Introduction to Vitamins
Vitamins are low-molecular-weight organic compounds that serve as essential micronutrients in human nutrition. Though they do not supply energy directly, they act as critical cofactors or coenzymes in a multitude of biochemical reactions necessary for cellular metabolism, growth, and physiological function. Deficiencies in specific vitamins can result in a broad spectrum of pathologies, many of which can have systemic and long-term health impacts.
Vitamins are broadly classified according to their solubility properties: fat-soluble (Vitamins A, D, E, and K) and water-soluble (Vitamin C and the B-complex group, including B1 through B12). Fat-soluble vitamins are absorbed alongside dietary lipids and stored predominantly in hepatic and adipose tissues. In contrast, water-soluble vitamins are absorbed via diffusion or carrier-mediated transport and are excreted readily, necessitating consistent dietary intake.
Chemistry and Function of Each Vitamin
1. Fat-Soluble Vitamins
Vitamin A (Retinoids and Carotenoids)
- Chemistry: Retinol, retinal, retinoic acid, and provitamin carotenoids like beta-carotene.
- Physiological Role: Visual pigment synthesis, epithelial differentiation, immune modulation, embryogenesis.
- Bioavailability: Improved with dietary fat; cooking may enhance absorption.
- Deficiency: Night blindness, xerophthalmia, keratinization, immune dysfunction.
Vitamin D (Cholecalciferol and Ergocalciferol)
- Chemistry: Vitamin D2 from plants; D3 synthesized in skin upon UVB exposure.
- Physiological Role: Calcium/phosphorus absorption, bone mineralization, immune regulation.
- Bioavailability: Sunlight and fats enhance absorption; malabsorption reduces it.
- Deficiency: Rickets, osteomalacia, osteoporosis.
Vitamin E (Tocopherols and Tocotrienols)
- Chemistry: Eight isomers; alpha-tocopherol is the most active form.
- Physiological Role: Lipid antioxidant, membrane stabilization, gene expression modulation.
- Bioavailability: Depends on fat intake; vulnerable to oxidative degradation.
- Deficiency: Hemolytic anemia, neuromuscular disorders.
Vitamin K (Phylloquinone and Menaquinones)
- Chemistry: K1 from plants; K2 synthesized by gut bacteria.
- Physiological Role: Blood clotting protein activation, bone health.
- Bioavailability: Enhanced by dietary fat; impaired by lipid malabsorption.
- Deficiency: Bleeding disorders, poor bone mineralization.
2. Water-Soluble Vitamins
Vitamin B1 (Thiamine)
- Chemistry: Pyrimidine + thiazole ring.
- Role: TPP coenzyme in energy metabolism, transketolase reactions.
- Deficiency: Beriberi, Wernicke-Korsakoff syndrome.
Vitamin B2 (Riboflavin)
- Chemistry: Isoalloxazine ring with ribitol side chain.
- Role: Precursor of FAD and FMN in oxidative metabolism.
- Deficiency: Glossitis, cheilitis, ariboflavinosis.
Vitamin B3 (Niacin)
- Chemistry: Nicotinic acid and nicotinamide.
- Role: Component of NAD/NADP in redox reactions.
- Deficiency: Pellagra (dermatitis, diarrhea, dementia).
Vitamin B5 (Pantothenic Acid)
- Role: Coenzyme A synthesis, fatty acid metabolism.
- Deficiency: Fatigue, numbness, rare symptoms.
Vitamin B6 (Pyridoxine)
- Role: PLP coenzyme for amino acid metabolism, neurotransmitter biosynthesis.
- Deficiency: Microcytic anemia, neuropathy, mood changes.
Vitamin B7 (Biotin)
- Role: Coenzyme in carboxylation reactions (gluconeogenesis, lipid metabolism).
- Deficiency: Dermatitis, alopecia, depression.
Vitamin B9 (Folate)
- Role: One-carbon metabolism for nucleotide synthesis and methylation.
- Deficiency: Megaloblastic anemia, neural tube defects.
Vitamin B12 (Cobalamin)
- Role: DNA synthesis, methylation, neurological function.
- Deficiency: Pernicious anemia, neuropathy, cognitive decline.
Vitamin C (Ascorbic Acid)
- Role: Antioxidant, collagen synthesis, iron absorption enhancer.
- Deficiency: Scurvy: bleeding gums, joint pain, anemia.
Dietary Requirements and Bioavailability
Vitamin requirements are defined by RDAs based on age, gender, and physiological status. Fat-soluble vitamins are stored and may accumulate to toxic levels, while water-soluble vitamins require daily intake due to urinary excretion (except B12).
Bioavailability depends on:
- Cooking methods (e.g., heat degradation vs. increased release)
- Presence of alcohol or tobacco (which inhibit absorption)
- Nutrient-nutrient interactions (e.g., Vitamin C enhances iron uptake)
- Health of the GI tract
Nutritional Deficiency Diseases Summary
| Vitamin | Deficiency Disorder |
|---|---|
| Vitamin A | Xerophthalmia, Night blindness, Keratomalacia |
| Vitamin D | Rickets, Osteomalacia, Osteoporosis |
| Vitamin E | Hemolytic anemia, Ataxia, Neuropathy |
| Vitamin K | Hemorrhagic disease, Osteopenia |
| Vitamin B1 | Beriberi, Wernicke-Korsakoff syndrome |
| Vitamin B2 | Glossitis, Cheilitis, Ariboflavinosis |
| Vitamin B3 | Pellagra (3 Ds) |
| Vitamin B5 | Fatigue, Numbness, Rare symptoms |
| Vitamin B6 | Microcytic anemia, Neuropathy |
| Vitamin B7 | Dermatitis, Hair loss |
| Vitamin B9 | Megaloblastic anemia, Neural tube defects |
| Vitamin B12 | Pernicious anemia, Cognitive decline |
| Vitamin C | Scurvy, Anemia, Hemorrhage |