Vitamin D

In the thirties of the twentieth century, vitamin D was first synthesized and studied. This vitamin is very interesting to the world of science, as it is both a vitamin and a hormone. It can enter the body both with food and be produced by the body when exposed to sunlight. Vitamin D is known as a vitamin associated with the development of rickets. Rickets was mentioned as early as 1650. The model of the vitamin was proposed in 1919, synthesized in 1932.

What you need to know about vitamin D?

In order not to get into a dead end when you are called a particular substance by scientific terms, you need to know its chemical name. For example, vitamin D has other names such as antirachitic vitamin, cholecalcefirol, ergocalcefirol and viosterol.

Vitamin D is divided into several vitamins of this group. Thus, vitamin D3 is called cholecalciferol, and simply vitamin D is called ergocalciferol. Both of these vitamins can only be found in animal food. Vitamin D is also produced directly by the body, and this happens due to the effect of ultraviolet rays on the skin.

Vitamin D is directly related to such a disease as rickets. The fact is that animal fats are able to release vitamin D if they are exposed to sunlight. Thus, already in 1936, pure vitamin D was isolated from tuna fat. Thus, it began to be used to combat rickets.

Chemical nature and biologically active forms of vitamin D

Vitamin D is a group designation for several substances that are chemically related to sterols. Vitamin D is a cyclic unsaturated high-molecular alcohol – ergosterol.

There are several vitamers of vitamin D. Among them, the most active are ergocalciferol (D2), cholecalciferol (D3), and dihydroergocalciferol (D4). Vitamin D2 is formed from a plant precursor (provitamin D) – ergosterol. Vitamin D3 – from 7-dehydrocholesterol (synthesized in human and animal skin) after exposure to ultraviolet light. Vitamin D3 is biologically the most active.

Less active vitamin D vitamers – D4, D5, D6, D7 – are formed by ultraviolet irradiation of plant precursors (respectively, dihydroergosterol, 7-dehydrositosterol, 7-dehydrostigmasterol and 7-dehydrocampesterol). Vitamin D1 does not occur in nature. Biologically active forms of ergo- and cholecalciferols are formed during metabolism.

Vitamin D metabolism

Dietary calciferols are absorbed in the small intestine with the participation of bile acids. After absorption, they are transported as part of chylomicrons (60-80%), partly in a complex with OC2-glycoproteins to the liver. Endogenous cholecalciferol also enters here with the blood.

In the liver, cholecalciferol and ergocalciferol undergo hydroxylation in the endoplasmic reticulum by cholecalciferol 25-hydroxylase. As a result, 25-hydroxycholecalciferol and 25-hydroxyergocalciferol are formed, they are considered the main transport form of vitamin D. They are transported with the blood as part of a special calciferol-binding plasma protein to the kidneys, where 1,25-dihydroxycalciferols are formed with the participation of the enzyme 1-a-hydroxylase of calciferols. They are the active form of vitamin D, which has a D-hormone-like effect - calcitriol, which regulates calcium and phosphorus metabolism in the body. In humans, vitamin D3 is more effective in increasing the level of serum 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D than vitamin D2.

In cells, vitamin D3 is localized in membranes and subcellular fractions - lysosomes, mitochondria, and the nucleus. Vitamin D does not accumulate in tissues, with the exception of adipose tissue. Both 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D are broken down by catalysis involving the enzyme 24-hydroxylase. This process occurs in various organs and tissues. In general, the amount of vitamin D circulating in the blood depends on exogenous sources (foods, nutraceuticals), endogenous production (synthesis in the skin), and the activity of enzymes involved in the metabolism of the vitamin.

It is excreted mainly in feces in unchanged or oxidized form or in the form of conjugates.

Biological functions of vitamin D

The biological activity of 1,25-hydroxycalciferols is 10 times greater than the activity of the original calciferols. The mechanism of action of vitamin D is similar to the action of steroid hormones: it penetrates the cell and regulates the synthesis of specific proteins by acting on the genetic apparatus.

