Vitamin D

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Occurrence and structure

Cholecalciferol/Vitamin D is the precursor of Calcitriol. It is synthesized from cholesterol. The cholesterol is split in the skin by exposure to sunlight (i.e. UV light) and thus becomes cholecalciferol, which is actually vitamin D. The active form, however, is calcitriol, whose chemical name is actually 1.25 – dihydroxycholecalciferol.

This means that cholecalciferol, which is produced from cholesterol, is hydroxylated in two places (at C 1 and C 25) (OH groups are added). This happens in the liver and kidney. The resulting calcitriol is active and acts as a hormone.

About 80% of the vitamin D in the body is produced by the body itself. The remaining 20% should be taken in with food. Vitamin D3 is found in animal foods, such as fish, eggs and milk.

In contrast, vitamin D2 is mainly found in plant foods, such as mushrooms. Vitamin D3, just like vitamin D2, is converted to the hormone calcitriol in the human body, which is why the vitamins are also called precursors of the hormone. We would like to point out that a separate topic on nutrition in cholesterol has been written on this subject.

Function of vitamin D

Calcitriol plays an important role in the calcium and phosphate balance. For the regulation of the concentration of these two substances there are three hormones, some of which act in opposite ways to be prepared for every situation. Therefore, here is a short digression: The parathyroid hormone is one of these three substances.

It is synthesized in the parathyroid glands and is released there when the calcium level in the blood drops. Once in the blood, it ensures that increased calcium is made available in the intestines as well as in the kidneys. This means that more calcium is absorbed in the intestine (taken up from food) and less calcium is excreted in the kidneys.

In addition the Parathormon releases strengthened calcium from the bones. At the same time, however, it ensures – in contrast – increased excretion of phosphate via the kidneys. Why?

Calcium and phosphate form complexes (e.g. in the bone substance), such complex formation in the blood would be extremely unfavorable, so that it is prevented by the increased elimination of phosphate. The opponent of the parathyroid hormone is calcitonin. It is synthesized in the C-cells of the thyroid gland and causes a reduction in the level of calcium and phosphate in the blood.

On the one hand, through their increased excretion via the kidneys, and on the other hand through the reintegration of both substances into the bones. This is called mineralization of the bone. The third in the bundle is calcitriol.

It originates from the kidney, as this is where the last step of its activation described above takes place. Parathormone increases the release of calcitriol, so the two work hand in hand, so to speak. Calcitonin ensures that more calcium and phosphate is absorbed in the intestine and less calcium and phosphate is excreted in the kidney. At the same time, it rebuilds both back into the bone substance, which results in increased mineralization. Calcitonin works together with the parathyroid hormone by re-integrating the calcium and phosphate that was taken from the bones, thus counteracting long-term bone loss.