Vitamin D – Production, Functions, and Reference Values
Who hasn’t heard of Vitamin D? At the latest since the Covid pandemic, everyone should have come across it. Several studies have shown that better vitamin D levels are associated with a lower risk of severe illness.1 But what exactly is Vitamin D? How is it produced, and what functions does it serve? How and when should it be tested, and what are considered “good” values? In this blog post, we answer all these questions.
What is Vitamin D?
First and foremost: Vitamin D is not actually a vitamin but a steroid hormone. This is because it can be produced by the body itself, unlike vitamins. It binds to the vitamin D receptor (VDR) and regulates a wide range of genes.2 Vitamin D is fat-soluble and is therefore stored in our fat tissue, as well as in the liver and muscles. This is important because these stores are depleted when there is insufficient sunlight. For this reason, a vitamin D test should ideally be conducted in September in our latitudes. This allows for an optimal determination of how many units should be supplemented over the winter months if desired (more on this later). Since fat-soluble vitamins and hormones are stored in the body, they can also be over-supplemented. In the case of vitamin D, this can lead to kidney stones or heart rhythm disturbances.
1 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10215038
2 https://www.usz.ch/krankheit/vitamin-d-mangel
How is Vitamin D produced?
The synthesis begins with 7-dehydrocholesterol (once again: cholesterol is not bad but one of the most important molecules in our body). Through UV radiation with a wavelength of 290-315 nm, it is broken down into pre-vitamin D in the keratinocytes of the skin and then isomerized into cholecalciferol.3 In the liver, it is converted into 25-hydroxycholecalciferol (25-OH vitamin D). This form serves as a storage form and is also the one most commonly measured in laboratory tests (more on this later). To become the active form, it must pass through the kidneys, where 1,25-dihydroxy-vitamin D is produced.4
The vitamin D content from food accounts for less than 20% of the total amount and is not sufficient to reach minimum levels (unless one were to eat a large fish meal every day). In food, it is primarily found in fish (the fattier, the better), as well as in veal or lamb. Eggs and mushrooms also provide a small amount, provided the mushrooms grew in sunlight (unlike cultivated mushrooms, which are often grown in the dark). Cod liver oil or fish roe are considered “natural” dietary supplements.
3 https://www.springermedizin.de/emedpedia/detail/dgim-innere-medizin/vitamin-d-stoffwechsel-stoerungen?epediaDoi=10.1007%2F978-3-642-54676-1_32#
4 https://www.osd-ev.org/files/3115/4679/5087/Vitamin_D.pdf
What functions does Vitamin D serve?
Vitamin D is essential for bone and tooth formation, as it supports the absorption of calcium and phosphate from the intestine. A deficiency can lead to rickets in children, a condition that results in improper bone growth. Fortunately, this is rare today, as vitamin D is recommended and provided to young children. However, in the 19th century, rickets was not uncommon, and Unterägeri was well known as a treatment center for rachitic children. In older adults, a deficiency can also lead to osteomalacia, a condition where bones become soft. This significantly increases the risk of fractures in case of falls.
Furthermore, Vitamin D enhances muscle growth and muscular coordination. Another study showed that Vitamin D concentration also increases bioactive free testosterone levels. Last but not least, higher levels improve recovery.5 Vitamin D also contributes to a healthy metabolism. It is believed to play a significant role in insulin resistance and related issues.6
5 https://www.osd-ev.org/files/3115/4679/5087/Vitamin_D.pdf 6https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9293580/#:~:text=Vitamin%20D%20seems%20to%20have,%2C%20oxidative%20stress%2C%20and%20apoptosis
Vitamin D supply in our latitudes
Vitamin D deficiency is very common, especially in our latitudes and among people with darker skin or hair. However, not only genetic factors but also the lifestyle of an industrialized work environment take their toll on vitamin D levels. Men and women are spending less and less time outdoors, and when they do, they often apply sunscreen from the very first minute. Studies have shown that with a sun protection factor (SPF) of 15 or higher, almost no vitamin D can be synthesized. To optimize production, it is recommended to sunbathe for a few minutes without sunscreen first (depending on skin type and season, between 10 and 40 minutes). It is also important to know that the higher the sun is in the sky, and the steeper the angle of sunlight, the more UV-B light reaches the skin and the more vitamin D can be produced. Therefore, exposure is most effective around midday and in regions closer to the equator. For a different perspective on the relationship between sunlight, sunscreen, and vitamin D, the article Sunlight and Vitamin D by Stephanie Seneff is worth reading.
