Vitamins, Classification, List, Function, Deficiency, Disease

Vitamins are organic micronutrients required for metabolic functions, immunity, growth, and cell repair. They cannot be synthesized in sufficient amounts by the human body and must be obtained from food or supplements. They act as coenzymes, antioxidants, hormone precursors and regulators of metabolic pathways to regulate enzyme activity, help maintain cell health, support immune mechanisms, and prevent several deficiency disorders.

Vitamins Classification

Vitamins consist of 13 essential micronutrients performing specific biochemical and physiological functions for human health. Vitamins are classified into fat-soluble and water-soluble groups based on their absorption, storage, and excretion behaviour within the human body:

Fat-Soluble Vitamins

Fat-soluble Vitamins dissolve in lipids, stored in liver and adipose tissues, and include vitamins A, D, E, and K.

Water-Soluble Vitamins

Water-soluble Vitamins dissolve in water, are not stored significantly in the body, and include Vitamin C and Vitamin B-complex group.

List of 13 Vitamins

The 13 essential Vitamins and their functions & characteristics has been given below:

1. Vitamin A (Retinol, Beta-carotene): Essential for vision, immunity, epithelial tissue health, and reproductive function. WHO reports Vitamin A deficiency as a major global problem. Sources- Liver, eggs, dairy, carrots, spinach, etc.
2. Vitamin D (Calciferol): Functions like a hormone regulating calcium-phosphate metabolism and bone health. NIH identifies Vitamin D deficiency as widespread, especially in high-latitude regions. Sources- Sunlight, fortified milk, eggs, fatty fish, etc.
3. Vitamin E (Tocopherol): Acts as a powerful antioxidant protecting cell membranes from oxidative damage. Needed for neurological health and immune function. Sources- Nuts, seeds, vegetable oils, green vegetables, etc.
4. Vitamin K (Phylloquinone, Menaquinone): Required for clotting factor synthesis. Deficiency increases bleeding risk. Newborns receive Vitamin K injections to prevent haemorrhagic disease. Sources- Green leafy vegetables, fermented foods, etc.
5. Vitamin C (Ascorbic Acid): Strong antioxidant required for collagen synthesis, immunity, and iron absorption. Deficiency causes scurvy. Sources- Citrus fruits, tomatoes, peppers, guava, etc.
6. Vitamin B1 (Thiamine): Critical for carbohydrate metabolism and nerve conduction. Deficiency causes beriberi and Wernicke-Korsakoff syndrome. Sources- Whole grains, legumes, etc.
7. Vitamin B2 (Riboflavin): Supports energy production, skin health, and enzyme activity. Deficiency presents as cheilosis and dermatitis. Sources- Milk, eggs, green vegetables, etc.
8. Vitamin B3 (Niacin): Involved in NAD/NADP formation needed for redox reactions. Deficiency causes pellagra with dermatitis, diarrhoea, and dementia. Sources- Meat, fish, grains, etc.
9. Vitamin B5 (Pantothenic Acid): Required for Coenzyme-A synthesis and fatty acid metabolism. Sources- Eggs, legumes, vegetables, etc.
10. Vitamin B6 (Pyridoxine): Needed for amino acid metabolism, neurotransmitter synthesis, and haemoglobin formation. Sources- Poultry, fish, bananas, etc.
11. Vitamin B7 (Biotin): Important for carbohydrate, fat, and protein metabolism. Deficiency causes hair loss and skin inflammation. Sources- Eggs, nuts, legumes, etc. 
12. Vitamin B9 (Folate): Key for DNA synthesis and red blood cell formation. Deficiency leads to megaloblastic anaemia and neural tube defects. Sources- Leafy vegetables, legumes, fortified grains, etc.
13. Vitamin B12 (Cobalamin): Required for myelin formation and DNA synthesis. Deficiency common among vegetarians, causing pernicious anaemia. Sources- Meat, dairy, fish, fortified cereals, etc.

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Vitamin Deficiency

Deficiency of Vitamin causes various disease, disorders and clinical symptoms as highlighted below:

• Vitamin A: Night blindness, xerophthalmia
• Vitamin D: Rickets (children), osteomalacia (adults)
• Vitamin E: Neuropathy, haemolytic anaemia
• Vitamin K: Excessive bleeding
• Vitamin C: Scurvy, poor wound healing
• Vitamin B1: Beriberi, Wernicke encephalopathy
• Vitamin B2: Cheilosis, glossitis
• Vitamin B3: Pellagra
• Vitamin B5: Fatigue, neuropathy
• Vitamin B6: Dermatitis, anaemia, seizures
• Vitamin B7: Dermatitis, hair loss
• Vitamin B9: Megaloblastic anaemia, neural tube defects
• Vitamin B12: Pernicious anaemia, neuropathy

Vitamin Toxicity

Vitamin Toxicity arises mainly from the excessive accumulation of fat-soluble vitamins (Hypervitaminosis) due to storage in the body, leading to several medical complications.

• Vitamin A toxicity: Liver damage, intracranial pressure
• Vitamin D toxicity: Hypercalcemia, kidney stones
• Vitamin E toxicity: Bleeding tendencies
• Vitamin K toxicity: Rare, but affects anticoagulant therapy
• Water-soluble vitamin toxicity: Usually mild; excess excreted in urine