Vitamins and Minerals

Question 1

describe vitamin A

Answer 1

• collectively called retinoids
• retinol: transport and storage
• 11-cis retinal:is required for vision
  
  • retinol and retinal can be easily interconverted
• retinoic acid: epithelial growth and differentiation
  
  • has steroid hormone like effects
  • can’t be converted back to other forms

Question 2

describe the absorption and transport of vit. A

Answer 2

Question 3

name the sources and functions of vit. A

Answer 3

• sources:
  
  • liver, kidney, butter and cream products; egg yolk
  • yellow vegetables
• functions:
  
  • vision: 11-cis retinal is a component of rhodopsin
  • maintenance of specialized epithelia, especially mucus secreting cells (retinoic acid)
  • growth (retinoic acid)
  • reproduction (retinol)

Question 4

describe the mechanism of action of retinoic acid in epithelial cells

Answer 4

• retinol enters the target cell and is oxidized to retinoic acid in the cytosol
• from the cytosol, the retinoic acid moves into the nucleus with the help of cellular retinoid binding proteins
• retinoic acid binds to nuclear receptors and activates transcription of specific genes (keratin)

Question 5

describe cause and signs of deficiency of vitamin A

Answer 5

• dietary deficiency is the commenest cause; fat free diets
• malabsorption of fats can lead to deficiency
• signs and symptoms:
  
  • night blindness – earliest symptom
    
    • regeneration of rhodopsin is delayed
  • xerophthalmia: dryness of the conjuctiva and cornea
  • keratomalacia: corneal erosion and ulceration
  • immune deficiency: weakened innate immunity

Question 6

describe clinical uses of vitamin A and what happens in hypervitaminosis A

Answer 6

• retinol or B-carotene: treatment of dietary deficiency
• retinoic acid used in treatment of acne
• hypervitaminosis A: raised intracranial pressure (headahces)
  
  • enlarged liver
  • pregnancy: spontaneous abortions and congenital malformations in the fetus

Question 7

describe the formation of calcitriol (1, 25 dihydroxy cholecalciferol) which is the active form of vit. D

Answer 7

Question 8

describe the actions of vitamin D on serum calcium

Answer 8

• intestine: stimulates intestinal absorption of calcium and phosphate by increased synthesis of calbindin
• bone: stimulates the mobilization of calcium and phosphate from the bone in the presence of PTH (when serum calcium is low)
• kidneys: inhibits calcium excretion by stimulating parathyroid dependent calcium reabsorption

Question 9

describe the actions of vitamin D on serum phosphate

Answer 9

• vitamin D increases serum phosphate by increasing its absorption from the intestine
• in the presence of vit. D, serum Ca and phosphate are elevated
  
  • increases the tendency to form bone mineral
• in vit. D deficiency, reduced absorption of Ca reduces serum Ca level–stimulates secretion of PTH which in turn causes demineralization of bone

Question 10

describe causes of vit. D deficiency

Answer 10

• nutritional deficiency: decreased intake, fat malabsorption, exclusively breast fed infants
• inadequate exposure to sunlight (common during winter)
• chronic renal disease, chronic liver disease

Question 11

describe effects of vit. D deficienccy

Answer 11

• decreased calcium absorption from diet -> decreased serum calcium -> increased PTH release -> demineralization of bone
  
  • soft pliable bones
  • bow-leg deformity
  • overgrowth at costochondral junction: rachitic rosary
  • pigeon chest deformity
  • frontal bossing

Question 12

what lab findings are seen in nutritional rickets?

Answer 12

increased serum alkaline phosphatase (ALP)

Question 13

describe mutations in the calcitriol receptor

Answer 13

• inheritied condition where calcitriol is unable to bind to its receptor in the intestinal mucosal cells
• as a result, these children have high levels of calcitriol (loss of negative feedback)
• serum calcium and phosphate levels are low and 1,25-hydroxyvitamin D levels are high
• compare to vit. D deficient rickets which has low serum calcium and phosphate and low levels of 25-hydroxyvitamin D

Question 14

describe osteomalacia

Answer 14

• vitamin D deficiency in an adult
• bones are demineralized and are susceptible to fracture
• osteomalacia can be secondary to dietary deficiency, renal disease or liver disease
• lab findings = high ALP

Question 15

describe hypervitaminosis D

Answer 15

• characterized by high serum calcium and high serum phosphate
  
  • this results in an increased tendency for ectopic (soft tissues) mineralization