Regarding the absorption and metabolism of fat-soluble vitamins, all of the following statements are TRUE EXCEPT:

Explanation

## Analysis of Fat-Soluble Vitamin Absorption and Metabolism ### Correct Statements (Options 0, 1, 2) **Option 0 — Vitamin A Absorption:** - Requires bile salts for micelle formation and dietary fat for solubilization - Carotenoid-binding proteins (e.g., β-carotene oxygenase) facilitate conversion of dietary carotenoids to retinol - ✓ TRUE **Option 1 — Vitamin D Metabolism:** - Skin synthesis: 7-dehydrocholesterol → previtamin D~3~ → vitamin D~3~ (via UVB) - Liver: vitamin D~3~ → 25-hydroxyvitamin D~3~ [calcifediol] (storage form) - Kidney: 25-hydroxyvitamin D~3~ → 1,25-dihydroxyvitamin D~3~ [calcitriol] (active form) - ✓ TRUE **Option 2 — Vitamin E Absorption:** - α-tocopherol is the most biologically active form - Acts as a chain-breaking lipophilic antioxidant (protects polyunsaturated fatty acids) - TPGS is a water-soluble derivative used therapeutically in fat malabsorption syndromes - ✓ TRUE ### Incorrect Statement (Option 3) — **CORRECT ANSWER** **Option 3 — Vitamin K Activation:** - Vitamin K is absorbed in the **proximal small intestine** (duodenum and jejunum), NOT terminal ileum and colon - Vitamin K undergoes **reduction by vitamin K reductase** (not vitamin C) to form the hydroquinone (reduced) form - The hydroquinone form is the active cofactor for γ-carboxylation of glutamic acid residues in clotting factors (II, VII, IX, X) and bone proteins - ✗ FALSE — contains two errors ### Key Point: **Vitamin K activation depends on the enzyme vitamin K reductase (also called vitamin K epoxide reductase), NOT vitamin C. Vitamin C is a water-soluble antioxidant and has no role in vitamin K reduction.** ### High-Yield: **All fat-soluble vitamins (A, D, E, K) share:** - Requirement for bile salts and dietary fat for absorption - Storage in adipose tissue and liver - Lipophilic nature - Potential for toxicity with excessive supplementation (except vitamin E, which has low toxicity) ### Clinical Pearl: **Vitamin K deficiency occurs in:** - Antibiotic use (disrupts gut flora that synthesize vitamin K) - Biliary obstruction (impaired fat absorption) - Malabsorption syndromes (celiac, Crohn's disease) - Warfarin therapy (inhibits vitamin K reductase, preventing γ-carboxylation)