Vitamin D, what it is and what functions it performs in the human body
Vitamin D is part of a group of molecules commonly called vitamins. Vitamins are micronutrients that we take in through food or that our bodies synthesize on their own
Their job is to speed up specific biochemical reactions that are critical for our cells.
In all, there are 13 of them, and each has specific functions and characteristics.
In particular, vitamin D has the characteristic of dissolving in organic solvents and fats, is said to be fat-soluble, and is essential for the proper functioning of our bodies.
Under the name vitamin D we identify 5 different types of molecules: vitamin D1, D2, D3, D4 and D5
The two most important forms in which we can find vitamin D are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol).
Ergocalciferol is taken with food, while cholecalciferol can either be taken with food or be synthesized through the action of UV rays from the sun.
Vitamin D accumulates in the liver and is released when it becomes necessary. There is no need, therefore, to take it regularly[1].
How do we take in vitamin D?
Ten to 20 percent of the daily requirement for vitamin D comes from food.
The foods in which more is found (besides those that are industrially fortified with it) are fatty fish (such as salmon, mackerel, and herring), egg yolk, and liver.
The rest of vitamin D is formed in the skin from a cholesterol-like fat (7-dehydrocholesterol) that is converted into cholecalciferol by exposure to a particular component of UV rays, UVB rays.
These rays are present more in the period from April to October and act on the first layer of the skin (epidermis).
In the summer months the increased exposure to the sun leads to a surplus of vitamin D that is stored for later use during the winter period[1, 2].
Cholecalciferol is transported through the bloodstream from the skin to the liver.
Here it undergoes its first change to calcifediol.
The latter is transported to the kidney where it is again modified into calcitriol.
Vitamin D thus modified is “active” and can perform its function by entering cells[1, 2].
What is the purpose of vitamin D?
Vitamin D is involved in processes that keep calcium levels in our bodies in balance.
Calcium is the most common mineral in the human body and is essential for the development and health of bones and teeth.
In addition, bones undergo a continuous remodeling process that involves the release and deposition of calcium in bone tissue.
Only 1% of this mineral participates in other functions:
- muscle contraction,
- nerve transmission,
- the secretion of hormones,
- vasodilation
- the contraction of blood vessels.
The functions of vitamin D are closely related to this mineral.
In case of low blood calcium concentration, vitamin D plays a key role in the processes of: renal calcium reabsorption, intestinal calcium absorption, and bone demineralization.
Should calcium be in short supply, this vitamin can stimulate the release of calcium from the kidney (where it accumulates) and/or increase its intestinal absorption during digestion.
As a last resort, it is involved in the processes of calcium release from bone[2, 3].
How does vitamin D act?
Vitamin D, like all vitamins, allows precise biochemical reactions to take place in the cell.
In particular, it acts as a hormone.
Hormones are different molecules from each other, but they all have the function of “transmitting signals” to cells by binding to structures on them, called receptors.
Each hormone binds to a specific receptor that may be present on the outer surface of the cell or inside it.
Vitamin D that is “activated” (calcitriol) by the various structural changes it undergoes enters the target cell and binds to its receptor (VDR).
Calcitriol binding to the VDR is the “signal” that the cell receives and, as a response, creates specific proteins[4].
Vitamin D allows the absorption of calcium from the gut
Only 1% of this mineral participates in other functions:
- muscle contraction,
- nerve transmission,
- the secretion of hormones,
- vasodilation
- the contraction of blood vessels.
The functions of vitamin D are closely related to this mineral.
In case of low blood calcium concentration, vitamin D plays a key role in the processes of: renal calcium reabsorption, intestinal calcium absorption, and bone demineralization.
Should calcium be in short supply, this vitamin can stimulate the release of calcium from the kidney (where it accumulates) and/or increase its intestinal absorption during digestion.
As a last resort, it is involved in the processes of calcium release from bone[2, 3].
How does vitamin D act?
