The most common form of vitamin D is vitamin D3 or cholecalciferol. It is usually produced in the skin of human and animals under the influence of solar or ultraviolet radiation. In plants it can be produced from provitamin ergosterol. Irradiation of ergosterol leads to the formation of ergocalciferol (vitamin D2), while irradiation of the skin in animals produces cholecalciferol (vitamin D3) from the precursor 7-dehydrocholesterol.
Ergocalciferol and cholecalciferol have the same biological value of vitamin D.
Vitamins D2 and D3 are absorbed from food in the small intestine. Related to specific protein they are transported to the liver.
It was found that vitamin D provides adsorption of calcium and phosphorus in the small intestine. Vitamin D enhances the adsorption of the phosphorus in the kidneys. It enhances the retention of calcium in bone tissue. It was found that the in animals that consume insufficient amount of calcium and phosphorus, rickets develops faster.
As with some other vitamins, in 1985. was shown that vitamin D may have a protective role against various tumors (primarily cancer of the colon and breast cancer). It is not yet determined whether the adsorption of calcium has a protective role or vitamin D by itself. Experimentally was determined that the active form of vitamin D inhibits metastasis of large intestine tumor cells. It is presumed that this vitamin has a positive effect with retinoblastoma and human leukemia.
Vitamin D increases the immune system. Decades ago (before the era of antibiotics) TB patients were able used to go to the sanatorium in the mountains, where they were exposed to fresh air, sunshine and good food. Later, it was assumed that vitamin D is likely to lead to an improvement of TB patients. There are several groups of researchers, who explain how vitamin D helps in the treatment of tuberculosis. The most numerous among them claim that the active form of vitamin D stimulates human macrophages (cells that are found in the blood and are used for the removal of foreign and dead cells from the blood). In this way, macrophages slow or completely prevent the reproduction of bacteria that cause disease. In this case, vitamin D is an immunomodulator.