1. Vitamin D

2. Vitamin D & metabolites are steroids D 3 is the most active metabolite D 3 is a product of Vitamin D metabolism in the liver and kidney D 3 diffuses into cells and binds to the vitamin D receptor, a protein found in the cytoplasm and nucleus Hormone-receptor complex then binds to specific nuclear DNA sequences, turning on or off transcription of genes This changing of proteins synthesized is the primary activity of Vitamin D

3. Vitamin D effects on calcium The changes in transcription promote calcium absorption from the intestine Vitamin D also stimulates mineralization of bone Rickets: formation of weak bones in children

4. Biosynthesis and metabolism Vitamin D can be acquired through diet (D 2 from plant sterols) and through action of sunlight on cholesterol-derived precursors in keratinocytes (D 3 from skin) Both are stored in fats and muscle Precursors are carried to the liver where it is formed into Vitamin D carried in the body (half-life of 2 weeks) Other forms of Vitamin D generated in proximal tubule of the kidney

5. When plasma calcium or phosphate levels are low, the activity of the enzymes that form Vitamin D is high (and visa versa) Calcitonin is secreted by the thyroid in response to excess calcium in the blood which inhibits bone resorption Recent research indicates that Vitamin D may have a broader role than in calcium metabolism (cancer, hypertension, renal failure, immune response)