Did you know that spending a short time in the sun can trigger a process in the body that affects everything from bone health to immune defence – and even mood? Vitamin D, which is formed naturally in the skin when the sun shines, is something we get far too little of during the darker months of the year.

Historical importance for health

The importance of vitamin D for human health has been known for more than a hundred years. As early as the beginning of the 20th century, the link between vitamin D deficiency and rickets was discovered – a disease that mainly affects children and leads to soft, deformed bones. When researchers began to understand the vitamin’s connection to sunlight and succeeded in identifying its two main forms – D₂ from plants and D₃ from animal sources, the latter being more biologically active – the foundation was laid for the knowledge we have today of its crucial role in the body (1, Jones G, 2022).

However, the history of vitamin D is about more than bone health alone. Long before the vitamin was identified, sunlight and cod liver oil were used to treat infections such as tuberculosis – at a time when antibiotics did not yet exist. Patients were sent to sanatoria to “breathe fresh air” and sunbathe, in the belief that the sun’s rays could kill bacteria. In reality, it was the body’s own production of vitamin D that helped strengthen the immune system (2, Aranow C, 2011).

The role of vitamin D in the body

Over recent decades, research into vitamin D has made major advances. Today we know that it affects far more functions than bone strength alone. The vitamin plays an important role in the immune system and has been linked to a reduced risk of several autoimmune diseases, such as type 1 diabetes, multiple sclerosis (MS), inflammatory bowel disease, rheumatism and lupus (3, Daryabor et al., 2023). Studies have also shown positive effects in asthma, type 2 diabetes, high blood pressure, depression, Alzheimer’s disease and various infectious diseases (4, Ghanaati S & Choukroun J, 2020).

To understand how vitamin D works, it is important to know its journey through the body. When the skin is exposed to the sun’s UVB rays, vitamin D₃ (cholecalciferol) is formed. This is then transported to the liver and converted into 25(OH)D (calcidiol) – an inactive form that serves as the body’s vitamin D store. In the kidneys, a final conversion then takes place into the active form 1,25(OH)₂D (calcitriol), which influences a wide range of biological processes, from cell growth to immune regulation (2, Aranow C, 2011).

Quick facts about vitamin D

• The amount of vitamin D in a supplement is stated in micrograms (µg) or international units (IU). The conversion is 1 µg = 40 IU.
• To determine how much vitamin D the body has stored, the level of 25(OH)D in the blood is measured. This analysis shows how well filled the vitamin D stores are.
• The blood test, which shows the serum level, is reported in nanomoles per litre (nmol/L) or nanograms per millilitre (ng/mL). 1 ng/mL corresponds to 2.5 nmol/L.

Evidence from research

The importance of these levels for health was demonstrated in a Swedish study conducted at Karolinska Hospital in Huddinge. Over the course of one year, 140 adults with recurrent respiratory infections were given either vitamin D (4,000 IU daily) or a placebo. Those who received vitamin D achieved an average serum level of 133 nmol/L and experienced fewer respiratory problems, as well as requiring less antibiotic treatment, compared with the control group, whose level was 67 nmol/L (5, Bergman, 2012).

According to Swedish and European guidelines, levels below 50 nmol/L are considered insufficient (6, 1177, 2021). During the latter part of the 20th century, the recommended daily intake of vitamin D to achieve sufficient levels was reduced from 4,000–5,000 IU to 400 IU, possibly as a result of incorrect statistical calculations (7, Papadimitriou D, 2017). At the same time, several studies indicate that higher levels may be associated with additional health benefits (8, Kimball et al., 2017), which is particularly relevant during the winter months, when vitamin D levels naturally decline by around 10–25 nmol/L (9, Giustina et al., 2024).

Safety and intake

Despite its positive effects, vitamin D can cause problems if taken in excessive doses. Very high levels of 25(OH)D can lead to hypercalcaemia – a condition in which the calcium level in the blood becomes too high, potentially causing symptoms such as nausea, muscle weakness and kidney stones. For this reason, a maximum daily intake of 4,000 IU (100 µg) is recommended for adults (10, Marcinowska-Suchowierska et al., 2018). At the same time, more recent research shows that the body can often tolerate significantly higher levels without adverse effects (11, EFSA, 2023).

