Researchers recently discovered a major facet of vitamin D. It is its modulating action on the immune system. However, its contribution to health is not new. The cold season was once associated with the spoonful of cod liver oil. A very tasty remedy! It was long before the use of antibiotics that this vitamin D-rich preventive potion against winter infections was circulating in our families, but also in the sanatoriums of the time to treat tuberculosis and drug use.

Vitamin D deficiency

Over the past decades, several studies have shown that vitamin D deficiency is a risk factor for various diseases, including osteoporosis and seasonal depression. Since then, epidemiological studies have shown strong associations between seasonal variations in vitamin D levels and the incidence of various infectious diseases, including respiratory infections and influenza. Indeed, in people with documented vitamin D deficiency, the immune response is believed to be weakened or abnormal, making them automatically more likely to fall victim to seasonal outbreaks and to develop an increased degree of severe respiratory symptoms.

Quick overview of the immune system

The main role of the immune system is to maintain the integrity of the body. It works to eliminate substances perceived as foreign, also called “antigens” (microorganisms, abnormal cells, debris) while ensuring the tolerance of the different structures that belong to us.

The immune system is made up of two types of defenses: innate and adaptive. Representing the first line of defense against antigens, innate immunity is made up of natural barriers (skin, mucous membranes and microbiota, tissue secretions such as saliva, sweat, sebum, hydrochloric acid from the stomach), inflammatory mechanisms (fevers , cytokines), production of antimicrobial substances and cleaning functions (phagocytosis). This response remains the same regardless of what type of antigen is left outside the body’s cells. Whereas adaptive immunity acts in the background. It is when the antigen enters cells that adaptive immunity will give a specific response to that antigen by creating antibodies (special proteins that neutralize antigens). Once the antibody is created, it stays in our body’s memory. This is called immunization. This is the basic principle aimed at by vaccinations.

Roles and characteristics of vitamin D

Vitamin D can be inactive or active (also called calcitriol). Recently, we discovered that the kidneys are not the only activating organs, but that immune cells are also important players. Directly at the site of infection, these cells are said to enhance their actions by changing vitamin D precursors to their biologically active form. When activated, calcitriol binds to its receptor (also found in these same immune cells) and triggers a cascade of events stemming from both innate and adaptive immunity, including increased antimicrobial secretions, the production of antiviral proteins, the activation of eliminator cells, the creation and release of antibodies, the modulation of the inflammatory response (neither too much nor less) and the repair of damaged tissues. These actions show us the importance of vitamin D for providing energetic immune defense.

Here are some useful functions related to vitamin D:

  • Reduction of pulmonary permeability to invaders;
  • Downward modulation of harmful inflammatory factors could cause narrowing of the bronchioles;
  • Muscle strengthening (including the intercostal muscles used during respiratory infections);
  • Improvement of the maximum volume of exhaled air (lung capacity);
  • Induction of the death of infected cells (apoptosis);
  • Synthesis of free radicals aimed towards destabilizing the intruder.

In the presence of a virus

During a viral infection, it is not the pathogen that determines the clinical severity or risk of mortality associated with diseases (e.g., influenza, Covid-19), but the exuberance of the immune response of the organism. SARS-CoV-2 produces a strong dysfunction of the immune system which creates an intense inflammatory response and the development of a potentially fatal condition; CRS (cytokine release syndrome). Deregulation of protective responses of the immune system as well as induction of ineffective activities in different types of immune cells can lead to acute respiratory distress syndrome (ARDS), mainly in the elderly.

An optimal level of vitamin D allows a marked improvement of the defense mechanisms, the reduction of the severity of the infection, the reduction of the convalescence and the promotion of the quality of life. These benefits then lead several researchers to consider the use of vitamin D as a strategic adjuvant to other therapies currently used. During the first wave of COVID-19, an inverse correlation between blood levels of vitamin D and the severity of the disease was found to be significant in more than 20 countries.

Epidemiological data shows that, compared to younger or healthier people, older or chronically ill people have a higher risk of death after being infected with SARS-CoV-2. In fact, these people present with a chronic weakening of the immune response induced by qualitative/quantitative dysfunctions of the various stakeholders resulting from both innate immunity and adaptive immunity. This results in an imbalance in favor of pro-inflammatory cytokines (IL-1α, IL-2, IL-6, IL-8, IL-12, IFN-γ) compared to anti-inflammatory drugs (IL-1 Ra, IL-4, IL-10, TGF-β), which constantly weakens their immune condition. This phenomenon is amplified on contact with SARS-CoV2 and can lead to a cytokine storm. The latter takes place both in the blood and in the lungs of infected people. The cytokine storm is the most severe form of the cytokine release syndrome mentioned above. The stronger the storm, the more alarming the disease as well as the lung damage.

Therefore, the regulation of the exaggerated inflammatory response observed during SARS-CoV-2 infection represents a key element in the strategy to counter the virus and prevent its potentially fatal effects. However, a vitamin D deficiency is associated with higher levels of pro-inflammatory cytokines, suggesting that an adequate status of this vitamin could help to limit their synthesis and therefore lessen the seriousness of the viral infection. Vitamin D also has a repairing action on the pulmonary alveolar tissues (with the contribution of vitamin A). Since about 20% of people with COVID-19 develop interstitial pneumonia with severe lesions, adding vitamin D to the conventional therapeutic arsenal appears to be beneficial to date.

The last few years have brought about a radical change in our perspective on how vitamin D influences health. The impact of vitamin D deficiency on the immune system (innate and adaptive) has become clearer. In such a situation, there appears to be an increased susceptibility to infections and a predisposition to toxic inflammatory outbreak. Several studies support the hypothesis that vitamin D sufficiency contributes to immunoregulatory functions and a balanced immune response, particularly in the context of respiratory infection of viral origin. In these times of pandemic, the state of current knowledge about vitamin D leads us to believe that its blood status is one of the factors that influence our fighting spirit against the disease.




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