Vitamin D exists in two forms: D2 (ergocalciferol) and D3
(cholecalciferol). Vitamin D3, “the sunshine vitamin,” is synthetized
in the human epidermis via ultraviolet irradiation, or
it may be consumed in the form of oily fish or supplements.
Vitamin D2 is found in plants, as a product of irradiation
of ergosterol [1]. The vitamin is converted in the liver and
kidney to calcidiol and calcitriol, respectively, and acts on
specific target tissues via vitamin D receptors. Calcitriol, the
active form of vitamin D, binds to vitamin D receptors in the
intestines, bones, and kidneys to increase calcium absorption
from the intestines, promote calcium deposition in bones,
and decrease parathyroid hormone concentrations (PTH).
Its extraosseous effects are less known. Vitamin D receptors
were found in other tissues, as well, including the brain,
cardiomyocytes, vascular smooth muscle cells, endothelial