Vitamin D belongs to the fat-soluble steroids. It occurs in several forms, which could include:
- calciferol– vitamin D1, occurring mainly in cod- liver oil,
- ergocalciferol– vitamin D2, occurring in a variety of plants, fungi and yeast,
- and cholecalciferol vitamin D3 of animal origin.
After metabolic activation, the activity of vitamin D is shown by cholecalciferol and ergocalciferol. Relative to vitamin D2 cholecalciferol shows longer duration of action and 2-10 times higher efficiency.
Vitamin D3 belongs to the vitamins that the human body is able to produce on their own. In the first stage of the synthesis in the skin of 7-dehydrocholesterol (provitamin D) under the influence of UVB (290-315 nm) is activated to cholecalciferol, which is released into the bloodstream. Then the activation takes place in two stages. The first stage occurs in the liver, where the cholecalciferol undergoes hydroxylation and 25-hydroxycholecalciferol is created. It is the main form of vitamin D that is present in the bloodstream, which is why in examining the status of the body’s supply of vitamin D is based on determination of this metabolite. The second phase occurs mainly in the tubules and leads to the creation of biologically active form-1α, 25dihydroxycholecalciferol (calcitriol) (1 mg = 40, 000 IU).
The main role of vitamin D in the human body is associated with calcium-phosphate management and modeling and mineralization of bones. Calcitriol stimulates increased reabsorption of calcium, impacting on the cells of the small intestine and is the promoter of calcium deposition in bone matrix and differentiates into osteoblasts. However, the presence of vitamin D receptor in tissues which are not related to the maintenance of calcium-phosphate homeostasis, points to a wider range of action of this vitamin-hormone, which includes participation in the production of insulin, increasing muscle strength, regulating the process of proliferation and differentiation of cells, as well as impact on the processes of angiogenesis and apoptosis.
The correct concentration of 25-OH-D3 in blood serum in healthy children is within the limits of 20-80 ng/ml (50 nmol/l-200 nmol/l), whereas in adults: from 30 ng/ml (75 nmol/l).
The contents of cholecalciferol in foods (fatty saltwater fish, cod- liver oil, mushrooms) are relatively small in comparison with the amount we get as a result of the short exposure to solar radiation. 90% of vitamin D present in the body is endogenous, but its synthesis in the skin is constrained by many factors, which include both environmental and geographical factors.
A significant impact in the intensity of the synthesis of vitamin D has:
- the degree of sunlight
- time of year of the day
- skin pigmentation
- sunscreen usage
- the amount of body fat
- body mass index (bmi)
- degree of air pollution
- and the thickness of the cloud cover
A condition in which the concentration of 25-hydroxychlorecalciferol is smaller than 75 nmol/l (30 ng/ml) is defined as the vitamin D deficiency. It is very common and populations of countries at higher latitudes are especially vulnerable e.g. Poland.
In a healthy population, the causes of the shortage can be many. It is associated with the insufficient supply in the diet, the lack of or impairment of skin synthesis process (low insolation, sunscreen usage, frequent presence in artificially lit rooms, etc.), aging, digestive diseases, kidney or liver disorders.
The correct supply of vitamin D is necessary in every period of life, but it seems, that it should be observed especially in children and adolescents.
Considering that the activities of vitamin D are very expansive, its deficiency increases the risk of occurring the diseases of different etiology. The most common effects of shortage include rickets in children, osteomalacia as well as osteopenia and osteoporosis in people at any age. Vitamin D deficiency also affects skeletal muscles weakness, which provides to falls and fractures of bones.
In recent years, it has been proven that deficiency of vitamin D leads to the development of cancers, e.g. breast cancer, prostate cancer, cancer of the colon and increases the incident of cardiovascular disease, hypertension, upper respiratory tract infections, as well as leads to a reduction of the activity of the immune system and increased susceptibility to autoimmune disease.
In Poland, skin synthesis takes place from April to September, from 10 AM to 3 PM, however, it is necessary for 18% of the surface area of the body (lower limbs, forearms) to provide a minimum of 15 minutes exposure to the Sun without the use of filters.
Nowadays, more and more often adequate vitamin D intake requires the use of dietary supplements.
The daily requirement of vitamin D3 is arguable. Prof. A. Milewicz believes that it is 2 000 IU, while prof. E. Sewerynek, that it is 30 000 IU.
The daily intake of vitamin D (from all sources) shouldn’t be lower than 400 IU in the entire development period. In the case of infants (0-1 year of age) the value of the recommended daily intake is 800 UI for children from 1-9 years of age- 600 IU, and for young people at the age of 10- 18 years of age- 400 IU. Polish standards do not specify the recommended daily intake for adults up to 60 years of age, while those over 60 years of age it is advisable to provide 400 IU of vitamin D3 a day.
Supplementation is recommended during puberty (except summer months) and growing up, as well as in people after the age of 65. It should be aware that the important role of calcium, which in an correct amount is necessary to calcemic action of vitamin D.
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