Vitamin A

Vitamin A

1. Generic term for a collection of 3 vitamins collectively called retinoids:

   • Retinol (alcohol form)
   • Retinal (aldehyde form)
   • Retinoic acid (acid form)
   • Each form performs a specific function
   • In metabolism, cis and trans forms have different roles

2. Retinol is the physiologically-active form

3. Provitamin A: β-carotene (a carotenoid or pigment) in yellow/ orange foods

4. Carotenoids

   • Carotenoids are phytochemicals that are not nutrients but have biological activity in body
     • Food from plant sources provide carotenoids, some of which have vitamin A activity
       • β-carotene is the most important carotenoid
         • Like retinol, carotenoids are absorbed dissolved in lipids
           • Biological activity and absorption depends on food nature (cooked or raw), and the amount of fat in food
   • Carotenoids Physiologic Effects
     • 1. Pro-Vitamin A
     • 2. Serving as an “antioxidant”
       • a) Protecting from cancer and heart disease
         • b) Protects cell membranes and LDL from oxidation
           • c) Protects skin from sun UV rays
     • 3. Enhances immune system

5. Functions:

   • Vision, especially night vision
     • Differentiation and maintenance of epithelial tissues
       • Gene expression
         • Support immune system
           • Fertility
             • Growth promotion
               • Maintenance of bone and teeth remodelling, intestinal and skin health.
                 • Essential for glycoproteins synthesis (GAGs and PGs) and mucus secretion.

6. Deficiency

   • Night blindness (the 1st symptom of vitamin A deficiency)
     • Rare in industrialized world but the leading cause of blindness in areas of poverty
       • Xerophthalmia
         • Keratinization
           • Acne and psoriasis
             • Impotence
               • Growth retardation
   • Signs and symptoms
     • Cornea kertainization, followed by ulceration → keratomalacia
       • Skin lesions, such as follicular hyperketatosis due to deficient mucus secretion.
         • Increased susceptibility to infectious diseases → impairment of immune responses
   • Diagnosis
     • Bitot’s spots may be seen (white plaques of Keratinized epithelial cells on conjunctiva)

7. Toxicity

   • Excessive intake lead to accumulation in liver and other tissues.
     • Children are most vulnerable to toxicity because they need less and are more sensitive to overdoses
       • Nausea, vomiting, abdominal pain, drowsiness and headache in acute poisoning.
         • In chronic poisoning, fatigue, insomnia, bone pain, loss of hair and skin desquamation are common. Skeletal malformations, spontaneous fractures, internal hemorrhages
           • Occurs commonly due to excessive use of vitamin preparations without medical advice.
             • Adolescents taking massive doses to fight acne can be toxic. Some prescribed medication is highly toxic, and has caused birth defects
               • Overconsumption of β-carotene is harmless may turn skin yellow

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