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Vitamin Physiology - Key Concepts & Models

Vitamins are micronutrients that the body cannot synthesize in adequate quantities and must obtain from the diet. They are divided into fat-soluble (A, D, E, K) and water-soluble (B-complex, C) groups, each with distinct absorption mechanisms, physiological roles, and deficiency states.

1. ABSORPTION MODELS

Fat-Soluble Vitamins (A, D, E, K) - Lipid Micelle Model

Fat-soluble vitamins are processed exactly like dietary lipids:

Incorporated into micelles in the intestinal lumen
Transported to the apical membrane of enterocytes
Diffuse across the membrane → packaged into chylomicrons
Extruded into lymph → delivered to systemic circulation via the thoracic duct

Vitamin A specifically uses membrane protein STRA6 for retinol uptake, while carotenoids enter via SR-B1, CD36, and NPC1L1. Inside the cell, retinol binds CRBP2 and is re-esterified by LRAT before chylomicron packaging.

Costanzo Physiology 7th Edition, p. 3098
Yamada's Textbook of Gastroenterology, p. 189

Water-Soluble Vitamins - Na⁺-Cotransport Model

Most water-soluble vitamins are absorbed via Na⁺-dependent cotransport in the small intestine.

Vitamin B12 (cobalamin) - the exception: This has a unique, multi-step model:

Pepsin in the stomach releases dietary B12 from food
Free B12 binds R proteins (haptocorrin) secreted in saliva
Pancreatic proteases in the duodenum degrade R proteins → B12 transfers to intrinsic factor (IF), a glycoprotein from gastric parietal cells
The B12-IF complex resists protease digestion and travels to the ileum, where specific receptors mediate absorption

Clinical consequence: After gastrectomy, loss of parietal cells = no intrinsic factor → B12 malabsorption → pernicious anemia. Oral B12 is useless - it must be given by injection.

Costanzo Physiology 7th Edition, p. 3103-3107

2. VITAMIN D ACTIVATION CASCADE

Vitamin D follows a two-step hydroxylation model before becoming biologically active:

Dietary Vitamin D3 (Cholecalciferol) — INACTIVE
         ↓  Liver (cytochrome P-450 enzyme)
25-Hydroxycholecalciferol (25-OHD) — INACTIVE (major circulating form)
         ↓  Kidney proximal tubule (1α-hydroxylase)
1,25-Dihydroxycholecalciferol (Calcitriol) — ACTIVE

Active Calcitriol actions:

Induces synthesis of calbindin D-28K (vitamin D-dependent Ca²⁺-binding protein) in intestinal epithelial cells → promotes Ca²⁺ absorption
Acts on kidney and bone for calcium homeostasis
The renal 1α-hydroxylation step is lost in CKD → declining calcitriol levels
Costanzo Physiology 7th Edition, p. 3112
Harrison's Principles of Internal Medicine 22E, p. 822-828

3. B-VITAMIN COENZYME MODELS (Harper's Biochemistry)

Each B vitamin functions as a coenzyme or coenzyme precursor in specific metabolic pathways:

Vitamin	Active Coenzyme	Key Metabolic Role	Deficiency Syndrome
B1 (Thiamin)	Thiamin diphosphate (TDP)	Pyruvate dehydrogenase, α-ketoglutarate DH, branched-chain keto acid DH; transketolase in pentose phosphate pathway	Beriberi (dry/wet), Wernicke-Korsakoff syndrome
B2 (Riboflavin)	FMN, FAD	Mitochondrial respiratory chain, fatty acid oxidation, citric acid cycle	Cheilosis, glossitis, seborrheic dermatitis (ariboflavinosis)
Niacin (B3)	NAD, NADP	Oxidation/reduction across metabolism; ADP-ribosylation for DNA repair; cyclic ADP-ribose signaling	Pellagra (3 Ds: Dermatitis, Diarrhea, Dementia)
B6 (Pyridoxine)	Pyridoxal phosphate	Transamination, decarboxylation of amino acids; heme synthesis	Seborrheic dermatitis, microcytic anemia, convulsions
B12 (Cobalamin)	Methylcobalamin, adenosylcobalamin	Methionine synthesis, methylmalonyl-CoA → succinyl-CoA	Megaloblastic anemia, subacute combined degeneration of spinal cord
Folate (B9)	Tetrahydrofolate (THF)	One-carbon transfers; nucleotide synthesis; cell division	Megaloblastic anemia; neural tube defects
Biotin	Biocytin	Carboxylation reactions (pyruvate carboxylase, acetyl-CoA carboxylase)	Alopecia, dermatitis, CNS abnormalities
Pantothenic acid	Coenzyme A (CoA)	Acyl group transfer in fatty acid and carbohydrate metabolism	Fatigue, GI symptoms, neurological symptoms