The half-life of vitamin D is only 3 to 6 weeks. This means that after 3 to 6 weeks, only half of the original amount remains, and after another 3 to 6 weeks, only half of that (a quarter) is left. This explains why vitamin D stores can be depleted quickly. A representative study from Germany (2016) showed that 60% of participants had a vitamin D level below 20 ng/ml (reference values further below). In Switzerland, the Federal Nutrition Commission also considers vitamin D deficiency to be widespread.
Measurement and Reference Values of Vitamin D
Vitamin D can be easily measured through blood analysis. At Personalworkout, we have a specially designed device for this purpose. Of course, vitamin D levels can also be tested in any laboratory. Typically, the storage form 25-OH vitamin D is measured, as it is about half the price of the active form 1,25-dihydroxy-vitamin D.
In recent years, the recommended intake for vitamin D has been increased three to fourfold.7 Not least because of this, there are varying recommendations for reference values. On one hand, a distinction must be made between deficiency levels and optimal values, as defined in functional medicine. On the other hand, it is important to note that there are two different measurement units: nmol/l and ng/ml. In Switzerland, nmol/l is most commonly used.
| Vitamin D Status | Vitamin D Status 25-Hydroxy-Vitamin D in Blood (nmol/l or ng/ml) | Clinical Significance / Effect |
| Severe Deficiency | < 25nmol/l resp. < 10ng/ml | Risk of bone mineralization disorders, rickets, osteomalacia |
| Insufficient Supply | 25-49nmol/l resp. 10-19ng/ml | Risk of increased bone resorption/metabolism and/or elevated parathyroid hormone levels |
| Vitamin D Deficiency | <50 nmol/l resp. < 20 ng/ml | Includes both vitamin D insufficiency and severe deficiency |
| Sufficient Vitamin D Supply (Minimum Concentration) | 50 nmol/l resp. 20 ng/ml | Low risk of bone resorption and elevated parathyroid hormone levels; neutral effect on falls and fractures |
| Target Value for Fall and Fracture Reduction | 75 nmol/l resp. 30 ng/ml | Suppression of parathyroid hormone increase and bone resorption, reduction of falls and fractures |
To increase vitamin D levels by 1 ng/ml or 2.5 nmol/l, a dose of 10,000 IU is required for a 70 kg person (this can be adjusted to your weight using a simple ratio calculation).8 However, this value should be taken with caution, as daily vitamin D consumption is highly individual. If you want to determine more precisely how much vitamin D is needed to maintain or replenish your stores, regular testing is essential. The following table shows the recommended safe daily dose based on age for people living in Switzerland.
| Population Group | Safe Upper Intake (Daily Dose) |
| 0-6 months (up to the end of the 6th month) | 1000 IE (25 μg) |
| 6-12 months (7th month up to and including the 12th month) | 1500 IE (37.5 μg) |
| 1-4 years (up to the 4th birthday) | 2500 IE (62.5 μg) |
| 4-9 years (up to the 9th birthday) | 3000 IE (75 μg) |
| From 18 years, including pregnant and breastfeeding women | 4000 IE (100 μg) |
7 https://www.sge-ssn.ch/media/tabula-3-14-d-report.pdf
8 https://www.zentrum-der-gesundheit.de/ernaehrung/vitamine/vitamin-d-uebersicht/vitamin-d-die-richtige-einnahme
Conclusion on Vitamin D
What is clear is that vitamin D is a very important hormone with many functions. However, some aspects remain uncertain. For example, some scientists question the correlation between 25-OH vitamin D and 1,25-OH vitamin D. Due to inflammation, it is possible that 25-OH vitamin D levels are low while 1,25-OH vitamin D levels are high, as it is needed to fight inflammation. Therefore, testing both values might make sense. For instance, many people living near the equator may have low 25-OH vitamin D levels without experiencing a deficiency, as they simply have high amounts of active 1,25-OH vitamin D.9
Many people are now aware that vitamin D should be supplemented together with vitamin K2. Vitamin K2 ensures that excess calcium is directed into the bones rather than being deposited in the blood vessel walls. We describe the major risk factors for arteriosclerosis here. However, few people know that vitamin D levels cannot increase without magnesium. Without magnesium, 25-OH vitamin D cannot be converted into its active form 1,25-OH vitamin D or be properly transported.10 Last but not least, magnesium is essential for parathyroid hormone (PTH), which in turn influences vitamin D metabolism. To learn more about the importance of magnesium and its different forms, check out the article “Magnesium – an Essential Mineral.”
