- Vitamins are organic compounds essential for normal growth and metabolism in living organisms.
- They are required in small amounts and play crucial roles in various biochemical processes, including energy production, immune function, and healthy skin and eye maintenance.
- Vitamins are typically obtained from the diet, as the body cannot synthesize most in sufficient quantities.
- They are classified into two main categories: water-soluble (like vitamin C and B vitamins) and fat-soluble (like vitamins A, D, E, and K).
Classification of vitamins
Fat-Soluble Vitamins
Fat-soluble vitamins are a class of vitamins that dissolve in fats and oils and are absorbed along with dietary fat. These vitamins are stored in the body’s fatty tissues and liver and are used as needed by the body. Because they can be stored, fat-soluble vitamins are not needed as frequently as water-soluble vitamins, but excessive intake can lead to toxicity.
The four main fat-soluble vitamins are:
- Vitamin A (Retinoids)
- Vitamin D (Calciferols)
- Vitamin E (Tocopherols and Tocotrienols)
- Vitamin K (Phylloquinones and Menaquinones)
General Properties of Fat-Soluble Vitamins
- Solubility: Fat-soluble vitamins are soluble in fats and oils but not water.
- Absorption: They are absorbed in the small intestine along with dietary fats and require bile salts for optimal absorption.
- Storage: Fat-soluble vitamins are stored in the liver and fatty tissues, meaning they do not need to be consumed daily.
- Toxicity Risk: Due to their ability to accumulate in the body, consuming fat-soluble vitamins in excessive amounts can lead to toxicity (hypervitaminosis).
- Transport: These vitamins are transported in the bloodstream via lipoproteins or specific binding proteins since they are not water-soluble.
1. Vitamin A (Retinoids and Carotenoids)
Types and Forms:
- Preformed Vitamin A (Retinoids): These are active forms found in animal products, including retinol, retinal, and retinoic acid.
- Retinol: The most usable form, converted to retinal (for vision) or retinoic acid (for cellular regulation).
- Retinoic acid: Critical for gene expression, growth, and development, including cell differentiation.
- Retinal: Key for the visual cycle in the retina, forming rhodopsin, which helps with night vision.
- Provitamin A (Carotenoids): Plant pigments that can be converted into Vitamin A in the body, such as beta-carotene, alpha-carotene, and beta-cryptoxanthin.
- Beta-carotene: A potent antioxidant and its conversion to retinol is carefully regulated to avoid toxicity from overconsumption.
Absorption and Metabolism:
- Vitamin A is absorbed in the small intestine, requiring fat and bile salts for optimal uptake.
- Retinol is transported in the bloodstream by retinol-binding protein (RBP) and is stored primarily in the liver, where it can be released when needed.
- Carotenoids are absorbed similarly but are less efficiently converted to retinol in humans.
Functions in Greater Detail:
- Vision:
- Vitamin A is critical for the synthesis of rhodopsin in the photoreceptor cells of the retina. Rhodopsin absorbs light and initiates the visual signal to the brain.
- Deficiency in Vitamin A deficiency hampers rhodopsin regeneration, leading to night blindness.
- Immune Function:
- Vitamin A supports the integrity of epithelial barriers (skin and mucous membranes), the body’s first defence against infections.
- It also regulates the immune system, enhancing the production and function of white blood cells like lymphocytes, which fight infections.
- Gene Expression and Cellular Growth:
- Retinoic acid regulates gene transcription by binding to nuclear receptors (RAR and RXR), which control gene expression in cell differentiation, embryonic development, and tissue repair.
- It is particularly important during fetal development and cell differentiation in tissues such as the skin, lungs, and gastrointestinal tract.
- Reproduction and Development:
- Vitamin A is vital for the development of sperm and the maintenance of pregnancy, influencing fetal growth and organ development.
Deficiency in Detail:
- Keratinization: Deficiency leads to improper skin cell development, causing the skin to become dry, rough, and scaly due to keratin buildup.
- Increased Risk of Infections: Mucosal integrity is compromised, leading to a higher risk of respiratory and gastrointestinal infections, especially in children.
- Xerophthalmia and Blindness: Prolonged deficiency can lead to corneal ulcers and blindness.
Toxicity (Hypervitaminosis A):
- Symptoms of Vitamin A toxicity include nausea, headaches, dizziness, blurred vision, and, in severe cases, liver damage and bone fractures.
- Teratogenic Effects: High doses of Vitamin A during pregnancy can cause severe birth defects.
