Vitamin A

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Introduction

First vitamin identified! (in 1912)

    Preformed Vitamin A (animal sources): Retinoids (retinal, retinol, retinoic acid)

     

    Provitamin A (plant sources): Carotenoids (beta-carotene, alpha-carotene, beta-cryptoxanthin) - Must be converted to retinoid form.

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    Daily Requirements

    The current, more precise method of measuring vitamin A is via Retinol activity equivalent (RAE).Food labels have not adapted to new way (RAE) so generally speaking any values listed on food labels provide less vitamin A than the RE or IU values suggest.

    RDA (for adults 19y+)

      • 900 µg (RAE) for men
      • 700 µg (RAE) for women

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      Sources

      Preformed Vitamin A Contributes ~70% of vitamin A intake for Americans

      It is found in:

      • liver
      • fish oils
      • fortified milk
      • eggs
      • other fortified foods

      Provitamin A Carotenoids: Dark leafy green and yellow-orange vegetables/fruits

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      Biological Roles

      • transported and stored in the liver
      • carried to nucleus, where it controls growth, reproduction, epithelial cell maitenance, visual cycle

      Vision

      • Retinal turns visual light into nerve signals in retina of eye
      • Retinoic acid required for structural components of eye
      • Cones in the retina
      • Responsible for vision under bright lights
      • Translate objects to color vision
      • Rods in the retina
      • Responsible for vision in dim lights
      • Translate objects to black and white vision

      Growth and differentiation of cells

      • Retinoic acid is necessary for cellular differentiation
      • Important for embryo development, gene expression
      • Retinoic acid influences production, structure, and function of  epithelial cells that line the outside (skin) and external passages (mucus forming cells) within the body

      Possible Roles of Carotenoids (due to antioxidant capabilities)

      • Cancer prevention
      • Cardiovascular disease prevention
      • Age-related macular degeneration
      • Cataracts

       

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      Deficiency

      Night blindness; number one cause of pediatric blindness worldwide

      Decreased mucus production

      Decreased immunity

      Bacterial invasion of the eye

      Conjunctival xerosis

      Bitot’s spots

      Xerophthalmia

      Irreversible blindness

      Follicular hyperkeratosis

      Poor growth

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      Overdose

      The upper limit is: 3000 µg retinol
      Acute – short-term megadose (100 x RDA); symptoms disappear when intake stops

      • GI effects: nausea and vomiting
      • Headaches
      • Blurred vision
      • Poor muscle coordination

      Chronic – long-term megadoses; possible permanent damage

      • Bone and muscle pain
      • Loss of appetite
      • Skin disorders: dryness, peeling skin, alopecia
      • Headache
      • Increased liver size
      • Vomiting

      Teratogenic (may occur with as little as 3 x RDA of preformed vitamin A)

      • Tends to produce physical defect on developing fetus as a result of excess vitamin A intake
      • Spontaneous abortion
      • Birth defects

      (Note: Toxicity of Carotenoids is not likely, as rate of conversion of carotenoids to retinoic acid by liver is slow and efficiency of absorption of carotenoids decreases as intake increases.)

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      Resources and References

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