Vitamin D: When to Test, How to Interpret Results and How Much to Supplement

Vitamin D is one of the most frequently tested nutrients in Poland, and deficiency is extremely common. Estimates suggest that most adults have insufficient vitamin D levels during the winter months. This is largely due to Poland’s geographical location: from October to March, the angle of the sun is too low for the skin to produce significant amounts of vitamin D. At the same time, vitamin D has become associated with many exaggerated health claims. This article explains what is reliably known: how and when to test vitamin D, how to interpret the results, how much to supplement and when supplementation can be harmful. Supplement doses and treatment of deficiency should be determined by a doctor; this article is for informational purposes only.

What does vitamin D do?

Its primary, well-established role is to regulate calcium and phosphorus absorption and support bone mineralisation. Severe deficiency causes rickets in children and osteomalacia in adults, while chronic moderate deficiency contributes to osteoporosis, muscle weakness and falls in older people. Vitamin D receptors are also found in immune cells, muscles, the pancreas and blood vessels. This has prompted research into its role in infections, autoimmune diseases, diabetes and depression. The findings are mixed: low vitamin D levels are associated with poorer health, but supplementation in people without deficiency has not shown clear benefits in most large clinical trials. The practical conclusion is that correcting a deficiency is worthwhile, whereas taking megadoses for general health is not supported by evidence.

Which test: 25-OH-D, not 1,25-OH-D

The body’s vitamin D status is reflected by 25-hydroxyvitamin D (25-OH-D, calcifediol), a metabolite with a long half-life that reflects vitamin D obtained through skin synthesis, diet and supplements. This is the test you should request: 25-hydroxyvitamin D (25 OH) – vitamin D test. 1,25-dihydroxyvitamin D (calcitriol) is the active form, tightly regulated by the kidneys and parathyroid glands. Its concentration can be normal or elevated even in vitamin D deficiency, so this test is reserved for investigating kidney disease, parathyroid disorders and certain rare conditions. Most laboratories measure vitamin D2 and D3 together, which is sufficient for routine assessment.

How to interpret your result

Laboratories may use different units: 1 ng/ml = 2.5 nmol/l. American medical societies, including the Endocrine Society in its 2024 guidelines, are moving towards less routine vitamin D testing and supplementation in the general population. Polish recommendations from 2023, developed by an expert group led by Professor Płudowski, maintain a target range of 30–50 ng/ml and recommend testing for people in high-risk groups. A practical compromise is to test when clinically indicated, take preventive supplements during winter and monitor vitamin D levels when using higher doses.

Who should be tested?

Measuring 25-OH-D is not necessary for every healthy adult taking a preventive supplement dose. Testing is recommended for people with obesity (vitamin D is stored in fatty tissue, so higher doses may be needed), osteoporosis or a history of fractures, people over 65, those with kidney or liver disease, and people with malabsorption disorders (coeliac disease, Crohn’s disease or following bariatric surgery). It is also recommended for people with autoimmune conditions (Hashimoto’s thyroiditis, rheumatoid arthritis or psoriasis), those taking antiepileptic medicines or glucocorticoids, pregnant and breastfeeding women, people with darker skin, those who work indoors or avoid sunlight, and anyone experiencing muscle or bone pain, chronic fatigue or recurrent infections. Monitoring is also necessary when taking doses above 4000 IU daily.

When should you test?

Vitamin D levels are lowest in February–March and highest in September. Testing in late autumn or early winter shows how much vitamin D your body has in reserve before the sun-poor months and helps determine an appropriate supplement dose. Retesting after 8–12 weeks helps assess whether the dose is suitable. Fasting is not required, and the test can be performed at any time of day. It is best to take your supplement after blood collection. High doses of biotin can interfere with the test, so stop taking biotin for 2–3 days beforehand.

Vitamin D supplementation: general guidelines

Polish guidelines for healthy adults recommend 1000–2000 IU daily from October to April, or throughout the year if sun exposure is insufficient. People with obesity may require twice this amount, while adults over 75 are advised to take 2000–4000 IU throughout the year. Treating vitamin D deficiency requires higher doses, determined according to the test result, together with monitoring. Vitamin D is fat-soluble, so it is absorbed more effectively when taken with a meal containing fat. Vitamin D3 raises blood levels more effectively than D2. Vitamin K2 is not required for safe vitamin D supplementation at recommended doses, although it is sometimes included in combined supplements.

When can vitamin D be harmful?

Vitamin D toxicity occurs almost exclusively after prolonged use of very high doses (tens of thousands of units daily) or dosing errors involving concentrated preparations. Symptoms result from hypercalcaemia (excessively high blood calcium levels) and include nausea, vomiting, constipation, increased thirst and urination, weakness, kidney stones and abnormal heart rhythms. If 25-OH-D exceeds 100 ng/ml, supplementation should be stopped and calcium and creatinine levels checked. Particular caution is required in sarcoidosis, hyperparathyroidism, kidney stone disease and granulomatous disorders, where even standard doses may increase blood calcium levels.

Additional tests

For vitamin D deficiency and bone disorders, additional tests may include Total calcium – laboratory test, phosphorus and alkaline phosphatase (bone health panel), parathyroid hormone (PTH) (which rises as a secondary response to vitamin D deficiency), Creatinine – laboratory test and Magnesium – laboratory test (required for vitamin D activation). Bone density scanning (DXA) provides additional information for postmenopausal women and men over 70.

Frequently asked questions

Do I need to take vitamin D supplements in summer?

People who spend 15–20 minutes in the sun each day, with their forearms and legs exposed, without sunscreen, between 10:00 and 15:00, generally do not need supplementation. However, people who work indoors, use sunscreen or have obesity often do. A blood test can help determine individual needs.

Is taking a higher dose once a week as effective as taking vitamin D daily?

Weekly doses (e.g. 7000–14000 IU) are equivalent to daily supplementation and may be more convenient. However, very large single doses taken once every few months are not recommended.

Do I need a referral for a vitamin D test?

No. The 25-hydroxyvitamin D (25 OH) – vitamin D test is available privately, either individually or as part of a package with calcium and phosphorus tests. Results are available within 1–2 days.

Sources

• Płudowski P. et al., Nutrients, Guidelines for Preventing and Treating Vitamin D Deficiency: A 2023 Update in Poland, 2023. mdpi.com
• Demay MB et al., J Clin Endocrinol Metab, Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline, 2024. academic.oup.com