Updated: 9/19/2019

Vitamin D

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Topic
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Questions
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Evidence
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Topic
Introduction
  • Calcium homeostasis controlled by a combination of
    • Vitamin D pathway (this topic)
    •  PTH pathway
Vitamin D Pathway

  • Synthesis
    • vitamin D3
      • synthesized in skin from sun exposure
    • vitamin D2
      • ingested from plants in the diet
    • both are prehormones that must be converted in 2 steps 
      • 25-hydroxylation in liver
      • 1-hydroxylation in the peritubular capilliaries of the kidney
    • only 1,25-(OH)2 vitamin D is active
    • 24,25-(OH)2 vitamin D is an inactive form
  • Function
    • ↑ serum Ca2+ and phosphate via 
      • ↑ absorption of calcium and phosphate from the intestine
      • ↑ bone resorption of Ca2+ and phosphate
    • recall PTH functions to ↑ serum Ca2+ but ↓ serum phosphate
  • Regulation
    • PTH stimulates 1,25-(OH)2 vitamin D production 
    • hypocalcemia/hypophoshatemia stimulates 1,25-(OH)2 vitamin D production
    • 1,25-(OH)2 vitamin D feedback negatively on itself
  • Pathology
    • rickets / osteomalacia
Clinical Conditions
  • Hypoparathyroidism 
  • Hyperparathyroidism
  • Rickets

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(M1.EC.15.75) The human body obtains vitamin D either from diet or from sun exposure. Darker-skinned individuals require more sunlight to create adequate vitamin D stores as the increased melanin in their skin acts like sunscreen; thus, it blocks the necessary UV required for vitamin D synthesis. Therefore, if these individuals spend inadequate time in the light, dietary sources of vitamin D are necessary. Which of the following requires sunlight for its formation?

QID: 106797
1

7-dehydrocholestrol

22%

(38/171)

2

Cholecalciferol (D3)

56%

(96/171)

3

25-hydroxyvitamin D

8%

(13/171)

4

1,25-dihydroxyvitamin D

8%

(14/171)

5

Ergocalciferol (D2)

5%

(9/171)

M 1 C

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Evidence (1)
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