Vitamin D (25-OH) Above or Below Normal: Causes in Athletes and What to Do

A vitamin D test result outside the reference range is one of the most common "findings" in athletes. The editorial team examines why the 25(OH)D level drops or, conversely, becomes excessive, which of the causes are characteristic specifically of athletes and what to do about it together with a doctor.

When a result is considered a deviation

Before talking about causes, it is worth agreeing on what counts as "below" and "above" normal. According to the position of the US Institute of Medicine (IOM), a 25(OH)D concentration below 12 ng/mL (30 nmol/L) means a risk of deficiency, a level of 20 ng/mL (50 nmol/L) is sufficient for the vast majority of people, and values above 50 ng/mL (125 nmol/L) are already linked to potential adverse effects.

In its 2011 guideline the Endocrine Society considered a level below 20 ng/mL to be deficiency and 21–29 ng/mL to be insufficiency. It is precisely this division that many laboratories still use, so on the report you may see the label "insufficiency" at a value that the IOM considers acceptable.

As for the upper limit, the clinical toxicity of vitamin D, according to Holick's review in NEJM (2007), is usually associated with levels above 150 ng/mL (375 nmol/L) and is accompanied by hypercalcemia. The intermediate zone between 50 and 150 ng/mL is not poisoning, but it indicates excessive intake that has no proven benefit.

Thus, for an athlete a deviation should be considered both a markedly low level and a persistently high one that has appeared through uncontrolled supplement use.

Causes of a low level in athletes

The most common cause is insufficient sun exposure. Training indoors, early or late sessions, the winter season and the use of sunscreens sharply limit synthesis in the skin. The review by Owens et al. (2018) shows that deficiency is especially widespread among athletes of indoor sports and in northern countries.

• Season and latitude:in Ukraine in winter there is almost no ultraviolet B for synthesis.
• Dark skin:melanin reduces the efficiency of synthesis.
• Excess adipose tissue:vitamin D is distributed in fat, and its blood level is lower, which is relevant for heavy weight categories.
• Malabsorption:celiac disease, inflammatory bowel diseases, states after bariatric surgery.
• Liver and kidney diseases:they disrupt the hydroxylation steps.
• Medications:some anticonvulsants, glucocorticoids and certain antiretroviral drugs accelerate the breakdown of vitamin D.

Diet rarely compensates for a lack of sun: vitamin D is present in significant amounts only in fatty fish, egg yolks and fortified products. Athletes with restricted diets, in particular vegan ones, have a higher risk.

Women with low energy availability — within the framework of RED-S syndrome — often combine low vitamin D with menstrual cycle disturbances and reduced bone density. In such a case the deficiency is part of a broader problem.

Finally, the cause of a "low" result may be the test itself: taken at the end of winter, it will show the seasonal minimum rather than the year-round average level.

Causes of a high level

Practically the only real cause of high 25(OH)D is excessive supplement use. The skin has a protective mechanism: with prolonged exposure the excess precursors break down, so the sun does not cause intoxication.

The most common scenarios in the sports environment are: simultaneous use of several products containing vitamin D (multivitamins, a separate D3, fortified protein); mistakes with drops, where a single drop contains a significantly larger dose than the person expects; prolonged use of loading doses once prescribed for a short course.

The Institute of Medicine has set an upper tolerable intake limit for adults of 4000 IU per day. It does not mean that exceeding it is immediately toxic, but prolonged consumption above this limit without monitoring is not recommended.

Certain diseases — sarcoidosis, tuberculosis, some lymphomas — raise the level specifically of the active form 1,25(OH)₂D and calcium, while 25(OH)D may be normal. This is another reason why interpretation requires a doctor.

Signs of excess are associated mainly with hypercalcemia: thirst, frequent urination, nausea, constipation, weakness and, in severe cases, kidney damage and heart rhythm disturbances.

What to do in case of a deviation

The first step is not to draw hasty conclusions from a single number. Compare the result with the season, supplement use and the laboratory's method. Further action depends on the direction of the deviation.

For prevention in adults the IOM recommends 600 IU per day (800 IU after age 70). These figures are calculated for minimal sun exposure. Corrective doses in case of deficiency are significantly higher; they are prescribed for a limited time under laboratory monitoring.

Since 25(OH)D has a half-life of about 2–3 weeks, a new stable concentration is established no earlier than a few weeks later. Repeating the test 10 days after starting intake makes no sense.

For athletes with a low level, the editorial team also advises assessing the diet with respect to calcium and energy availability: without sufficient calcium and calories, correcting vitamin D will not protect the bones.

Editorial conclusions

Low vitamin D in athletes is most often explained by a lack of sun, the season, skin characteristics, body mass or a restricted diet. High vitamin D is almost always caused by excessive supplements.

Both deviations are correctable, but in different ways: deficiency — by finding the cause and adequate repletion, excess — by review and stopping intake.

Rely on a repeat test in the same laboratory and consult a doctor instead of independently "ramping up" doses.

We also recommend reading our materials on what the vitamin D test shows, on magnesium in the blood and on vitamin B12 deviations from the norm.

References

1. Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academies Press; 2011.
2. 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.
3. 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.
4. Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266–281.
5. Owens DJ, Allison R, Close GL. Vitamin D and the athlete: current perspectives and new challenges. Sports Med. 2018;48(Suppl 1):3–16.
6. Mountjoy M, Sundgot-Borgen JK, Burke LM, et al. IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. Br J Sports Med. 2018;52(11):687–697.