Vitamins, in the general sense of the term, are obtained exclusively through food.
Due to its mode of action, vitamin D is actually a prohormone: our body can also synthesize it, in the skin. It circulates through the blood to activate or deactivate certain cellular mechanisms. Vitamin D acts especially on the immune system , at different levels 2 and 3 :
- Vitamin D allows the maintenance of the integrity of the intestinal mucosa , due to its action on the proteins that make up the junctions between intestinal cells. The intestine is the place of transit for pathogens, so their passage into the bloodstream can only occur through the intestinal wall. When the wall is altered, it presents a hyper-permeability that allows these pathogens to pass through. A healthy intestinal wall is essential to prevent its passage, as well as a diversified intestinal microbiota .
- Vitamin D has an action on the innate immune system : it stimulates macrophages and dendritic cells, which are the “front-line soldiers” of the immune system. In this way, our body reacts quickly in case of aggression.
- Vitamin D also stimulates the adaptive immune system : it increases the number of Th2 lymphocytes, producing an anti-inflammatory effect .
- Vitamin D increases pathogen clearance mechanisms , such as the synthesis of antimicrobial agents by immune cells.
- Finally, vitamin D has been studied extensively in the context of the prevention of acute lung infections . Vitamin D3 supplementation has been shown to reduce the risk of getting a respiratory infection at least once.
The different forms of vitamin D: vitamin D2 and vitamin D3
Vitamin D is in two forms:
- The vitamin D2 , present in plants.
- The vitamin D3 , present both in animals and plants.
The vitamin D that our skin makes is vitamin D3. Both vitamin D2 and vitamin D3 can be used for supplementation, with amounts expressed in international units.
The international units allow to establish a universal reference in the dosage of the active forms of vitamins and hormones. For information, one international unit = 0.025 micrograms of vitamin D3 and 1 ug of vitamin D = 40 IU .
The D3 form, better metabolized by the body
Once ingested, the whole of vitamin D3 binds to a transporter protein in the blood that carries it to the liver. Upon reaching the liver, it undergoes a change in its chemical structure and gives rise to a new form of vitamin D (25 (OH) D3). This form, called circulating, is determining when a blood test is performed to know the concentration of vitamin D .
From this circulating form, the active form of vitamin D is synthesized, according to the needs of the body. Indeed, in response to the signals that indicate a need, circulating vitamin D will undergo a new change in its chemistry, which will take place mainly in the kidneys (but also in other cells). This stage is what allows the formation of active vitamin D (calcitriol).
Vitamin D3 is better metabolized and leads to the active form of vitamin D more efficiently than vitamin D2 . 
The concentration of vitamin D
An adequate concentration of vitamin D is defined by a circulating level greater than or equal to 75 nmol / l, that is, 30 ng / ml . The determination is made in late winter or early spring.
Vitamin D deficiency, on the other hand, is defined by a concentration below 10-12 ng / ml, which is much rarer and mainly affects the very elderly who suffer from diseases such as kidney failure.
There are different parameters that can explain this general deficit:
- An exposure to insufficient sunlight during the winter; Indeed, in a healthy adult, exposing the hands, forearms and face for 10 to 15 minutes two or three times a week, from April to October, would be enough to cover the needs 1 . However, this synthesis is not always optimal in France, especially in winter, when sun exposure is insufficient .
- The air pollution and cloud cover , which filter UVB rays, used by skin for the synthesis of vitamin D3.
- A diet low in vitamin D2 and vitamin D3 (one of the major sources of vitamin D3 is found in fatty fish).
- It should be noted that the elderly and people with pigmented skin (pigmentation acts as an anti-UVB filter) have a lower capacity to synthesize vitamin D3.
Prevention: how much vitamin D3 should be provided with supplementation?
The French Academy of Medicine recommends a daily intake of 800 to 1000 IU . This data comes from a reminder of its recommendations published in May 2020 and related to the COVID-19 pandemic 5.
Concomitant magnesium supplementation may be recommended , as it contributes to the activation of vitamin D 6 . Finally, according to scientific publications, vitamin D is only toxic when the doses provided are excessive, that is, of the order of 50,000 IU / d, over a period of several months .
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