Vitamin D deficiency: prevention, diagnosis and when to see a doctor

Vitamin D is one of the most popular supplements and at the same time one of those surrounded by the most confusion: who should get tested, what levels to consider normal, how much to take and whether you can 'overdo it'. The editorial team gathered practical answers based on clinical guidelines.

Who needs examination

The 2024 Endocrine Society guidelines do not recommend routine screening of 25(OH)D levels in the general population of healthy adults. However, this does not apply to people with risk factors and symptoms, for whom the test remains appropriate.

Examination should be discussed with a doctor if there is osteoporosis or osteopenia, stress fractures, intestinal diseases with impaired absorption, chronic kidney or liver disease, long-term therapy with glucocorticoids or anticonvulsants, or obesity.

For athletes, the appropriateness of examination is increased by indoor training, dark skin, restrictive diets, low energy availability and bone pain. The IOC consensus on supplements stresses: it is better to base the decision about a supplement on a test.

Symptoms of deficiency are nonspecific: fatigue, pain in the bones and muscles, muscle weakness. Therefore relying on well-being alone is unwise.

The 25(OH)D test: how to read it

To assess status, 25-hydroxyvitamin D (25(OH)D) is measured. The active form 1,25(OH)₂D is not used for this: it may be normal or even elevated in deficiency due to the compensatory action of parathyroid hormone.

Results are expressed in ng/mL or nmol/L (1 ng/mL ≈ 2.5 nmol/L). The US Institute of Medicine (IOM, 2011) considers a level of 20 ng/mL (50 nmol/L) sufficient for the bone health of most people. The Endocrine Society in 2011 defined deficiency as below 20 ng/mL and insufficiency as 21-29 ng/mL.

The discrepancies between organizations are explained by different approaches to the evidence, so there is no unambiguous 'ideal' level. Levels above 100-150 ng/mL are already linked with a risk of toxicity.

Results of different laboratories may differ due to measurement methods. To monitor dynamics it is better to take the test in one laboratory, and a repeat test - no earlier than a few months after a dose change, when the level stabilizes.

Prevention: sun, food, supplements

In summer, a short stay in the sun with exposed arms and legs is sufficient for vitamin D synthesis for many people. However, a prolonged stay without protection increases the risk of skin cancer, so the sun should not be regarded as an 'unlimited' source.

Food sources are fatty fish (salmon, mackerel, herring, sardines), egg yolks, fish liver, fortified dairy products and plant-based drinks. Regularly including fatty fish in the diet is also beneficial from the standpoint of omega-3 fatty acids.

The IOM recommended intake for adults up to 70 years is 600 IU per day, and for people over 70 - 800 IU. The Endocrine Society in 2011 noted that to maintain a level above 30 ng/mL adults may need 1500-2000 IU per day. The upper safe limit for adults, according to IOM, is 4000 IU per day.

Vitamin D3 (cholecalciferol) raises the 25(OH)D level more effectively than D2. It is better to take the supplement with food that contains fat. Correction of pronounced deficiency with higher doses is carried out as prescribed by a doctor with repeat monitoring.

Excess vitamin D: risks

Vitamin D toxicity is rare and is almost always linked with excessive supplements rather than with sun or food. Cases have been described due to manufacturer errors, incorrect dosing or prolonged intake of very high doses 'for immunity'.

The main danger is hypercalcemia. Its signs are nausea, vomiting, thirst, frequent urination, constipation, weakness, confusion, disturbance of heart rhythm. Prolonged hypercalcemia can lead to kidney stones and kidney damage.

• nausea, vomiting, loss of appetite;
• strong thirst and frequent urination;
• weakness, confusion;
• lower back pain, renal colic;
• irregularities in the heart's work.

People with sarcoidosis, some lymphomas, hyperparathyroidism or kidney diseases should take vitamin D only under a doctor's supervision, because their risk of hypercalcemia is higher even at moderate doses.

A practical rule: do not combine several products with vitamin D (multivitamins, 'sports' complexes, separate capsules) without counting the total daily dose.

When to see a doctor

You should see a doctor if you have bone pain, muscle weakness, repeated fractures or stress fractures at usual loads. Such symptoms require examination not only of vitamin D but also of calcium, phosphorus, parathyroid hormone and kidney function.

A consultation is also needed if you have a chronic disease that affects vitamin D metabolism, or you take drugs that lower it. The doctor will determine whether a test is needed and what dose of supplement is safe.

Seek help urgently if, against a background of taking high doses, signs of hypercalcemia have appeared - severe weakness, vomiting, confusion, disturbance of heart rhythm.

For athletes with signs of REDs - menstrual cycle disturbances, frequent injuries, weight loss - a consultation with a sports doctor will help assess bone health comprehensively.

Editorial conclusions

Vitamin D deficiency is common, but not everyone needs a test. Examination is appropriate for people with risk factors, including athletes who train indoors, have stress fractures or restrictive diets.

For assessment, 25(OH)D is used, and different organizations interpret the thresholds differently. Preventive doses are relatively small, and the upper safe limit for adults is 4000 IU per day.

Excess vitamin D is dangerous because of hypercalcemia, so 'the more the better' does not work here.

We also advise getting acquainted with our articles on the causes of vitamin D deficiency in athletes, on calcium and bone health and on osteoporosis in men.

References

1. Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: The 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. 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.