Vitamin D

Vitamin D is a fat soluble vitamin. The generic name of this group of vitamins is calciferols, the main representatives of which are vitamer D2 or ergocalciferol and vitamer D or colecalciferol. Those are the two vitamers which are biologically active. Colecalciferol is the stronger form of the two and thus mostly used in food supplements.

Which foods contain the vitamin?

• Fish liver oil
• Salmon
• Herring
• Sardine
• Flounder
• Egg
• Butter

Biological functions of the vitamin

The biological function of the vitamin is the metabolism of calcium and thus the promotion of the development of bones. Therefore, vitamin D is also called an anti-rachitic vitamin.

The active form is 1.25 (OH)2D3 which functions in cooperation with parathyroid hormone (parathyroid gland hormone) which is reproduced when there is little calcium in blood and where the increase in the level of this hormone in blood stimulates the creation of an active form of vitamin D in kidneys. The cooperation of the two hormones ensures the regulation of the metabolism of calcium, whereas the central places of functioning are kidneys, small intestine mucosa and bones — this ensures the increased absorption of calcium from small intestine and the reduced excretion thereof from kidneys.

Vitamin D also plays an important role in the regulation of blood pressure. It has been observed that an additional intake of vitamin D lowers blood pressure up to 6 mmHg.

The active form of vitamin D produced in kidneys also has other targets such as the pancreas, pituitary gland, placenta, uterus, brain tissue, blood formed elements (lymphocytes, monocytes) and skin keratocytes.

Additionally, vitamin D is also needed for the normal clotting of blood, functioning of the cardiac muscle and the nervous tissue.

Absorption, storage and excretion

Vitamin D is absorbed from the small intestine. Absorption is inhibited by fibre-rich diet, corticosteroids, oral contraceptives, excessive alcohol and mineral oils.

Vitamin D is also synthesised in skin by UV radiation.

The vitamin is stored in liver and the reserves last for about 1–5 months.

Excreted from the organism in both free form as well as conjugates.

Problems resulting from deficiency

• Rickets in children
• Osteomalacia in adults
• Osteoporosis
• Increased occurrence of malignant tumors
• Increased occurrence of autoimmune diseases
• Falling ill with tuberculosis, influenza, upper respiratory infections

Risk groups for developing the deficiency

• Elderly people
• People living in the Nordic countries
• Pregnant women
• Persons with gastrointestinal diseases
• Obese persons
• Persons using glucocorticoids, statins and medications against cramps
• People with liver functioning diseases
• People with chronic kidney failure
• Chronic alcoholics
• Pre-school children
• Menopausal women

Toxicity

Vitamin D functions similarly to steroid hormones.

Excessive amounts cause hypercalcemia, and it it happens during pregnancy, it may cause mental and growth retardation and aortic stenosis for the fetus.

At first, the symptoms of toxicity include loss of appetite, vomiting, muscle weakness and diarrhoea. Severe toxicity may result in growth retardation, mineralisation of bones, hypertension and calcification of soft tissues.

Use of vitamin preparations

As an additional treatment component, vitamin D can be used for treating alcoholism, psoriasis, acne, tetanus, tuberculosis, allergy, hepatitis, jaundice, arthritis, cystic fibrosis, diabetes, glaucoma, cataract and hypoparathyroidism.

Optimal vitamin D level in blood lowers the risk of type 2 diabetes, cardiovascular diseases and metabolic syndrome.

The effect of vitamin D is stronger when administered with calcium, phosphorus, vitamin A, vitamin C and boron.

Figure 1. Vitamin D2: ergocalciferol

Figure 2. Vitamin D3: colecalciferol

Figure 3. Vitamin D4: 22-Dihydroergocalciferol

Figure 4. Vitamin D5: sitocalciferol