Vitamin D, like all vitamins, allows precise biochemical reactions to take place in the cell.
In particular, it acts as a hormone.
Hormones are different molecules from each other, but they all have the function of “transmitting signals” to cells by binding to structures on them, called receptors.
Each hormone binds to a specific receptor that may be present on the outer surface of the cell or inside it.
Vitamin D that is “activated” (calcitriol) by the various structural changes it undergoes enters the target cell and binds to its receptor (VDR).
Calcitriol binding to the VDR is the “signal” that the cell receives and, as a response, creates specific proteins[4].
Vitamin D allows the absorption of calcium from the gut
A low blood calcium concentration is interpreted as an alarm signal by the parathyroid glands, glands attached to the thyroid gland.
Thus alerted, they produce parathyroid hormone (PTH), which stimulates the kidney to produce activated vitamin D (calcitriol).
Calcitriol travels from the kidney to the intestinal cells and enters them, resulting in the production of certain proteins, such as TRPV6 and calbindin.
The first is a protein that acts as a “tunnel” and allows calcium to pass from the intestine to the inside of the cell.
The second is found in the cell and carries calcium to the blood vessels. With the help of these proteins, calcium is absorbed to a greater extent from the intestine and ends up in the circulation[4].
Vitamin D allows the release of calcium from the kidney.
Increased absorption of calcium from the gut may not be enough to restore blood calcium levels.
Therefore, in addition to intestinal absorption, calcitriol plays a role in calcium release from the kidneys.
How? By increasing the production by kidney cells of certain proteins (TRPV5, NCX1 and calbindin D28k).
Their function is to allow the transport of calcium outside the kidney[4].
TRPV5, for example, helps release calcium into the bloodstream so that it is not eliminated with urine[5].
Vitamin D enables bone resorption of calcium
Vitamin D also serves to release calcium stored in our bones.
How? Calcitriol, produced in large quantities due to low blood calcium levels, acts on the cells that “build” bone, the osteoblasts, and initiates a series of responses that lead to the activation, instead, of the cells that “crumble” bone, the osteoclasts.
This “crumbling,” called bone resorption, reshapes the bone structure and, in the process, releases calcium.
Calcium released from bone ends up in blood vessels, increasing blood calcium levels[2, 5, 6].
Vitamin D and bone wellness
One might think that by withdrawing calcium from bones they weaken.
This is actually not the case: calcium and vitamin D help maintain bone health and reduce the risk of osteoporosis and fractures in old age.
Bones also contain about 99 percent of the calcium in the body, and their mineralization depends mainly on the concentration of calcium in the blood.
The regulation of calcium levels is mainly controlled by PTH and vitamin D.
PTH causes activation of vitamin D in the kidney leading to increased calcium absorption in the gut and release from the kidney, raising blood calcium levels.
There is much confirmation from the literature that low vitamin D levels are a risk condition for bone loss and fractures.
We can conclude, therefore, that vitamin D is also concerned with the well-being of our bones[2].
References
1.“Vitamin D physiology” P. Lips, Progress in Biophysics and Molecular Biology, 2006.
- “Vitamina D: tutto ciò che avreste voluto sapere e che non avete mai osato chiedere”M.L. Brandi, R. Michieli, Disease Managment, SIMG, 2015.
- “The role of calcium in health and disease”M.L. Power, R.P. Heane, H.J. Kalkwarf, R.M. Pitkin, J.T. Repke, R.C. Tsang, J. Schulkin, 1999.
- “A model of calcium homeostasis in the rat” David Granjon,Olivier Bonny, Aurélie Edwards, 2016.
5.“Kidney and calcium homeostasis” Un Sil Jeon, M.D., Electrolyte; Blood Pressure, 2008.
6.“Vitamin D for skeletal and non-skeletal health: What we should know.” Nipith Charoenngam, Arash Shirvani, Michael F. Holick, Journal of Clinical Orthopaedics and Trauma, 2019.
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