Vitamin D , also called cholecalciferol , belongs to the group of fat-soluble vitamins (along with vitamin E and vitamin K) even though it actually works just like a hormone.
The main source of Vitamin D is the sun: following exposure to the sun , under the stimulus of UVB rays , Vitamin D is synthesized starting from 7-dehydrocholesterol , in fact it shares the cyclopentaneperhydrophenanthrenic form with cholesterol.
Vitamin D it is introduced with food only in a very small part and the best sources are egg yolk , fish , liver and butter . In the past , cod liver oil was drunk precisely because it had a high concentration of Vitamin D.
Both Vitamin D synthesized in the skin and that which we introduce with food are biologically inactive , therefore, to exercise its function, the vitamin must be transformed into the active form by specific enzymes belonging to the cytochrome family (CYP 2R1, CYP27A1 and CYP27B1). This process occurs first in the liver , then in the kidneys and leads to the formation of calcitriol (or 1,25-dihydroxycholecalciferol) which is the active form of Vitamin D.
The vitamin is degraded quite rapidly by light , oxygen and acids . It is quite heat stable .
Pharmacological functions and properties
Vitamin D is one of the most important regulators of phosphorus and calcium metabolism , minerals that play a fundamental role in bone health.
Acting at the intestinal, bone and renal level, Vitamin D has a hypercalcemic action and the purpose of its various actions is to maintain an adequate pool of calcium and phosphorus for correct bone mineralisation.
Although its function on bone is the best known, Vitamin D performs many other functions, no less important, as the Vitamin D receptor and its activating enzyme (CYP27B1) are present in many other cell types.
This has led the scientific community in recent decades to discover that vitamin D:
- Modulates the immune system: numerous animal studies have shown that Vitamin D is able to suppress the activation of Th1 cells and therefore consequently inhibit the production of pro-inflammatory cytokines, especially interferon-γ, interleukin-6, interleukin-2 and tumor necrosis factor-α. Furthermore, the vitamin is also able to suppress the activity of Th17, the main cells producing interleukin-17, involved in the pathogenesis of many autoimmune diseases. A stimulating action , however, is exerted on Treg cells which block inflammatory processes. Finally, in addition to Treg cells, the vitamin is also able to stimulate and modulate the activity of Th2 cells which increase the production of anti-inflammatory cytokines such as interleukin-10 and interleukin-4.
- It has antioxidant activity : Vitamin D is able to induce the expression of numerous enzymes (in particular superoxide dismutase and thioredoxin reductase) involved in the detoxification of reactive oxygen species.
- It has antimicrobial activity . Vitamin D is in fact an important regulator of innate immunity : it appears to improve chemotaxis, phagocytosis, the production of proteins with antimicrobial action and strengthen the physical barrier function of epithelial cells. It is therefore able to stimulate a whole series of actions that allow our body to fight infections.
- It is an important modulator of the intestinal microbiota : studies on mice have shown that Vitamin D deficiency generates a situation of intestinal dysbiosis , responsible for various metabolic disorders.
The scientific studies conducted so far would seem to suggest that Vitamin D deficiency could negatively influence the onset and course of numerous pathologies:
- Autoimmune diseases (rheumatoid arthritis, systemic lupus erythematosus, type 1 diabetes mellitus, inflammatory bowel diseases such as Crohn's disease and ulcerative colitis)
- Asthma and respiratory infections
- Intestinal diseases
- Cardiovascular diseases
- Neuro-psychiatric disorders (Parkinson's, Alzheimer's, depression, bipolar disorder).
Vitamin D , being a fat-soluble vitamin, after being synthesized in the skin or introduced as a supplement or with the diet, is accumulated in adipose tissue and muscle . When the accumulation becomes excessive it can cause intoxication. Excess vitamin D, a fairly rare condition in our country, causes hypercalcemia and widespread calcification in various organs.
Vitamin D deficiency, however, causes bone diseases called rickets (typical of children and adolescents) and osteomalacia and osteoporosis (typical of menopausal women). Other consequences of vitamin deficiency could be impaired immune system function, lower antioxidant defenses and a greater risk of developing cancer.
Vitamin D deficiency is now universal and affects almost the entire population , especially in countries above the 35th parallel such as Italy. To enjoy the sun safely and to produce the quantity of Vitamin D we need, it is necessary to follow the following rule: expose 25% of the skin (hands, arms and lower parts of the legs) for a period of time ranging from 25 to 50% of the time it is assumed it takes for the skin to redden.
Therefore, if you do not live in the Tropics and if you do not spend most of your days naked in the sun, it is almost impossible for your body to produce enough vitamin D for all its needs and it therefore becomes necessary to take it as a supplement.