And what do you do to maintain an optimal level of vitamin D? In Denmark, several studies have shown that approx. 50% of the population has mild and approx. 15% moderate vitamin D deficiency. Here you can read what you can do to ensure an optimal vitamin D level and why vitamin D is important for your immune system.
Many people experience being tired, lacking energy and in a bad mood, especially in winter. It can be a sign that the level of vitamin D3 is low. By having a blood test taken by your doctor, you can quickly find out whether you lack Vitamin D3. The way to check if the vitamin D level is low is by measuring (blood) plasma 25-OH-vitamin D - i.e. the concentration of vitamin D in plasma, measured in nanomoles per liter (nmol/l).
In Denmark, the following divisions are used for deficiency states of vitamin D:
- Vitamin D insufficiency (slight deficiency): 25-50 nmol/l (insufficient level)
- Vitamin D deficiency: 12-25 nmol/l
- Severe vitamin D deficiency: <12 nmol/l
According to head physician Jens-Erik Beck Jensen, Hvidovre Hospital, 75 nmol/l is an optimal level to provide a broad and beneficial effect on general health.
What effect does vitamin D3 have?
Vitamin D3 is one of the most important vitamins that maintains our immune system. In fact, vitamin D is absolutely essential for your immune system to work. It is because the immune system consists of a lot of different immune cells that protect us against diseases. They can be divided into two different types of immune cells: Macrophages and T cells. 
The amount of vitamin D in the blood is absolutely crucial for whether your immune system can fight bacteria, viruses and other infections.
Macrophages act like little automatic robot vacuum cleaners, constantly eating all the foreign bodies and cellular debris they come across. They are part of the innate immune system. The T cells, on the other hand, are highly specialized fighting and supply cells, which are first activated by hardy bacteria or viruses, such as e.g. pneumonia or influenza. The T cells are part of the adaptive and learning immune system. They are in the bloodstream, but cannot detect danger until a macrophage shows the T cell that it has found something suspicious.
Then T cells divide and build an "army" of specialized T cells that are trained to track down and eradicate anything in the body that contains the bacteria or viruses that the macrophage displays. The information is then stored in the immunological memory, so that the immune system is prepared for the next encounter with the bacteria or virus, after which the T cells in question die.
And precisely in the T-cell defense, vitamin D is absolutely essential. The T cells cannot be activated at all without vitamin D. The first antenna that the T cell opens in its wake-up blast is the vitamin D receptor. That is when T cells begin to be activated, it must have vitamin D, otherwise the awakening process stops. If there is insufficient vitamin D in the blood, the T cells are not activated. If, on the other hand, there is sufficient vitamin D in the blood, the inactive T cell is transformed into either a killing machine that fights diseased and infected cells, or an activation machine that activates other immune cells.
The amount of vitamin D in the blood is therefore absolutely crucial for whether your immune system can fight bacteria, viruses and other infections.
In addition to ensuring the function of the immune system, vitamin D3 also ensures that an appropriate calcium-phosphorus balance is maintained in the blood and at the same time promotes the formation of bone tissue by helping the body absorb calcium. It is, among other things, therefore legally required that calcium supplements must contain Vitamin D3.
Vitamin D3 therefore contributes to maintaining:
- immune system
- cell division
- teeth and bones
- normal calcium level in the blood
- the uptake of calcium and phosphorus
How do I get enough vitamin D and what is the best source?
Vitamin D3 is also known as the sunshine vitamin, as vitamin D3 is formed in the skin when the sun hits the skin. In addition, vitamin D3 is also found in fatty fish such as salmon, herring and mackerel, and can of course also be taken as a vitamin D3 supplement. Vitamin D3 is available both vegan (plant-based) and extracted from sheep's wool (lanolin). But which method is best?
In 2016, DR's Sundhedsmagasin did a small experiment to find out which source of vitamin D3 was the best. In the experiment, DR tested 37 participants from the travel agency Spies. Here they measured the participants' plasma 25-OH-vitamin D level before the start of the experiment. The blood tests showed that 16 of the 37 employees lacked vitamin D3, and thus had a level below 50 nmol/l. Then the 16 participants who lacked vitamin D3 were divided into 3 groups:
- group had to eat 100 g of fatty fish 3 times a week (6 participants)
- group had to take 25 mcg daily (5 participants)
- group was going on a sunny holiday (5 participants) - the participant got approx. 30 t of sun per week.
The trial ran over 14 days.
The result of the experiment
The result of the trial showed that after 14 days, 11 out of the 16 participants had reached a level of vitamin D3 of 50 nmol/l or more, and were no longer in deficit. The two groups that did best and saw the greatest increase in vitamin D3 levels were in the supplement group and the sunshine group.
In fact, 2 out of the 16 reached a Vitamin D3 level of 75 nmol/l, which according to head doctor Jens-Erik Beck Jensen, Hvidovre Hospital, is an optimal level to provide a broad and beneficial effect on general health. The two participants had been on a 14-day sun holiday.
The best source of vitamin D3
Solar therefore immediately seems like the best solution. The problem, however, is that we in the Nordics do not get enough sun during the winter months, as there are very few hours of sunshine. If you therefore want to use the sun to get enough vitamin D3, you must make sure to build up a store in the summer. And here the next challenge arises. Because when you are in the sun, it is recommended to use sunscreen and be partially covered to protect the skin from the sun's harmful rays, which can lead to skin and mole cancer. But a sunscreen with sun factor 15+ blocks more than 99% of the formation of vitamin D3 in the skin.  And the skin cannot therefore form and build up the necessary storage.
A supplement of vitamin D3 is therefore the best and most stable way to cover your daily need for vitamin D3 all year round. This is also the conclusion in DR's Sundhedsmagasin about vitamin D deficiency. You can see the entire program here: https://youtu.be/ZMmce4TF7ZE
If you are in doubt about how to find the best supplement, you can read our article "Which vitamins are best? - Understand your label"
- Plasma (blood plasma): 55% of blood is plasma. Plasma is a yellowish liquid in which the blood cells are located. Among other things, the plasma's function is to transport the blood cells as well as nutrients and waste substances to and from the body's organs. Plasma consists of 90% water with a salt concentration similar to that of seawater. The rest are various proteins, salts, hormones, vitamins, nutrients, waste and sugar substances. 
- 25-OH-vitamin D: 25-hydroxy-vitamin D, also called 25OHD and Calcidiol.
 University of Copenhagen - news and research: https://nyheder.ku.dk/alle_nyheder/2010/2010.3/dvitamin/