However, vitamin A metabolism also influences vitamin D levels (and vice versa). If there is too little vitamin A in the body, vitamin D levels cannot increase to the same extent.11
Wie immer ist der Körper komplexer als vielfach angenommen und wir sind noch weit davon entfernt alle Zusammenhänge zu verstehen. Aus diesem Grund plädieren wir von Personalworkout mehrheitlich für Supplemente aus natürlichen Quellen, welche eine Kombination verschiedener Stoffe beinhaltet. Für die Ergänzung von Vitamin D sind Fischeier eine gute Möglichkeit. Sie beinhalten neben Mineralstoffen wie Zink und Jod auch die Vitamine A, D, E und K in einem natürlichen Verhältnis (plus wertvolles Omega-3) und liefern ca. 3200 IE «per serving» von 6 Kapseln.
As always, the human body is more complex than often assumed, and we are still far from fully understanding all the interconnections. For this reason, at Personalworkout, we primarily advocate for supplements from natural sources that contain a combination of various nutrients. For vitamin D supplementation, fish roe is a great option. In addition to minerals like zinc and iodine, it naturally provides vitamins A, D, E, and K in a balanced ratio, along with valuable omega-3 fatty acids. A serving of six capsules delivers approximately 3,200 IU of vitamin D.
Interested in your vitamin D level? As of now, we offer the vitamin D test for 35.- CHF at our facility. You’ll receive your result in just 15 minutes.
9 Morley M. Robbins – Cure: your fatigue, Chapter 9
10 https://www.zentrum-der-gesundheit.de/ernaehrung/vitamine/vitamin-d-uebersicht/vitamin-d-magnesium
11 https://www.zentrum-der-gesundheit.de/ernaehrung/vitamine/vitamin-d-uebersicht/vitamin-d-vitamin-a
SOURCES
- Nicolai Loboda – Vortrag bei Personalworkout
- Bundesamt für Lebensmittelsicherheit und Veterinärwesen BLV, Fachinformation zu Vitamin D., abgerufen am 18.4.24
- https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.blv.admin.ch/dam/blv/de/dokumente/lebensmittel-und-ernaehrung/ernaehrung/fachinformation-vitamin-d.pdf.download.pdf/fachinformation-vitamin-d.pdf&ved=2ahUKEwj-xLarj8uFAxWDhv0HHQBVAowQFnoECCkQAQ&usg=AOvVaw3T4bLBh0CV1NR-j-OB8OaE
- https://www.osd-ev.org/files/3115/4679/5087/Vitamin_D.pdf
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9293580/#:~:text=Vitamin%20D%20seems%20to%20have,%2C%20oxidative%20stress%2C%20and%20apoptosis
- https://www.zentrum-der-gesundheit.de/ernaehrung/vitamine/vitamin-d-uebersicht/vitamin-d-magnesium
- https://www.zentrum-der-gesundheit.de/ernaehrung/vitamine/vitamin-d-uebersicht/vitamin-d-die-richtige-einnahme
- https://www.zentrum-der-gesundheit.de/ernaehrung/vitamine/vitamin-d-uebersicht/vitamin-d-vitamin-a
- https://www.springermedizin.de/emedpedia/detail/dgim-innere-medizin/vitamin-d-stoffwechsel-stoerungen?epediaDoi=10.1007%2F978-3-642-54676-1_32#
- https://www.usz.ch/krankheit/vitamin-d-mangel/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10215038/
- https://www.sge-ssn.ch/media/tabula-3-14-d-report.pdf
- Morley M. Robbins – Cure: your fatigue, chapter 9
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