Vitamin D: test first or take it blind? Levels and doses
Sonia Biecka
Dietitian
Medical review: lek. Wojciech Sierocki
Content medically reviewed by an Optimals team physician.
Vitamin D is one of the most commonly supplemented nutrients, especially in autumn and winter. At the same time, measuring its blood level has become a popular part of preventive lab work. Many people test it regularly, even when they have no symptoms and no risk factors for deficiency.
Do you really need to measure your vitamin D level before you start supplementing? Current guidelines indicate that it is not always necessary. What matters most is age, health status, sun exposure and the presence of conditions that can disrupt its absorption or metabolism.
What does vitamin D do in the body?
Vitamin D is a fat-soluble compound that is primarily involved in regulating calcium and phosphate metabolism. An adequate supply is essential for proper bone mineralisation and muscle function.
Vitamin D also affects the immune system and takes part in regulating various cellular processes. That does not mean, however, that raising its level above values considered sufficient automatically improves immunity or protects against chronic disease.
The body obtains vitamin D mainly through skin synthesis driven by UVB radiation. Food is another source, especially oily sea fish, egg yolks and fortified foods.
In Poland, skin synthesis is insufficient in autumn and winter, which is why supplementation plays an important role in preventing deficiency [1].
Should everyone have their vitamin D level tested?
Vitamin D status is assessed by measuring 25-hydroxyvitamin D, or 25(OH)D, in the blood. It is the main metabolite used to diagnose deficiency.
It should not be confused with 1,25(OH)₂D, the active form of vitamin D, which is measured only in specific clinical situations.
According to the 2023 Polish guidelines, routine 25(OH)D testing in all healthy people is not recommended. The test should be done mainly in groups at increased risk of deficiency or when there are specific medical indications [1].
The Endocrine Society's 2024 guideline takes a similar position. The authors do not recommend routine 25(OH)D testing in healthy adults, because there is not enough evidence that population-wide screening improves meaningful health outcomes [2].
This means that a healthy adult who wants to take a standard preventive dose usually does not need a lab test first. For a broader look at which tests belong in a yearly check-up, see our article on preventive tests worth doing once a year.
When is it worth measuring 25(OH)D?
There are situations in which the test can matter for diagnosis and for deciding on the right course of action.
This applies in particular to people with:
- Conditions affecting nutrient absorption, such as coeliac disease, Crohn's disease or a history of bariatric surgery.
- Calcium and phosphate metabolism disorders, including hypocalcaemia, hyperparathyroidism or suspected osteomalacia.
- Osteoporosis, low-energy fractures or symptoms suggesting impaired bone mineralisation.
- Chronic kidney and liver disease, which can affect vitamin D metabolism.
- Certain medications, including some anti-epileptic drugs and glucocorticoids.
The Polish recommendations also list obesity as a factor that increases the risk of inadequate vitamin D status [1]. It is worth noting, though, that the Endocrine Society's 2024 guideline does not recommend routine testing on the basis of obesity alone in people with no other indications [2].
In these situations, the decision to test should come from a clinical assessment, not from automatically ordering an extensive panel.
How to interpret vitamin D results
In Poland, the interpretation ranges set out in the 2023 recommendations are the most widely used.
| 25(OH)D level | Interpretation according to Polish guidelines |
|---|---|
| ≤20 ng/ml | Deficiency |
| >20 to <30 ng/ml | Suboptimal level |
| 30–50 ng/ml | Level considered optimal |
| >50 to 100 ng/ml | High level |
| >100 ng/ml | Increased risk of toxicity |
Source: Płudowski et al., 2023 [1].
It helps to clarify that a "suboptimal level" does not automatically mean disease or symptomatic deficiency. It refers to values below the target range adopted in those guidelines.
Not all scientific organisations use the same thresholds, however. The Endocrine Society's 2024 guideline moved away from specifying a single universal target 25(OH)D level for disease prevention in the general population, because no value has been established whose attainment clearly improves health outcomes [2].
That is why a result should be interpreted in the context of a person's health, not only by comparing it with a single reference value.
When is preventive supplementation enough?
In Poland's climate, vitamin D supplementation is often justified, especially in the months when skin synthesis is limited. For when to start, see our article on vitamin D in autumn.
According to the 2023 Polish recommendations, healthy adults aged 19–65 who do not get enough sun exposure are advised to take 1000–2000 IU of vitamin D a day. The dose should take into account body weight and dietary vitamin D intake, among other things [1].
Recommendations for older people are somewhat different, because the skin's ability to synthesise vitamin D declines with age.
| Age group | Polish preventive recommendations |
|---|---|
| Infants 0–6 months | 400 IU/day |
| Infants 6–12 months | 400–600 IU/day, depending on dietary intake |
| Children 1–3 years | 600 IU/day all year round |
| Children 4–10 years | 600–1000 IU/day when skin synthesis is insufficient |
| Adolescents 11–18 years | 1000–2000 IU/day when skin synthesis is insufficient |
| Adults 19–65 years | 1000–2000 IU/day when skin synthesis is insufficient |
| People aged 65–75 | 1000–2000 IU/day all year round |
| People over 75 | 2000–4000 IU/day all year round |
Source: 2023 Polish recommendations [1]. The values refer to prevention, not to the treatment of diagnosed deficiency. Choosing a dose requires taking the individual clinical situation into account.
During pregnancy and breastfeeding, the Polish guidelines prefer supplementation guided by the 25(OH)D result. If testing is not available, they indicate a dose of 2000 IU a day [1].
