The vitamin D precursor in our skin is 7-dehydrocholesterol, which is made in the body from cholesterol. Ultraviolet-B (UVB) energy converts 7-dehydrocholesterol into cholecalciferol, which is the name of natural vitamin D. Cholecalciferol (pronounced koh· luh· kal· sif· uh· rawl) is then carried in the bloodstream to the liver, where it is
converted into the prohormone 25-hydroxy vitamin D (calcidiol). Some of that 25-hydroxy vitamin D is then transported to the kidneys where it is hydroxylated further to form 1,25-hydroxy vitamin D (calcitriol). This form is the physiologically active form of vitamin D. The calcitriol made in the kidneys is to fulfill only one of vitamin D’s functions: its endocrine function, which is to maintain the calcium economy. Technically not a “vitamin,” vitamin D is in a class by itself. Its second metabolic product, 1,25-hydroxy vitamin D (calcitriol), is actually a hormone, a key that fits binding sites on the human genome. The human genome contains more than 2,700 binding sites for 1,25-hydroxy vitamin D; those binding sites are near genes involved in virtually every known major disease of humans.
