Vitamin D (25-OH): What the Test Shows and Why It Matters for an 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 an athlete needs it, 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 in two ways: it is synthesized in the skin under the action of ultraviolet B and comes from food or supplements (the D3 and D2 forms). On its own 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 "reserve". 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 can be normal or even elevated in deficiency. Therefore the 1,25(OH)₂D test is not used to assess reserves — 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 calcium and phosphorus absorption in the intestine and the maintenance of bone mineralization. For athletes this is a direct matter of preventing stress fractures, which often occur in runners, in military personnel during the preparation phase and in sports with heavy impact loads.
Vitamin D receptors have also been found in skeletal muscle and immune cells. The review by Owens, Allison and Close (2018) describes a link between low status and worse muscle recovery, muscle weakness in pronounced deficiency and a higher frequency of upper respiratory tract infections.
At the same time the editorial team emphasizes: 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 aid — it is a condition of normal functioning, the deficiency of which can worsen results.
The IOC consensus on dietary supplements (2018) classifies vitamin D as a supplement that is appropriate specifically in the case of a detected deficiency or a high risk of its development. Hence the practical value of the test.
Immunity deserves a separate mention. In athletes who train in gyms in winter and get little sun, deficiency combines with a high training load, which by itself temporarily suppresses immune responses.
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 reasonably higher risk for whom the test makes practical sense.
- athletes who train predominantly indoors (gymnastics, basketball, hockey, weightlifting);
- athletes with dark skin who live at northern latitudes;
- people with repeated stress fractures or low bone mineral density;
- athletes with frequent respiratory infections;
- individuals with restricted diets, intestinal diseases, obesity;
- those who already take high doses of supplements and want to check safety.
In Ukraine there is little ultraviolet B sufficient for vitamin D synthesis in the skin from roughly October to March. Therefore the 25(OH)D level has a pronounced seasonality: the minimum falls at the end of winter — the beginning of spring, and 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, tell 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 started from different tasks: the IOM focused on bone health in the population, while the Endocrine Society in 2011 focused on patients in risk groups.
| 25(OH)D level | IOM (2011) | Endocrine Society (2011) |
|---|---|---|
| < 12 ng/mL (< 30 nmol/L) | Risk of deficiency | Deficiency |
| 12–20 ng/mL (30–50 nmol/L) | Possibly 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 necessary, parathyroid hormone. High parathyroid hormone against a background of low 25(OH)D confirms a functional deficiency.
The methods of determination (immunoassay or liquid chromatography with mass spectrometry) can 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 reserves. It is useful for athletes in risk groups, especially those who train indoors, have repeated bone injuries or frequent infections.
Vitamin D does not improve results on its own, but its deficiency can worsen them. Therefore the goal is not the "maximum figure" but the elimination of the shortfall.
Take the season into account, take repeat tests in the same laboratory and discuss any correction with your doctor.
We also recommend reading our materials on the causes of vitamin D deviations 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.