The Endocrine Society's 2024 guideline takes a more conservative approach to routinely using doses above the recommended intake. In healthy adults under 75, there was not enough evidence of benefit from such doses taken solely to prevent chronic disease [2].
This discrepancy does not mean that either organisation questions the importance of vitamin D. It stems mainly from differences in the scope and purpose of the recommendations.
Does a higher dose of vitamin D bring greater benefits?
Vitamin D is essential for health, but that does not mean the higher its level, the better.
The large randomised VITAL trial, published in The New England Journal of Medicine in 2019, assessed the effect of 2000 IU of vitamin D a day on the risk of cancer and cardiovascular disease. It included more than 25,000 participants.
Supplementation did not significantly reduce the risk of invasive cancer or major cardiovascular events compared with placebo [3].
Likewise, an analysis published in 2022, covering more than 25,000 VITAL participants, found no significant reduction in fracture risk with vitamin D compared with placebo. These results apply to the general population, however, not to treating people with specific bone diseases or severe deficiency [4].
The latest meta-analysis from 2025, on preventing acute respiratory infections, also found no statistically significant overall protective effect of vitamin D supplementation [5]. For more on what is known about immunity products, see our article on immune supplements.
The goal of supplementation should therefore be an adequate supply and the prevention of deficiency, not reaching the highest possible levels.
Can you overdose on vitamin D?
Yes. Vitamin D is fat-soluble, and excessive intake can lead to toxicity.
The main mechanism of harm is hypercalcaemia, an excessive level of calcium in the blood. It can cause nausea, weakness, increased thirst, more frequent urination and, in more severe cases, kidney damage.
In its 2023 assessment, the European Food Safety Authority (EFSA) set the tolerable upper intake level of total daily vitamin D intake for adults at 100 µg, or 4000 IU [6].
This is not a dose recommended for everyone, but the upper level of intake considered safe for most healthy adults.
In practice, watch out for combining several products that contain vitamin D, for example a single-ingredient supplement, a multivitamin and a calcium preparation. Therapeutic doses above standard prevention should be taken as advised by a doctor.
How to approach supplementation in practice
For a healthy adult with no additional risk factors, preventive supplementation matched to age, sun exposure and diet is usually enough.
If there are chronic conditions, absorption disorders, bone mineralisation problems or other medical indications, measuring 25(OH)D can help determine the right course of action.
It is also worth remembering that a low vitamin D result does not automatically explain every complaint. Fatigue, weakness or feeling unwell can have many causes, so diagnostics should be tailored to the clinical picture.
Frequently asked questions
Do I need a blood test before starting vitamin D supplements?
Not always. Most healthy people do not need routine 25(OH)D testing before starting standard preventive supplementation. Testing is more justified in specific conditions, with certain symptoms or when there are risk factors for deficiency.
Do I need to take vitamin D all year round?
It depends on age, sun exposure and health. Adults with insufficient skin synthesis may need supplements all year round. The Polish guidelines also recommend year-round supplementation for, among others, children aged 1–3 and people over 65.
Does a 25(OH)D result of 25 ng/ml mean deficiency?
According to the 2023 Polish recommendations, it is a suboptimal level, not deficiency, which is defined as ≤20 ng/ml. Bear in mind, though, that international organisations differ in how they assess optimal vitamin D thresholds.
Does vitamin D improve immunity?
Vitamin D helps regulate the immune response, but that does not mean extra supplementation in people with adequate status automatically lowers the risk of infection. The most recent meta-analysis does not confirm clear preventive benefits in the general population [5].
Do people with obesity need a higher dose of vitamin D?
Obesity can affect how vitamin D is distributed in the body and how it responds to supplementation. Polish guidelines indicate that some people with obesity may need higher doses. That does not mean increasing the dose on your own. The dose should be set individually.
Should vitamin D be taken with a meal?
Vitamin D is fat-soluble, so taking it with a meal that contains fat can support absorption. The form of the product and taking it regularly also matter.
Does vitamin D supplementation need to be monitored with tests?
For healthy people on standard preventive doses, repeating 25(OH)D tests regularly is usually unnecessary. Monitoring may be indicated when treating confirmed deficiency, when taking higher doses or with conditions that affect vitamin D metabolism.
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References
- Płudowski P, Kos-Kudła B, Walczak M, et al. Guidelines for preventing and treating vitamin D deficiency: a 2023 update in Poland. Nutrients. 2023;15(3):695. doi:10.3390/nu15030695.
- 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. doi:10.1210/clinem/dgae290.
- Manson JE, Cook NR, Lee IM, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med. 2019;380(1):33–44. doi:10.1056/NEJMoa1809944.
- LeBoff MS, Chou SH, Ratliff KA, et al. Supplemental vitamin D and incident fractures in midlife and older adults. N Engl J Med. 2022;387(4):299–309. doi:10.1056/NEJMoa2202106.
- Jolliffe DA, Camargo CA Jr, Sluyter JD, et al. Vitamin D supplementation to prevent acute respiratory infections: systematic review and meta-analysis of stratified aggregate data. Lancet Diabetes Endocrinol. 2025;13(4):307–320. doi:10.1016/S2213-8587(24)00348-6.
- EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA), Turck D, Bohn T, et al. Scientific opinion on the tolerable upper intake level for vitamin D, including the derivation of a conversion factor for calcidiol monohydrate. EFSA J. 2023;21(8):e08145. doi:10.2903/j.efsa.2023.8145.
- US Preventive Services Task Force, Krist AH, Davidson KW, et al. Screening for vitamin D deficiency in adults: US Preventive Services Task Force recommendation statement. JAMA. 2021;325(14):1436–1442. doi:10.1001/jama.2021.3069.