Fat-soluble vitamins (A, D, E, K)

Vitamin A - Retinoid Rave

• Functions: Antioxidant; essential for vision, immune function, and epithelial cell differentiation.
  • Vision: As retinal, a component of rhodopsin.
  • Gene Transcription: As retinoic acid, binds nuclear receptors.
• Deficiency:
  • Ocular: Nyctalopia (night blindness), xerophthalmia (dry eyes), Bitot's spots (keratin debris on conjunctiva).
  • Immune: Impaired immunity, especially to measles.
• Toxicity (Hypervitaminosis A):
  • Acute: Nausea, vomiting, vertigo.
  • Chronic: Alopecia, dry skin, hepatosplenomegaly, pseudotumor cerebri.
  • ⚠️ Teratogenic: Requires negative pregnancy test before isotretinoin prescription.

⭐ All-trans-retinoic acid (ATRA) is a key treatment for Acute Promyelocytic Leukemia (APL), promoting differentiation of malignant promyelocytes.

Vitamin D - Sunshine Steroid

• Forms: D₂ (ergocalciferol) from plants; D₃ (cholecalciferol) from sun exposure on skin (7-dehydrocholesterol).
• Activation Pathway: A steroid hormone, requires hydroxylation in liver, then kidney.

• Function: ↑ intestinal absorption of Ca²⁺ & PO₄³⁻; ↑ bone resorption to maintain blood Ca²⁺ levels.
• Deficiency:
  • Children: Rickets (soft, bowing bones).
  • Adults: Osteomalacia (soft bones, ↓ mineralization).
  • Presents with hypocalcemia, hypophosphatemia.
• Toxicity (Hypervitaminosis D): Hypercalcemia, hypercalciuria (kidney stones), confusion.

⭐ Granulomatous diseases (e.g., sarcoidosis, TB) can cause hypercalcemia via extra-renal activation of Vitamin D by macrophages.

Vitamin E - Antioxidant Ace

• Function: Major lipid-soluble antioxidant; protects cell membranes and RBCs from oxidative damage (lipid peroxidation).

  • Neutralizes free radicals, preventing propagation of cell injury.

• Deficiency: Rare, typically seen in fat malabsorption.

  • Hemolytic anemia: Due to ↑ RBC fragility.
  • Neurologic dysfunction:
    • Demyelination of posterior columns → ↓ proprioception, vibration sense.
    • Spinocerebellar tract demyelination → ataxia.

⭐ Vitamin E deficiency can mimic Friedreich ataxia.

📌 E is for Erythrocyte protection.

Vitamin K - Clotting King

• Function: Cofactor for γ-glutamyl carboxylase, which carboxylates glutamate residues on clotting factors II, VII, IX, X, and anticoagulant proteins C & S.
  • This carboxylation is essential for Ca²⁺ binding and function.
• Source: Synthesized by intestinal flora; also in green leafy vegetables.
• Deficiency: Leads to coagulopathy.
  • Causes: Newborns (sterile gut), prolonged broad-spectrum antibiotic use, malabsorption (e.g., cystic fibrosis, celiac disease).
  • Labs: ↑ PT/INR, normal bleeding time.
  • Presents as hemorrhagic disease of the newborn; easy bruising, mucosal bleeding.

⭐ Warfarin inhibits vitamin K epoxide reductase, the enzyme that recycles vitamin K to its active, reduced form. This prevents the activation of clotting factors.

High‑Yield Points - ⚡ Biggest Takeaways

• Vitamin A is crucial for vision; deficiency causes night blindness and xerophthalmia. Excess is teratogenic.
• Vitamin D regulates calcium & phosphate; deficiency leads to rickets (children) and osteomalacia (adults).
• Vitamin E is a key antioxidant; deficiency can cause hemolytic anemia and neuromuscular deficits.
• Vitamin K is essential for coagulation factors II, VII, IX, X; deficiency results in bleeding. Warfarin inhibits its action.
• Newborns receive vitamin K to prevent hemorrhagic disease.