OVERVIEW oF VITAMIN A DEFICIENCY
- Vitamin A: A family of lipid-soluble compounds (retinols, β-carotenes [provitamin A], and carotenoids) known as retinoic acids.
- Key roles: Phototransduction (particularly night vision), epithelial tissue growth/differentiation, bone growth, and immune function.
DIETARY SOURCES & ABSORPTION
- Food sources:
- Retinol-rich foods: Liver, egg yolk, kidneys, fish oils, and butter.
- β-Carotene sources: Green leafy vegetables (e.g., spinach), carrots, sweet potatoes, apricots, tomatoes, pimentos.
- β-Carotene → Vitamin A: In the GI tract, 1 molecule β-carotene can yield 2 molecules of vitamin A.
- Absorption:
- Occurs in jejunum and ileum.
- After absorption, enterocytes form retinyl-esters → packaged into chylomicrons → lymphatics → plasma.
- Chylomicron remnants (with apo B and apo E) are taken up by the liver, and vitamin A is stored in hepatic stellate cells.
- 50-85% of total body vitamin A is stored in the liver.
- Upon release, vitamin A circulates bound to retinol-binding proteins (RBPs).
FUNCTIONS
- Vision:
- Necessary for retinal photoreceptor function (especially rod cells for night vision).
- Deficiency → decreased retinal 11-cis-retinol and rhodopsin, hence night blindness (nyctalopia).
- Epithelial Tissue:
- Involved in growth/differentiation of epithelial cells.
- Deficiency → dryness (xerosis) and keratinization of mucosal surfaces (e.g., eyes).
- Bone Growth:
- Supports normal bone development.
- Immune Function:
- Involved in maintaining immune competence.
CLINICAL MANIFESTATIONS OF DEFICIENCY
- Night Blindness (nyctalopia): Earliest symptom of vitamin A deficiency.
- Xerophthalmia: Progressive eye disorder with:
- Xerosis (dryness of the conjunctiva)
- Bitot spots (triangular grayish patches of keratinized epithelium on sclera)
- Keratomalacia (corneal thinning) → can lead to perforation and blindness.
- Other Symptoms:
- Follicular hyperkeratosis on skin.
- Impaired bone growth.
DIAGNOSIS
- Often due to inadequate dietary intake or malabsorption.
- Serum vitamin A (retinol) level <100 μg/L indicates severe deficiency (normal reference range: ~325–780 μg/L in adults).
TREATMENT
- Severe deficiency (e.g., xerophthalmia):
- High-dose therapy: ~60 mg of vitamin A immediately, then repeated 1 and 14 days later.
- Milder deficiency (night blindness, Bitot spots):
- Lower doses: ~3 mg daily for 3 months.
- Also address underlying causes (poor diet or malabsorption).
PREVENTION
- Recommended daily allowance:
- Men: 900 μg retinol/day
- Women: 700 μg retinol/day
- Pregnancy/lactation: ~1.4 mg/day
- 1 μg retinol = 12 μg β-carotene.
- Met by balanced diet rich in milk, eggs, fish, butter, yellow/dark green vegetables.