Thiamin - Structure

Thiamin (Vitamin B1) molecular structure

Harper's Illustrated Biochemistry 32nd Ed., p. 553-554

Niacin note: Niacin is not strictly a vitamin - it can be synthesized from the essential amino acid tryptophan (60 mg Trp = 1 mg niacin). Its NAD is also the substrate for ADP-ribosylation (DNA repair) and produces cyclic ADP-ribose, which mobilizes intracellular calcium in response to hormones/neurotransmitters.

4. VITAMIN K - CARBOXYLATION MODEL

Vitamin K acts as cofactor for γ-carboxylase, which converts glutamate (Glu) residues to γ-carboxyglutamate (Gla) in clotting factors and other proteins.

Proteins dependent on this reaction:

Coagulation factors: II (prothrombin), VII, IX, X (all require Gla for Ca²⁺ binding and membrane interaction)
Anticoagulant proteins: Protein C, Protein S
Bone proteins: Osteocalcin, matrix Gla protein - bind calcium, causing a conformational change to interact with membrane phospholipids
Gas6: Controls apoptosis and differentiation in nervous system

Osteocalcin levels in circulation serve as an index of vitamin D status.

Harper's Illustrated Biochemistry 32nd Ed., p. 321-323

5. VITAMIN A - VISUAL CYCLE MODEL

Vitamin A (retinol) is essential for:

Vision (rhodopsin cycle in rods)
Cell differentiation and proliferation (via retinoic acid nuclear receptors RAR/RXR)
Immune integrity

The retinol is stored in large quantities in the liver. Mutations in LRAT (the esterification enzyme) cause Leber congenital amaurosis and retinitis pigmentosa; mutations in RBP (retinol-binding protein) cause night blindness.

6. VITAMIN E - ANTIOXIDANT MODEL

Vitamin E (α-tocopherol) acts as a lipid-soluble chain-breaking antioxidant that scavenges free radicals and protects cell membranes from lipid peroxidation.

Uptake: via NPC1L1 and SR-BI at the apical enterocyte membrane
Secretion: via ABCA1 (apoA1 pathway) and chylomicrons (apoB pathway)
Deficiency causes: reduced RBC lifespan, spinocerebellar ataxia, loss of deep tendon reflexes, myopathies, night blindness

Ataxia with isolated vitamin E deficiency (AVED) is caused by mutations in the α-tocopherol transfer protein (α-TTP), preventing vitamin E incorporation into hepatic lipoproteins.

Yamada's Textbook of Gastroenterology, p. 194-198

7. VITAMIN C - COLLAGEN SYNTHESIS MODEL

Vitamin C (ascorbic acid) is the cofactor for prolyl hydroxylase and lysyl hydroxylase - enzymes that hydroxylate proline and lysine residues in procollagen. This hydroxylation is essential for:

Cross-linking of collagen fibers (structural integrity)
Stability of the triple helix

Scurvy (vitamin C deficiency) results from defective collagen formation, leading to:

Subcutaneous hemorrhage, perifollicular hemorrhages
Swollen, bleeding gums
Poor wound healing
Aching bones and joints
Basic Medical Biochemistry - A Clinical Approach 6e

Quick Reference - Deficiency Models

Vitamin	Classic Deficiency Mnemonic/Model
A	Night blindness → Xerophthalmia → Bitot's spots
D	Rickets (children) / Osteomalacia (adults)
E	Spinocerebellar ataxia, areflexia
K	Bleeding diathesis (PT prolonged)
B1	Beriberi (wet = cardiac; dry = neural) / Wernicke-Korsakoff
B2	Ariboflavinosis - "cheilosis, corneal vascularization"
B3	Pellagra - "4 Ds: Dermatitis, Diarrhea, Dementia, Death"
B6	Sideroblastic anemia, peripheral neuropathy
B9	Megaloblastic anemia + neural tube defects
B12	Megaloblastic anemia + subacute combined degeneration
C	Scurvy - perifollicular hemorrhage, corkscrew hairs

These models cover the major physiological frameworks used to understand vitamins: absorption (micelle vs. Na⁺-cotransport), activation cascades (Vitamin D), coenzyme function (B-vitamins), antioxidant protection (E), carboxylation (K), and biosynthetic cofactor roles (C). Each provides a distinct teaching model that integrates biochemistry, physiology, and clinical medicine.

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