Vitamin D, or calciferol, belongs to the group of fat-soluble vitamins. There are two forms of vitamin D, vitamin D2 (ergocalciferol) synthesised by plants and vitamin D3 (cholecalciferol) present in animals (HAS, 2013). Vitamin D3 comes mainly from skin synthesis under the action of UVB rays. The amount synthesised depends in particular on the time of day, season, latitude, exposed skin surface, skin pigmentation and whether or not sunscreen is used (HAS, 2013). This is why the risk of deficiency is greater in winter, particularly at the latitudes of metropolitan France (Futura Sciences).

Vitamin D3 intake in the diet

Vitamin D can also be obtained from food, but in small quantities. Some foods do indeed naturally contain vitamin D. These are mainly fatty fish (cod liver, salmon, sardines, mackerel…), egg yolk and mushrooms (shiitake) (Futura Sciences, HAS 2013). In France, around 80% of the population does not have enough vitamin D (Futura Sciences). As the body’s ability to absorb or synthesise vitamin D decreases with age, the elderly are a particularly vulnerable population with regard to vitamin D deficiency (ANSES).

Vitamin D is essential for the proper functioning of our body (ANSES). It is known for its role in regulating calcium and phosphorus levels, as well as bone mineralization (SFMPL). However, vitamin D has a number of biological effects and is particularly involved in the differentiation and activity of immune system cells (ANSES).

A major role in the immune system

The first clue to the important role of vitamin D on the immune system comes from the discovery that the vitamin D receptor is present on almost every cell in the immune system! (Martens et al. 2020). In general, vitamin D3 activates the immune system. The presence of vitamin D3 will promote the expression of antimicrobial molecules by epithelial cells and macrophages in the presence of an infectious agent. Macrophages play a key role in initiating immune responses against a broad spectrum of pathogens. This role consists of phagocyting, i.e. enveloping and destroying pathogens (viruses or bacteria) or cellular debris and then eliminating them. (SFMPL)

In recent decades, a growing number of scientific studies have revealed the association between vitamin D deficiency and an increased risk of various infectious diseases. Vitamin D3 deficiency is associated with an increased risk of viral infections, including HIV and influenza. Low vitamin D3 levels also appear to be a risk factor for tuberculosis (SFMPL). A large systematic review (covering 10,933 subjects) found that vitamin D supplementation (D2 and D3) would protect against acute respiratory tract infections in a vitamin D-deficient population, particularly in people receiving daily or weekly supplementation. (Martens et al. 2020, Martineau et al. 2017).

HAS, Utilité clinique du dosage de la vitamine D, Janvier 2013 (Société Française de Médecine et de Physiologie de la Longévité)

Martens et al. Vitamin D’s Effect on Immune Function, Nutrients 2020, 12(5), 1248

Martineau et al. Vitamin D supplementation to prevent acute respiratory tract infections: Systematic review and meta-analysis of individual participant data. BMJ 2017, 356, i6583.

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