Vitamin D is a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium , magnesium , and phosphate , and many other biological effects. [1] [2] [3] In humans, the most important compounds in this group are vitamin D 3 ( cholecalciferol ) and vitamin D 2 ( ergocalciferol ). [2] [3] [4]
The major natural source of the vitamin is synthesis of cholecalciferol in the lower layers of epidermis of the skin, through a photo-chemical reaction of UVB light, from the sun exposure (specifically UVB radiation ) or UVB lamps. [1] Cholecalciferol and ergocalciferol can be ingested from the diet and supplements . [1] [2] Only a few foods, such as the flesh of fatty fish, naturally contain significant amounts of vitamin D. [2] [5] In the U.S. and other countries, cow's milk and plant-derived milk substit
Vitamin D from the diet, or from skin synthesis, is biologically inactive. It is activated by two protein enzyme hydroxylation steps, the first in the liver and the second in the kidneys. [1] [4] Because vitamin D can be synthesized in adequate amounts by most mammals if they get enough sunlight, it is not essential and therefore is technically not a vitamin . [3] Instead it can be considered a hormone , with activation of the vitamin D pro-hormone resulting in the active form, calcitriol , which then produ