Vitamin A: Sources, RDA, Functions, and Deficiency

Characterization and Forms of Vitamin A

  • Vitamin A is classified as a fat-soluble vitamin.
  • The active forms of the vitamin are found exclusively in animal tissues.
  • The pro-vitamin form, known as beta-carotene, is found in plant tissues.
  • The collective term for all compounds possessing vitamin A activity is retinoids.
  • There are three distinct chemical compounds recognized for their vitamin A activity:
    • Retinol: The alcohol form of vitamin A.
    • Retinal: The aldehyde form of vitamin A.
    • Retinoic acid: The acid form of vitamin A.

Dietary Sources of Vitamin A

  • Animal-based sources include:
    • Milk, butter, cream, and cheese.
    • Egg yolk.
    • Liver.
    • Fish liver oils, which are considered exceptionally rich sources of the vitamin.
  • Plant-based sources include:
    • Carrots, papaya, mango, and pumpkins.
    • Green leafy vegetables, specifically spinach and amaranth, are excellent sources of vitamin A activity.
  • Notably, the biological activity of vitamin A is not destroyed during the cooking process.

Recommended Dietary Allowance (RDA)

  • Children: 400−600 μg/day400 - 600\,\mu g/day
  • Men: 750−1000 μg/day750 - 1000\,\mu g/day
  • Women: 750 μg/day750\,\mu g/day
  • Pregnancy: 1000 μg/day1000\,\mu g/day

Biochemical Functions in Vision (Wald's Visual Cycle)

  • Vitamin A is a fundamental requirement for the maintenance of normal vision. Deficiency results in significant visual disturbances.
  • In the eye, the retinal form of vitamin A combines with a protein called opsin to synthesize rhodopsin.
  • Rhodopsin is a critical light-absorbing molecule necessary for color vision and for visual acuity in dim light.
  • The visual cycle (Wald's cycle) involves the following components and steps:
    • Rhodopsin (11-cis-retinal-opsin11\text{-cis-retinal-opsin}) is struck by light (photons).
    • This interaction triggers a nerve impulse and causes the molecule to split into Opsin and All-trans-retinal.
    • All-trans-retinal is converted to All-trans-retinol by the enzyme Alcohol dehydrogenase, utilizing NADH+H+→NAD+NADH + H^+ \rightarrow NAD^+.
    • All-trans-retinol is transported to the liver, where an Isomerase converts it to 11-cis-retinol11\text{-cis-retinol}.
    • In the liver, 11-cis-retinol11\text{-cis-retinol} is converted to 11-cis-retinal11\text{-cis-retinal} via Alcohol dehydrogenase, utilizing NAD+→NADH+H+NAD^+ \rightarrow NADH + H^+.
    • Alternatively, All-trans-retinal can be converted back to 11-cis-retinal11\text{-cis-retinal} within the eye via an Isomerase.
    • 11-cis-retinal11\text{-cis-retinal} recombines with Opsin to reform Rhodopsin, completing the cycle.

Systemic Biochemical Functions

  • Immune System: Vitamin A is essential for the maintenance of healthy immune function. A deficiency state directly leads to increased susceptibility to infections.
  • Cell Growth and Reproduction: Retinoic acid functions as a hormone-like growth factor, playing a key role in the development of epithelial cells and other various cell types throughout the body.
  • Dermatological and Ocular Health: The vitamin prevents the drying of the skin and the eyes.
  • Structural Development: It is required for the proper formation of teeth and soft tissues.
  • Hematopoiesis: It is involved in the formation of white blood cells.
  • Antioxidant Properties: Specifically in the form of beta-carotene, vitamin A acts as an effective antioxidant.

Vitamin A Deficiency Disorders (VADD)

  • Etiology (Causes):
    • Decreased dietary intake.
    • General malnutrition.
    • Malabsorption syndromes.
  • Clinical Manifestations:
    • Night Blindness: Patients fail to see objects in dim or poor light because the rod cells in the retina become less sensitive to light due to the depletion of the visual pigment rhodopsin.
    • Xerophthalmia: Progressively severe drying of the eye.
    • Keratomalacia: Damage and softening of the cornea.
    • Keratinization: Hardening of epithelial structures throughout the body.
    • Bitot's Spots: Visible lesions on the conjunctiva.
    • Anemia.
    • Decreased resistance to infections.
    • Poor physical growth.
    • Impaired reproductive system functionality.
  • Diagnosis:
    • A clinical increase in dark adaptation time serves as a primary diagnostic indicator.

Hypervitaminosis A (Toxicity)

  • Because vitamin A is fat-soluble and can be stored in the liver, excessive intake over a long period results in toxic effects.
  • Toxicity Threshold: Chronic daily intake of 25,000−50,000 μg/day25,000 - 50,000\,\mu g/day can result in hypervitaminosis A.
  • Symptoms of Toxicity:
    • Nausea and vomiting.
    • Diarrhea.
    • Rough, abnormal skin texture.
    • Hepatomegaly (enlargement of the liver).
    • Splenomegaly (enlargement of the spleen).