More and more often we hear about Vitamin D in the most varied areas of food supplementation. From osteoporosis to immunostimulation, this vitamin boasts several beneficial activities on the body. But what is Vitamin D? In reality it is part of a very large group of natural fat-soluble substances (which dissolve in a fatty environment) necessary to carry out numerous vital functions. The main natural source of vitamin D is the endogenous production of cholecalciferol (vit D3) in the skin.
It all starts with cholesterol, the very substance that we try to keep controlled with the diet. Through a series of chemical reactions, induced by Ultraviolet B (UVB) rays, cholesterol becomes Vitamin D. However, cholecalciferol can also be taken with the diet and food supplements. Among the foods that can be considered an excellent source of vitamin D we include above all fish, liver and egg yolk; unfortunately, vegetables contain very low quantities of Vitamin D, only some mushrooms contain moderate quantities.
Whenever we introduce Vitamin D with food or with a food supplement, but also the one that is produced by the skin exposed to UVB rays, the body must convert it into a biologically "active" form. This conversion occurs in both the liver and the kidneys.
We have said that vitamin D can be synthesized in adequate quantities by most mammals sufficiently exposed to sunlight, for this reason it should not be considered an essential dietary element or considered a vitamin in the strict sense. So what else is Vitamin D?
This very important substance can be considered as a pro-hormone, convertible into the hormone calcitriol, responsible for its effects by interacting with a specific receptor found in several cells of different tissues.
Cholecalciferol (vit D3) is converted into calcifediol (also called calcidiol), while ergocalciferol (vit D2) is transformed into 25-hydroxyergocalciferol. These two metabolites of vitamin D can be measured in the blood serum to measure a person's total vitamin D level.
As mentioned, the most significant transformations take place in the liver and kidneys to activate this vitamin and make it useful for the body. Calcifediol is transformed into calcitriol, which is the biologically active form of vitamin D.
Activated vitamin D is released into the blood circulating in all tissues and exercising different functions depending on the tissue or organ in which it interacts. In the intestine, for example, it is capable of increasing the absorption of calcium and phosphate, regulating their concentrations in the blood and promoting the physiological growth of bones, their remodeling and preventing degeneration in old age. Calcitriol also has other biological effects, on the brain it assists some neurotransmitters, on the tissues it has a role on cell growth, on the immune system it is capable of impressing cell replication and response.
What is vitamin D used for?
- it acts with a hormone-like mechanism of action, as:
- it is synthesized directly by the body
- It acts on one or more target organs
- It has a chemical structure very similar to that of steroid hormones.
- Vitamin D is essential for the homeostasis of calcium and phosphate, and is crucial for the growth and maintenance of bones.
Once activated, Vitamin D promotes the following functions:
- Absorption of calcium and phosphate in the intestine
- Deposition of calcium from bones (along with another hormone produced by the thyroid)
- Maintenance of the trophism and elasticity of the cartilages
- Renal reabsorption of calcium and phosphorus in the proximal convoluted tubule
- Activation of the immune system
Vitamin D and calcium
Vitamin D stimulates the synthesis of a particular protein that carries calcium inside the cells that line the intestine. This increases the ability to absorb calcium from food.
Vitamin D and the immune system
The immune system of mammals, including humans, is very articulated and has numerous cell populations that participate in the defense of the organism from attack by bacteria, fungi, viruses and cancer cells.
A particular type of cell, the T lymphocyte, when activated to fight a pathogen exposes on its surface a particular receptor able to bind vitamin D. If the T cells cannot find an adequate quantity of vitamin D in the blood, they cannot will activate effectively or will not activate at all.
Vitamin D and depression
The importance of vitamin D for the Central Nervous System is related to both neuro-immune modulation and neuroplasticity. These two elements suggest that this vitamin could also play an important role in psychiatric diseases such as depression.
This hypothesis that links vitamin D and depressive illness has recently been strengthened by the identification of the vitamin receptors in the areas of the brain involved in depression. It is known that vitamin D modulates the hypothalamus-pituitary-adrenal axis, that biochemical highway that regulates the production and prevents the exhaustion of neurotransmitters such as adrenaline, noradrenaline, dopamine and serotonin, as well as cortisol, a typical hormone of stress. Numerous studies have shown that vitamin D deficiency may be associated with an increased risk of depression and suicide; a 2014 study compared levels in depressed patients with a history of suicide attempt, depressed patients without suicidal tendencies, and healthy patients: in the former, vitamin D levels were significantly below the physiological range.