Vitamin D is synthesized in the skin upon exposure to sunlight and regulates calcium absorption and homeostasis. It is metabolized from 7-dehydrocholesterol to previtamin D to cholecalciferol with sunlight exposure, and absorbed into the bloodstream. Deficiency can lead to rickets in children or osteomalacia in adults by disrupting gene expression regulated by vitamin D nuclear receptors.

1. Calcitriol Biosynthesis and
metabolism
• Vitamin D is not strictly a vitamin, it can be
synthesized in the skin, and under most
conditions that is the major source of the
vitamin.
• Only when sunlight exposure is
inadequate is a dietary source required.
• Its main function is in the regulation of
calcium absorption and homeostasis

2. Calcitriol Biosynthesis and
metabolism
• Most of its actions are mediated by way of
nuclear receptors that regulate gene
expression.
• Deficiency, leading to rickets in children
and osteomalacia in adults.