Vitamin D regulates the transport of calcium and phosphorus ions through cell membranes and thus their levels in the blood. It acts as a synergist with parathyroid hormone and as an antagonist with thyrocorticotropic hormone. This regulation is based on at least three processes in which vitamin D participates:

1. Stimulates the absorption of calcium and phosphate ions through the epithelium of the small intestinal mucosa. Calcium absorption in the small intestine occurs by facilitated diffusion with the participation of a special calcium-binding protein (CaBP - calbindin D) and active transport with the help of Ca2+-ATPase. 1,25-Dihydroxycalciferols induce the formation of CaBP and protein components of Ca2+-ATPase in the cells of the small intestinal mucosa. Calbindin D is located on the surface of the mucosa and, due to its high ability to bind Ca2+, facilitates its transport into the cell. Ca2+ enters the bloodstream from the cell with the participation of Ca2+-ATPase.
2. Stimulates (together with parathyroid hormone) the mobilization of calcium from bone tissue. The binding of calcitriol to osteoblasts increases the formation of alkaline phosphatase and the Ca-binding protein osteocalcin, and also promotes the release of Ca+2 from the deep apatite layers of bone and its deposition in the growth zone. At high concentrations, calcitriol stimulates the resorption of Ca+2 and inorganic phosphorus from bone, acting on osteoclasts.
3. Stimulates the reabsorption of calcium and phosphorus in the renal tubules, due to the stimulation of Ca2+-ATPase of the renal tubule membranes by vitamin D. In addition, calcitriol suppresses its own synthesis in the kidneys.

In general, the effect of vitamin D is expressed in an increase in the content of calcium ions in the blood.

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How much vitamin D do you need per day?

The dose of vitamin D increases depending on the person's age and their expenditure of this vitamin. Thus, children should consume 10 mcg of vitamin D per day, adults - the same amount, and elderly people (after 60 years) - about 15 mcg of vitamin per day.

When does the need for vitamin D increase?

Elderly people should increase their daily dose of vitamin D, as should people who spend little time in the sun. Children should take vitamin D to prevent rickets. Pregnant and breastfeeding women, as well as menopause, should definitely increase their intake of this vitamin.

Vitamin D absorption

With the help of bile juices and fats, vitamin D is better absorbed in the stomach.

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Interaction of vitamin D with other elements of the body

Vitamin D helps to absorb calcium (Ca) and phosphorus (P), and with its assistance magnesium (Mg) and vitamin A are well absorbed.

What determines the presence of vitamin D in food?

You don't have to worry about cooking foods properly because vitamin D is not lost during heat treatment, but factors such as light and oxygen can completely destroy it.

Why does vitamin D deficiency occur?

The absorption of the vitamin may be impaired by poor liver function (liver failure and mechanical jaundice), since the supply of the required amount of bile is severely disrupted.

Since vitamin D is produced in the human body exclusively by the skin and sunlight (the fat on the skin synthesizes vitamin D under the influence of the sun, and then the vitamin is absorbed back into the skin), you should not immediately shower after being in the sun. Otherwise, you will wash off all the vitamin D from your skin, which will cause a deficiency in the body.

Signs of Vitamin D Deficiency

In small children, a lack of vitamin D can cause sleep disturbances, increased sweating, delayed teething, and softening of the bone tissue of the ribs, limbs, and spine. Children become irritable, their muscles relax, and in infants, the fontanelle may take a long time to close.

In adults, the signs of vitamin deficiency are slightly different: although their bones also soften, such people can still lose a lot of weight and suffer from severe fatigue.

Foods that contain vitamin D

If you eat more foods rich in vitamin D, you can fully maintain the amount of this vitamin in your body. These foods include liver (0.4 mcg), butter (0.2 mcg), sour cream (0.2 mcg), cream (0.1 mcg), chicken eggs (2.2 mcg) and sea bass (2.3 mcg of vitamin D). Eat these foods more often to keep your bones and body safe!

Vitamin D is found in a number of animal products: liver, butter, milk, as well as yeast and vegetable oils. Fish liver is the richest source of vitamin D. Fish oil is obtained from it, which is used to prevent and treat vitamin D deficiency.

Signs of Vitamin D Overdose

Overdosing on vitamin D can cause nausea, diarrhea, abdominal cramps, extreme fatigue, and headaches. People who suffer from vitamin D overload often have very itchy skin, impaired heart and liver function, high blood pressure, and severe eye inflammation.

Treatment of hypervitaminosis D:

• drug withdrawal;
• low Ca2+ diet;
• consumption of large amounts of liquid;
• administration of glucocorticosteroids, a-tocopherol, ascorbic acid, retinol, thiamine;
• in severe cases - intravenous administration of large quantities of 0.9% NaCl solution, furosemide, electrolytes, hemodialysis.