In fact, an intake of up to 10,000 IU (250 µg) daily for five months has been shown to be safe for healthy men. Even higher doses, up to 40,000 IU (1,000 µg) per day, have been considered safe as long as the serum concentration does not exceed 500 nmol/L (12, Hossein-nezhad & Holick, 2013). In one case, an infant was accidentally given 12,000 IU daily for 20 days – without any signs of toxicity, despite very high blood levels (1,055 nmol/L) (13, Rajakumar et al., 2014).

Vitamin D dosage

The sun – the body’s best source

The most natural and effective way to meet the body’s need for vitamin D is still through sunlight. How much vitamin D is produced in the skin depends on factors such as the season, geographical location, skin type and how much skin is exposed. For people with fair skin, around 15 minutes in the sun is often enough to stimulate significant production. With more extensive sun exposure, the body can produce between 10,000 and 25,000 IU in a single day – entirely without any risk of overdose (14, NIH ODS, 2025).

Sources

1. 100 years of vitamin D Historical aspects of vitamin D, Jones G, 2022, https://pmc.ncbi.nlm.nih.gov/articles/PMC9066576/
2. Vitamin D and the immune system, Aranow C, 2011, https://pmc.ncbi.nlm.nih.gov/articles/PMC3166406/
3. A review of the critical role of vitamin D axis on the immune system, Daryabor G Gholijani N Kahmini F, 2023, https://www.sciencedirect.com/science/article/pii/S0014480023000175
4. One hundred years after Vitamin D discovery Is there clinical evidence for supplementation doses, Ghanaati S Choukroun J, 2020, https://www.researchgate.net/publication/340668162_One_hundred_years_after_Vitamin_D_discovery_Is_there_clinical_evidence_for_supplementation_doses
5. Vitamin D3 supplementation in patients with frequent respiratory tract infections: a randomised and double-blind intervention study. Bergman P et al., 2012, https://pubmed.ncbi.nlm.nih.gov/23242238/
6. Vitamin D-brist, 1177, 2021, https://vardpersonal.1177.se/kunskapsstod/kliniska-kunskapsstod/vitamin-d-brist/
7. The Big Vitamin D Mistake, Papadimitriou D, 2017, https://doi.org/10.3961/jpmph.16.111
8. Evaluation of vitamin D3 intakes up to 15,000 international units/day and serum 25-hydroxyvitamin D concentrations up to 300 nmol/L on calcium metabolism in a community setting, Kimball SM Mirhosseini N Holick MF, 2017, https://pmc.ncbi.nlm.nih.gov/articles/PMC5402701/
9. Consensus Statement on Vitamin D Status Assessment and Supplementation Whys Whens and Hows, Giustina A et al., 2024, https://academic.oup.com/edrv/article/45/5/625/7659127
10. Vitamin D Toxicity-A Clinical Perspective, Marcinowska-Suchowierska E, Kupisz-Urbańska M, Łukaszkiewicz J, Płudowski P, Jones G, 2018, https://pmc.ncbi.nlm.nih.gov/articles/PMC6158375/
11. Scientific opinion on the tolerable upper intake level for vitamin D, including the derivation of a conversion factor for calcidiol monohydrate, EFSA Panel on Nutrition, 2023, https://www.efsa.europa.eu/en/efsajournal/pub/8145
12. Vitamin D for Health A Global Perspective, Hossein-nezhad A Holick MF, 2013, https://pmc.ncbi.nlm.nih.gov/articles/PMC3761874/
13. Dosing error with over-the-counter vitamin D supplement: a risk for vitamin D toxicity in infants, Rajakumar K, Reis EC, Holick MF, 2014, https://pmc.ncbi.nlm.nih.gov/articles/PMC3396753/
14. Vitamin D Fact Sheet for Health Professionals, National Institutes of Health (NIH) Office of Dietary Supplements (ODS), 2025, https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/