Vitamin D (25-OH): what the test shows and why it matters for the athlete
Vitamin D is one of the most popular tests among athletes, but it is often ordered and interpreted incorrectly. The editorial team explains which exact form of the vitamin is measured, why it matters to an athlete, who should take the test, and why the «normal» thresholds differ between guidelines.
What exactly the 25-OH test measures
Vitamin D enters the body by two routes: it is synthesized in the skin under the action of ultraviolet B and comes from food or supplements (forms D3 and D2). By itself it is biologically inactive. First, in the liver, the vitamin is converted into 25-hydroxyvitamin D — 25(OH)D, or calcidiol.
It is precisely 25(OH)D that is the main circulating «store». Its half-life is about two to three weeks, so the concentration reflects the total intake of vitamin D over the preceding weeks and months. The clinical guidelines of the Endocrine Society and the US Institute of Medicine (IOM) recognize 25(OH)D as the best marker of status.
The active form — 1,25-dihydroxyvitamin D (calcitriol) — is formed mainly in the kidneys. Its level is tightly regulated by parathyroid hormone and may be normal or even elevated in deficiency. That is why the 1,25(OH)₂D test is not used to assess stores — this is a common mistake when ordering on one's own.
Most laboratories report total 25(OH)D (the sum of D2 and D3) in ng/mL or nmol/L. The conversion is simple: 1 ng/mL = 2.5 nmol/L.
The role of vitamin D in an athlete's body
The classic function of vitamin D is the regulation of the absorption of calcium and phosphorus in the intestine and the support of bone mineralization. For athletes this is a direct matter of preventing stress fractures, which often occur in runners, military personnel during the training stage, and in sports with high impact loads.
Vitamin D receptors are also found in skeletal muscle and immune cells. The review by Owens, Allison, and Close (2018) describes a link between low status and poorer muscle recovery, muscle weakness in pronounced deficiency, and a higher frequency of upper respiratory tract infections.
At the same time, the editorial team stresses: there is no convincing evidence that raising 25(OH)D in people with a normal level improves strength or endurance. Vitamin D is not an ergogenic agent — it is a condition for normal function, a deficiency of which can worsen performance.
The IOC consensus on dietary supplements (2018) classifies vitamin D among supplements that are appropriate precisely in cases of identified deficiency or a high risk of its development. Hence the practical value of the test.
Immunity deserves separate mention. In athletes who train indoors in winter and get little sun, a deficiency is combined with a high training load, which in itself temporarily suppresses immune reactions.
Which athletes should take the test
The updated 2024 Endocrine Society guidelines do not recommend routine screening of all healthy people. However, in the athletic population there are groups with a justifiably higher risk, for whom the test makes practical sense.
- athletes who train mainly indoors (gymnastics, basketball, hockey, weightlifting);
- athletes with dark skin who live at northern latitudes;
- people with recurrent stress fractures or low bone mineral density;
- athletes with frequent respiratory infections;
- individuals with restricted diets, intestinal diseases, or obesity;
- those who already take high doses of supplements and want to check safety.
In Ukraine, ultraviolet B sufficient for the synthesis of vitamin D in the skin is scarce from about October to March. Therefore the level of 25(OH)D has a pronounced seasonality: the minimum falls at the end of winter and beginning of spring, the maximum at the end of summer.
The time of testing affects interpretation. A normal result in September does not guarantee the same in March, and a low reading in February may partly recover in summer.
The test does not require fasting, but it is convenient to combine it with other morning tests. If you take biotin in high doses, inform the laboratory: some immunoassays are sensitive to it.
How to read the result
There is no single global threshold, and this is a source of confusion. Different organizations proceeded from different tasks: the IOM was oriented toward bone health in the population, the Endocrine Society in 2011 — toward at-risk patients.
| Level of 25(OH)D | IOM (2011) | Endocrine Society (2011) |
|---|---|---|
| < 12 ng/mL (< 30 nmol/L) | Risk of deficiency | Deficiency |
| 12–20 ng/mL (30–50 nmol/L) | May be insufficient for some people | Deficiency |
| 20–29 ng/mL (50–75 nmol/L) | Sufficient for most | Insufficiency |
| ≥ 30 ng/mL (≥ 75 nmol/L) | Sufficient | Sufficiency |
| > 50 ng/mL (> 125 nmol/L) | Possible adverse effects | — |
The 2024 Endocrine Society guideline abandoned specific target thresholds for healthy people, acknowledging insufficient evidence. In sports medicine a level of no less than 30 ng/mL is often used as a reference, but this is expert practice, not a strict standard.
The result should always be assessed together with calcium, phosphorus and, if needed, parathyroid hormone. A high parathyroid hormone against a background of low 25(OH)D confirms a functional deficiency.
Methods of determination (immunoassay or liquid chromatography with mass spectrometry) may give different values, so it is better to monitor the trend in a single laboratory.
Editorial conclusions
The 25(OH)D test is a reliable way to assess vitamin D stores. It is useful for athletes in at-risk groups, especially those who train indoors, have recurrent bone injuries, or frequent infections.
Vitamin D does not improve performance on its own, but its deficiency can worsen it. So the goal is not the «maximum number» but the elimination of a shortage.
Take the season into account, take repeat tests in a single laboratory, and discuss correction with a doctor.
We also recommend reading our materials on the causes of deviations of vitamin D from the norm in athletes, on magnesium in the blood, and on the vitamin B12 test.
References
- 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.
- Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2024;109(8):1907–1947.
- Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academies Press; 2011.
- Owens DJ, Allison R, Close GL. Vitamin D and the athlete: current perspectives and new challenges. Sports Med. 2018;48(Suppl 1):3–16.
- Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266–281.
- Maughan RJ, Burke LM, Dvorak J, et al. IOC consensus statement: dietary supplements and the high-performance athlete. Br J Sports Med. 2018;52(7):439–455.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
