What is vitamin D?
A pro-hormone
The vitamin D, also called “calciferol”, is a pro-hormone. Thus, it is the precursor of the hormone calcitriol which increases calcium absorption.
That is why it is mainly known for its role in bone growth.
This vitamin is produced naturally by the body. It is synthesized by the skin under the action of UVB rays.
In addition to sun exposure, it is also provided through diet and/or supplements. There are two types of vitamin D:
- D2, or ergocalciferol, from plant sources.
- D3, or cholecalciferol, which is found in animal products.
Among foods rich in vitamin D are cod liver oil, fatty fish, eggs, dairy products and mushrooms.
A major role in immunity
Beyond its importance for bone health, it is also essential to the immune system. Numerous studies have thus confirmed the link between vitamin D and immunity.
Accordingly, EFSA (European Food Safety Authority) authorizes the health claim: “Vitamin D contributes to the normal functioning of the immune system.”
This fat-soluble vitamin circulates in the blood. Its receptors are present in almost all cells of the body. They can regulate certain processes, activating or deactivating them.
They notably modulate the expression of HLA genes involved in immunity. Thus, vitamin D contributes to a healthy and functional immune response.
In addition, it boosts the body’s defensive capacity against assaults and pathogens.
That’s why a vitamin D deficiency leads to an increased likelihood of contracting illnesses and infections. Supplementation may therefore be necessary and beneficial in winter.
Also read | How to choose the best vitamin D?
Its benefits for the immune system
It activates immune defenses
Vitamin D stimulates the body’s immune defenses and, in particular, T lymphocytes (white blood cells). As a result, these killer cells are better able to fight bacteria, viruses, and other infections.
This study, for example, confirms its relevance against acute respiratory tract infections : rhinopharyngitis, bronchitis, pneumonia…
In cases of vitamin D deficiency, these white blood cells enter a dormant phase. Vitamin D helps mobilize them and reactivate the immune response.
It also activates the expression of the enzyme PLC-gamma 1 (phospholipase C gamma 1). It helps the cell mount an appropriate response based on the pathogen.
Finally, it stimulates dendritic cells. These cells detect infectious agents, capture the antigen, and alert T lymphocytes.
As shown by this study, vitamin D regulates the maturation of dendritic cells, which corresponds to the moment when the cell detects the pathogen and becomes activated.
It supports both adaptive and innate immunity
As shown by this study, vitamin D supports the adaptive immune system and the innate (or acquired) immune system.
The innate immune system reacts immediately when under attack. It defends the body quickly by mobilizing cells and activating various mechanisms. It is the first line of defense.
The adaptive immune system, for its part, provides a later but long-lasting response. It is made up of T and B lymphocytes. It targets the infectious agent and retains memory of the antigen. It ensures an increasingly effective and targeted immune response.
On the one hand, vitamin D helps frontline defenses such as macrophages. It also increases the number of neutrophils, other white blood cells that support macrophages.
On the other hand, it supports T cells of the adaptive immune system, as shown by this study. In addition, it increases the number of Th2 lymphocytes that fight parasites.
Finally, according to this study, it even prevents a possible overreaction of the immune system in the event of a COVID-19 infection.
It supports the elimination of pathogens
In addition to acting early and stimulating the immune system, it also supports mechanisms for eliminating pathogens.
For this, and as shown by this study, it promotes the synthesis of antimicrobial peptides by immune cells. These agents act during the innate response.
They include cathelicidin and defensins. They can pierce and alter the membranes of viruses to better destroy and eliminate them.
