About Vitamin D
Vitamin D is classified as a lipophilic vitamin, meaning it is fat-soluble. It is a very important micronutrient that plays a crucial role in the functioning of the human body. It is naturally found in a limited number of foods; therefore, it is often necessary to supplement it, even though the human body can partially produce it on its own.
TIP: If you are interested in vitamins and their importance in human nutrition, you can also read about B-complex vitamins.
Vitamin D Metabolism
Although we consume vitamin D through our diet, the human body is capable of producing a certain amount itself. It is synthesized in the body from precursors derived from cholesterol and can also be formed by yeasts and molds. The vitamin is produced in the skin through exposure to UVB radiation. The amount of vitamin produced is directly proportional to the duration of skin exposure to sunlight; therefore, there are certain periods when a sufficient amount is produced, and conversely, periods when it is not. In the Czech Republic, the most abundant months are June, July, and August. During the rest of the year, vitamin D must be obtained through food, and supplementation is recommended for all age groups, especially during the winter months. It appears optimal to obtain 80% of the recommended daily allowance of the vitamin from sunlight and the remaining 20% from the diet. To cover the daily intake of vitamin D, approximately 20 minutes in the sun is sufficient, provided that at least 40% of the body is exposed. However, this also depends on latitude, season, time of day, and skin pigmentation. Sunscreens can also reduce vitamin production, though they are a necessity for skin protection.
There are 2 forms of vitamin D: D2 – ergocalciferol, which we consume through food, and D3 – cholecalciferol, which we also consume through food and is produced in the skin. Both of these forms travel to the liver and subsequently to the kidneys, where the active form of vitamin D – calcitriol – is created.
Functions of Calcitriol
Calcitriol, together with the hormones calcitonin and parathyroid hormone, regulates calcium metabolism, meaning it participates in the absorption of calcium from the diet, its storage in bones, and its excretion from the body. Vitamin D also stimulates the absorption of calcium and phosphorus from the small intestine. It plays a role in bone construction, cell division, and differentiation, and contributes to maintaining a healthy immune system. It also positively influences brain synapses and renal function.
Sources
D3 – fish oil, fatty fish meat (tuna, salmon, mackerel), the liver of certain fish (cod), egg yolks, butter, milk, fortified margarines, UVB radiation (not a source itself, but it triggers vitamin production in the skin)
D2 – mushrooms, yeast
In the Czech Republic, the main source of vitamin D for the adult population is eggs, while for children, the most significant source is milk and dairy products. Fish are also noteworthy, although their consumption in the Czech Republic is relatively low.
Daily Requirement
5–20 µg per day; the requirement is higher for children under 1 year and for seniors over 65, as biosynthesis in the skin decreases significantly with age.
Deficiency
Dietary sources of vitamin D do not cover the recommended daily allowance for up to 95% of the Czech population (for individuals aged 4–90). However, vitamin D deficiency is a problem not only in the Czech Republic but worldwide. In modern times, a sedentary lifestyle and obesity contribute significantly to this. The portion of vitamin D stored in adipose tissue is functionally inactive, thereby reducing the overall activity of vitamin D in the body. A sedentary lifestyle limits outdoor movement and, consequently, exposure to sunlight.
In children, vitamin D deficiency causes rickets. In older individuals, it can cause osteomalacia and osteoporosis, and it also increases the risk of cancerous, infectious, and autoimmune diseases, such as type 1 diabetes mellitus.
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Excess
Vitamin D is among the fat-soluble vitamins, which are vitamins that a person can overdose on. They are stored in our bodies and are not as easily excreted as water-soluble vitamins. However, this only applies to dietary sources. Long-term exposure to sunlight does not cause vitamin D intoxication.
Doses of vitamin D higher than 50 µg are toxic to us. Poisoning is only a risk with oral intake. This subsequently leads to hypercalcemia, increased neuromuscular irritability, and an increased risk of acute pancreatitis, gastric ulcers, and kidney stones.
Author: Ing. Kristina Heřmanová
