Vitamin A


Introduction to Vitamin A

  • Other names
    • Preformed Vitamin A – retinyl esters
      • Retinol, Retinal, Retinoic acid
      • Sources: animal foods, fortified foods, pharmaceutical supplements
  • Provitamin A – Precursors = carotenoids
    • Beta-carotene and other carotenoids
    • Sources: Plant foods

SOURCES

  • Beta-carotene
    • Dark leafy green vegetables, Spinach, Broccoli
    • Deep orange veggies
      • Carrots, Pumpkin, Squash, Sweet potato
    • Deep orange fruits
      • Apricots, Cantaloupe
  • Retinol

– Fortified milk, Butter,     

Cheese, Cream

– Fortified margarine

– Eggs

– Liver

Required daily allowance

Men: 900 μg RAE/day

Women: 700 μg RAE/day

RAE = Retinol Activity Equivalents

1 microgram of retinol

12 micrograms of beta-carotene

24 micrograms of other precursor carotenoid

Upper level for adults: 3000 μg /day

BIOCHEMICAL FUNCTIONS

  • Chief functions in the body

i. Vision

ii. Maintenance of cornea, epithelial cells, 

    mucous membranes, skin

iii. Bone and tooth growth

iv. Reproduction

v. Immunity

vi. Antioxidant effect of beta-carotene

VITAMIN A IN VISION

VITAMIN A IN VISION

VITAMIN A IN EPITHELIUM

  • Function in protein synthesis and cell differentiation
VITAMIN A IN EPITHELIUM

DEFICIENCY

  • Night blindness
  • Xerosis (corneal drying)
  • Bitot’s spots
  • Karatomalacia
  • Xerophtalmia
  • Hyperkaratosis
  • Impaired immunity

TOXICITY

  • Toxicity from provitamin A impossible

– Conversion of carotenoids to retinal highly regulated by the body

– Homeostatic mechanisms control tightly

  • Toxicity from preformed A inevitable

– Efficient absorption and hepatic storage of A

– Storage continues until pathologic condition develops; liver stores ~80% of body reserves

 

TOXICITY

  • Large % of population in developed nations 

have intakes of preformed vitamin A higher than the RDA

75% of people may be routinely ingesting more than RDA

  • Some studies suggest that as little as twice the 
    • RDA intake may contribute to subclinical  Vitamin A toxicity.

ACUTE TOXICITY

  • Occurs when adults ingest >100x RDA of preformed Vitamin A for a period of hours or several days
  • Occurs when children ingest >20x RDA of preformed Vitamin A for same period
  • Less of a problem than chronic toxicity
  • Acute toxicity symptoms
    • Blurred vision
    • Nausea, vomiting, vertigo
    • Increase of pressure inside skull, 
    • mimicking brain tumor
    • Headaches

CHRONIC TOXICITY

  • Occurs when adults ingest
    • >25,000 IU preformed Vitamin A for >6 years
    • >100,000 IU preformed Vitamin A for >6 months
  • Wide individual variabilty
  • Children particularly sensitive to daily intakes of 1500 IU/kg body weight
  • Elderly at significantly greater risk
  • Chronic toxicity symptoms
    • Increased activity of osteoclasts causing reduced bone density
    • Liver abnormalities
    • Birth defects

MACULAR DEGENERATION 

  • Studies in the elderly suggest that the use of large doses of certain vitamins and minerals are beneficial in the prevention of macular degeneration
  • Some evidence exists that improvement in existing damage may be seen
VITAMIN A &
MACULAR DEGENERATION
  • It directs the process of borrowing and redepositing calcium in the bone
  • Too much preformed Vitamin A (retinol) can promote fractures
  • It is in form of beta-carotene, a pre-curser form which does not increase fractures

Other Biochemistry Notes :-

Pyrimidine metabolism

Purine Metabolism

Cardiac Biomarkers

. Blood Metabolism (Heme synthesis and breakdown)

Haemoglobinopathies

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