Vitamin D (25-OH) Above or Below Range: Causes in Athletes and What to Do
The 25-hydroxyvitamin D test, usually written as 25(OH)D, is one of the most frequently ordered blood tests among people who train. Results outside the reference range are common in the UK, particularly in winter. The editorial team explains what the numbers mean, why athletes end up too low or, occasionally, too high, and what sensible next steps look like.
What the 25(OH)D test measures
Vitamin D from sunlight or food is first converted in the liver into 25-hydroxyvitamin D. This is the main circulating form and has a half-life of around two to three weeks, which makes it the best available marker of vitamin D status over recent weeks. The active hormone, 1,25-dihydroxyvitamin D, is made mainly in the kidneys, but it is tightly regulated and not useful for judging status.
UK laboratories report results in nanomoles per litre (nmol/L). Some international sources and supplement websites use nanograms per millilitre (ng/mL). To convert, multiply ng/mL by about 2.5 to obtain nmol/L. Mixing up the units is a frequent cause of confusion when people compare their results with online advice.
Thresholds vary between organisations. UK practice, influenced by the Scientific Advisory Committee on Nutrition (SACN) and the National Osteoporosis Society guidance, generally treats levels below 25 nmol/L as deficient. Many clinicians consider 25–50 nmol/L as possibly inadequate for some people, and levels above 50 nmol/L as sufficient for most.
| 25(OH)D level (nmol/L) | Approx. ng/mL | Common UK interpretation |
|---|---|---|
| Below 25 | Below 10 | Deficient; treatment usually recommended |
| 25–50 | 10–20 | May be inadequate in some people |
| 50–125 | 20–50 | Sufficient for most adults |
| 125–220 | 50–88 | Higher than needed; no added benefit shown |
| Above ~220–250 | Above ~88–100 | Potentially excessive; review supplementation |
The upper boundaries are less standardised than the lower ones, and different labs flag "high" at slightly different points. What matters most is the overall picture, including calcium levels, symptoms and the supplements you are taking.
Why athletes are often below range
The UK lies at a latitude where, from roughly October to March, sunlight does not contain enough UVB for the skin to make meaningful amounts of vitamin D. That is why the UK government advises everyone to consider a 10-microgram (400 IU) daily supplement during autumn and winter.
Athletes are not protected by being active. Research on professional UK athletes tested in winter found that a large proportion had inadequate levels. Indoor sports such as basketball, swimming, gymnastics, boxing and combat sports carry the highest risk, as do athletes who train early in the morning or late in the evening.
- Season and latitude: winter months in the UK are the main driver.
- Indoor training: little skin exposure to sunlight even in summer.
- Darker skin: more melanin means slower vitamin D production.
- Clothing and sunscreen: covering skin reduces synthesis.
- Higher body fat: vitamin D is stored in fat tissue, lowering blood levels.
- Restricted diets: few oily fish, eggs or fortified foods.
- Malabsorption: coeliac disease, inflammatory bowel disease, some medications.
A systematic review of athletes worldwide found that inadequate vitamin D is common across sports, with higher prevalence in winter and spring and at northern latitudes. For a British athlete, a low result in February is therefore more the rule than the exception.
Why levels can be above range
High 25(OH)D almost never comes from sunlight, because the skin has built-in mechanisms that limit production. Elevated results nearly always reflect supplementation, sometimes from several products at once, such as a vitamin D capsule, a multivitamin and a fortified sports drink.
The most worrying cause is a dosing error: taking a weekly or monthly high-dose product every day, misreading micrograms and IU, or using poorly manufactured supplements that contain far more vitamin D than declared. Case reports of vitamin D toxicity from such errors are published every year.
True toxicity is rare and generally associated with levels well above 250 nmol/L. It causes hypercalcaemia, raised calcium in the blood, with symptoms such as nausea, vomiting, thirst, frequent urination, constipation, confusion and, in severe cases, kidney damage and heart rhythm disturbances.
EFSA has set a tolerable upper intake level of 100 micrograms (4,000 IU) per day for adults. Moderately elevated results, say 150–200 nmol/L, are not dangerous in themselves, but they provide no proven additional benefit and are a signal to review how much you are taking.
Does vitamin D status affect performance?
Vitamin D receptors are present in skeletal muscle, and deficiency is associated with muscle weakness, bone pain and a higher risk of stress fractures. Correcting a genuine deficiency is clearly beneficial for musculoskeletal health and may support training availability.
Whether raising levels from adequate to high improves strength, power or endurance is a different question. Some early studies suggested benefits in athletes with low levels, but more recent controlled trials have generally failed to show performance gains once a person is sufficient.
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A study in UK professional athletes found that correcting low winter levels raised 25(OH)D effectively, while improvements in muscle function were less consistent. Reviews conclude that the goal is to avoid deficiency, not to chase very high numbers.
There is also interest in vitamin D and respiratory infections. Meta-analyses suggest a modest protective effect, particularly in people who are deficient and take regular daily doses rather than large intermittent boluses. For athletes who cannot afford to miss training, this is another reason to avoid winter deficiency.
What to do with your result
If your result is below 25 nmol/L, discuss it with your GP. NICE Clinical Knowledge Summaries describe treatment regimens using higher loading doses followed by maintenance, and the GP will decide on the regime and whether further tests, such as calcium, parathyroid hormone or coeliac screening, are needed.
If your result is between 25 and 50 nmol/L and you have no symptoms, a daily maintenance supplement, commonly 10–25 micrograms (400–1,000 IU), particularly through autumn and winter, is a reasonable approach for most adults. Some sports physicians use somewhat higher maintenance doses for athletes with documented low levels.
If your level is above the upper range, stop or reduce vitamin D supplements and check every product you take, including multivitamins and fortified foods. If you have symptoms such as persistent nausea, thirst or confusion, or the level is very high, seek medical advice promptly, as calcium and kidney function should be tested.
Retesting is usually sensible about three months after a change, because levels take several weeks to reach a new steady state. Testing in the same season each year also makes it easier to compare results meaningfully.
Conclusions of the editorial team
Low 25(OH)D is common in UK athletes, especially in winter, in indoor sports and in people with darker skin. Following the national advice to supplement through autumn and winter prevents most deficiency.
High levels almost always stem from supplements, often from multiple products or dosing errors. Moderately high results warrant a review, while very high levels with symptoms need medical attention.
The aim is to stay in the sufficient range, not to push values as high as possible. Retest around three months after any change.
The editorial team also suggests our articles on vitamin K2 combinations, on ferritin and iron status in athletes, and on how to prepare for a routine blood test.
References
- Scientific Advisory Committee on Nutrition. Vitamin D and Health. London: Public Health England; 2016.
- Owens DJ, Allison R, Close GL. Vitamin D and the athlete: current perspectives and new challenges. Sports Med. 2018;48(Suppl 1):3–16.
- Farrokhyar F, Tabasinejad R, Dao D, et al. Prevalence of vitamin D inadequacy in athletes: a systematic review and meta-analysis. Sports Med. 2015;45(3):365–378.
- Close GL, Russell J, Cobley JN, et al. Assessment of vitamin D concentration in non-supplemented professional athletes and healthy adults during the winter months in the UK: implications for skeletal muscle function. J Sports Sci. 2013;31(4):344–353.
- Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911–1930.
- Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583.
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on the tolerable upper intake level of vitamin D. EFSA J. 2012;10(7):2813.
