Updated: 9/19/2019

Vitamin D

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Topic

Review Topic

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Evidence

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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Questions (5)

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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

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7-dehydrocholestrol

20%

(49/249)

Cholecalciferol (D3)

53%

(131/249)

25-hydroxyvitamin D

(20/249)

1,25-dihydroxyvitamin D