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vita

vitamin A

see also:

introduction

  • although known that eating liver can Rx night blindness since ancient times, vitamin A was discovered in the 1920's
  • a family of compounds known as retinoic acids
  • 2 main forms:
    • pro-vitamin A carotenoids:
      • mainly found in plants and conversion to vitamin A is controlled thus overdosage does not occur
      • beta carotene is the main one converted to vitamin A in mammals
    • pre-formed vitamin A:
      • mainly found in animal livers and most vitamin A supplements
      • these are hydrolyzed into retinol in the lumen of the small intestine by retinyl ester hydrolases
      • there is no feedback regulation of this GIT absorption mechanism thus excessive ingestion may cause toxicity causing hypervitaminosis A
      • retinol, retinal, retinoic acid, and retinyl esters
  • 50 to 85% of total body retinol is stored in the liver
  • recommended daily intake:
    • adult males is 3000 IU (900 micrograms retinol) daily
    • adult females 2300 IU (700 micrograms retinol) daily.

physiologic actions

  • in retinal rod cells, it combines with a membrane-bound protein called opsin to make rhodopsin which is critical for night vision
  • in retinal cone cells, it forms iodopsin
  • it is crucial in the fetal development of tissues, in particular, the eye

vitamin A deficiency

aetiology

clinical features

  • night blindness
  • xerophthalmia with Bitot's spots and keratomalacia
  • poor bone growth
  • replacement of hair follicles with mucus-secreting glands
  • hyperkeratosis
  • phrynoderma (follicular hyperkeratosis)
  • impaired humoral and cell mediated immune system

hypervitaminosis A

  • serum levels do not correspond with toxicity as it is stored in the liver mainly
  • fasting serum retinyl ester concentrations >10 percent of the total vitamin A pool may indicate toxicity

aetiology

  • excessive vitamin A intake:
    • eating food rich in pre-formed vitamin A:
      • livers
      • kidney
      • egg yolk
    • overdose of pre-formed vitamin A supplements, or use with retinoids such as isotretinoin (Roaccutane)
  • toxicity may be increased in patients with:

acute toxicity syndrome

  • single dose of >660,000 IU (>200,000 micrograms) of vitamin A is ingested in adults
    • nausea, vomiting
    • vertigo
    • blurry vision
    • if very high dose: drowsiness, malaise, and recurrent vomiting
  • infants under 6 months may develop toxicity even if given doses > 20,000IU or 6000mcg for a few weeks

chronic toxicity syndrome

  • doses > 10x recommended daily dose
  • dry skin
  • nausea
  • headache (benign raised intracranial hypertension)
  • ataxia
  • hyperlipidaemia
  • hepatotoxicity and cirrhosis
  • bone and muscle pain
  • osteoporosis in post-menopausal women
  • visual impairment
  • hepatomegaly and elevated ALT, AST
  • many other symptoms

teratogenicity

  • doses only several times the recommended daily intake in the 1st trimester may cause:
    • spontaneous abortions
    • fetal malformations, including microcephaly and cardiac anomalies
vita.txt · Last modified: 2019/07/13 17:12 by wh