Vitamin D is not really a vitamin, but one of the oldest prohormones, having been produced by life forms for over 750 million years. In humans, vitamin D is critically important for the development, growth, and maintenance of a healthy body, beginning with gestation in the womb and continuing throughout the lifespan.
Vitamin D is produced endogenously when ultraviolet rays from sunlight strike the skin and trigger vitamin D synthesis. Vitamin D obtained from sun exposure, food, and supplements is biologically inert and must undergo two hydroxylations in the body for activation. The first occurs in the liver and converts vitamin D to 25-hydroxyvitamin D [25(OH)D], also known as calcidiol. The second occurs primarily in the kidney and forms the physiologically active 1,25-dihydroxyvitamin D [1,25(OH)2D], also known as calcitriol.
Vitamin D promotes calcium absorption. Together with calcium, vitamin D also helps protect older adults from osteoporosis.
Vitamin D plays a role in help to regulating blood glucose level. In diabetes patients it increases insulin production by regulating blood levels of calcium and improving pancreatic beta-cell function.