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Updated: Mar 24 2021

Vitamin A

  • Snapshot
    • A 33-year-old man presents to a free clinic with eye itchiness. He notices these abnormal white spots over the sclera of his eyes. He constantly experiences dryness in his eyes without clear cause. He is currently homeless with poor access to food and water for the past several months. Physical examination is notable for conjunctival keratinazation.
  • Introduction
    • Overview
      • a lipid-soluble vitamin that is also known as retinoic acid
        • provitamin A (beta-carotene)
          • derived from plants
        • preformed vitamin A (retinol, retinal, retinoic acid, and retinyl esters)
          • derived from animal sources
          • hydrolyzed into retinol in the small intestine
    • Function
      • vision
        • xerophthalmia prevention
        • phototransduction
          • rod cells
            • all-trans-retinol is converted to 11-cis-retinol to combine with opsin to form rhodopsin
              • a similar reaction occurs in the cone cells to form iodopsin
      • cellular differentiation
        • important for cellular differentiation of the cells in the conjunctiva and retina
          • these tissues have retinol-binding proteins
    • Deficiency
      • xerophthalmia
        • corneal and conjunctival dryness
          • secondary to impaired lacrimal gland function
          • can result in Bitot spots
            • abnormal conjunctival squamous cell proliferation and keratinization
      • night blindness (nyctalopia)
      • follicular hyperkeratosis
        • dry skin due to loss of sebaceous gland function
      • humoral and cell-mediated immune system impairment
    • Excess
      • idiopathic intracranial hypertension
      • teratogenicity
        • a pregnancy test must be done before isotretinoin is prescribed for severe acne and the patient should use contraception if sexually active
      • carotenemia
        • yellowing of the skin
    • Therapeutic uses
      • acute promyelocytic leukemia
        • treated with all-trans retinoic acid (ATRA)
      • mealses
        • treated with vitamin A in resource-limited countries
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