Vitamin D

Question 1

What are the characteristics of Vitamin D?

Answer 1

● Secosteriod produced from steroid ring system by opening of the B ring
● Occurs only under catalytic conditions involving ultraviolet light from sun
● Needed to make active Vit D = skin (UV light), liver, kidney. Note that in the kidney, PTH, ↓serum Ca, ↓ serum PO 4 all enhance synthesis of the active 1,25 form of vitamin D (and decrease 24,25 formation). On the other hand, fibroblast growth factor 23 (FGF23) is synthesized and secreted by OB’s and OC’s and decreases formation of 1,25 Vitamin D in the kidney, and additionally, FGF23 inhibits phosphate reabsorption.
● Transport via a specific carrier protein – Vitamin D binding globulin Fat soluble, so see adipose storage
● Binds to a nuclear receptor (transcription factor) similar to other steroid hormones → affects transcription of genes (i.e., effects are genomic)

Question 2

What are the major effects of vitamin D?

Answer 2

1. Increases intestinal absorption of Ca and PO4: a poorly understood process
   ▪TRPV6 Ca2+ channels mediate mucosal brush border Ca2+ entry in the intestine and this appears to be a Vitamin­D dependent process
   ▪ Vit. D ↑ the expression of a Ca2 ATPase on the serosal membrane of the intestinal mucosal cell (genomic effects). The Ca2+ ­ATPase helps to extrude Ca2+ across the serosal membrane into the bloodstream. Calbidin­ D9K, whose expression is also upregulated by Vitamin D, enhances the extrusion of Ca2+ by the Ca­ATPase
2. Decreases renal excretion of Ca and PO4
3. Increases both bone resorption and formation in part by stimulating the proliferation and differentiation of osteoblasts and osteoclasts

Question 3

What is the source of Vitamin D3?

Answer 3

Animal

Question 4

What is the source of Vitamin D2?

Answer 4

Plant

Question 5

What are the differences between Vitamin D2 and D3?

Answer 5

Although there are chemical differences there is no physiologic difference

Question 6

Is Vitamin D a hormone or a prohormone?

Answer 6

a prohormone

Question 7

What makes Vitamin D elimination long?

Answer 7

Binding

Question 8

Is Vitamin D water or fat soluble?

Answer 8

Fat (adipose storage)

Question 9

Where is Vitamin D cleared?

Answer 9

Liver

Question 10

What do Vitamin D metabolites do?

Answer 10

• Increase intestinal absorption of Calcium and Phosphate
• Decrease excretion of calcium and phosphate by the kidney
• Increase bone resorption
• Increase bone formation