stress fractures in athletes?

  The athlete evidence does not support that, which surprises most people. Research has consistently failed to demonstrate a relationship between vitamin D levels and bone health in athletes, and one proposed explanation is that the mechanical loading of training is itself a strong enough stimulus for bone to compensate. An important exception is athletes in non weight bearing sports such as swimming, cycling and rowing, who do not get that loading stimulus and are at higher risk of low bone density when vitamin D is low. Anyone who has had a stress fracture needs clinical assessment rather than a vitamin D result alone.

• Is the standard vitamin D blood test accurate for everyone?

  Not equally. The standard test measures total 25-hydroxyvitamin D, most of which is bound to vitamin D binding protein rather than biologically available. Reviewers have set out evidence that Black athletes frequently show low total vitamin D without the expected effects on bone, likely because genetic differences in binding protein leave a similar amount of the free fraction available. This means the standard measurement can misclassify people, and it is a limitation of the assay rather than of any individual result.

Related reading

• Blood tests for athletes in the UK: what to test and why
• Ferritin levels for athletes: what the evidence supports
• Blood testing for cyclists and mountain bikers
• Blood test results explained for athletes