2. Vitamin D (Calciferols)
Types:
- Vitamin D2 (Ergocalciferol): Synthesized by plants and fungi when exposed to UV light.
- Vitamin D3 (Cholecalciferol): Produced in the skin when exposed to sunlight (UVB rays) or obtained from animal sources like fatty fish, liver, and fortified dairy products.
Synthesis and Activation:
- Skin synthesis: UVB light converts 7-dehydrocholesterol in the skin to Vitamin D3.
- Both D2 and D3 are converted to their active form through two hydroxylation steps:
- The liver, where it is converted to 25-hydroxyvitamin D (calcidiol), is the main circulating form used to assess Vitamin D levels.
- In the kidneys, where it is converted to 1,25-dihydroxyvitamin D (calcitriol), the biologically active form.
Functions in Greater Detail:
- Calcium and Phosphorus Metabolism:
- Vitamin D stimulates the absorption of calcium and phosphate from the intestines.
- It promotes calcium reabsorption in the kidneys and the mobilization of calcium from bones to maintain normal blood calcium levels.
- Parathyroid hormone (PTH) works alongside Vitamin D to maintain calcium balance.
- Bone Health:
- Essential for bone mineralization, preventing softening of bones (rickets in children and osteomalacia in adults).
- Increases the activity of osteoblasts (bone-forming cells) and regulates bone resorption by osteoclasts.
- Immune Modulation:
- Vitamin D modulates innate and adaptive immune responses, enhancing the ability to fight respiratory infections.
- It helps reduce inflammation by suppressing pro-inflammatory cytokines.
- Cell Growth and Cancer Prevention:
- Regulates cellular proliferation and differentiation, with potential protective roles against certain cancers, such as colorectal, breast, and prostate cancer.
Deficiency in Detail:
- Rickets: In children, this leads to soft, weak bones and skeletal deformities (bowed legs, thickened wrists, and ankles).
- Osteomalacia: In adults, deficiency causes softening of the bones, leading to bone pain and muscle weakness.
- Osteoporosis: Chronic deficiency contributes to decreased bone mass and increased risk of fractures in older adults.
Toxicity (Hypervitaminosis D):
- Over-supplementation of Vitamin D can lead to hypercalcemia (elevated blood calcium levels), causing nausea, vomiting, weakness, and kidney damage.
3. Vitamin E (Tocopherols and Tocotrienols)
Types:
- The two main classes of Vitamin E are tocopherols and tocotrienols, each with four forms (alpha, beta, gamma, delta).
- Alpha-tocopherol is the most biologically active form in humans.
Absorption and Transport:
- Vitamin E is absorbed with dietary fat and requires bile for emulsification. It is incorporated into chylomicrons and transported to the liver, where it is repackaged into VLDL (very low-density lipoproteins) for distribution to tissues.
Functions in Greater Detail:
- Antioxidant:
- Vitamin E neutralizes free radicals, particularly lipid peroxides, preventing oxidative damage to cell membranes, lipoproteins, and other cellular structures.
- Protects polyunsaturated fatty acids (PUFAs) in cell membranes from lipid peroxidation.
- Cardiovascular Protection:
- It may help reduce the risk of cardiovascular diseases by preventing oxidation of LDL cholesterol (oxidized LDL is more likely to lead to atherosclerosis).
- Some studies suggest Vitamin E reduces platelet aggregation and improves blood vessel function.
- Skin and Eye Health:
- Vitamin E helps protect skin cells from UV damage and reduces the risk of age-related macular degeneration (a leading cause of blindness in older adults).
- Immune Function:
- Vitamin E enhances T-cell function and may improve immune response, particularly in the elderly.
Deficiency in Detail:
- Hemolytic Anemia: Due to the destruction of red blood cells from oxidative stress.
- Neurological Symptoms: Prolonged deficiency can cause peripheral neuropathy, ataxia (loss of coordination), muscle weakness, and impaired vision.
Toxicity:
- High doses of Vitamin E from supplements may interfere with Vitamin K’s role in blood clotting, increasing the risk of haemorrhage, particularly in individuals on blood-thinning medications.
4. Vitamin K (Phylloquinones and Menaquinones)
Types:
- Vitamin K1 (Phylloquinone): Found in green leafy vegetables.
- Vitamin K2 (Menaquinones): Found in fermented foods, animal products, and produced by gut bacteria.
Functions in Greater Detail:
- Blood Clotting (Coagulation):
- Vitamin K is essential for synthesizing clotting factors (II, VII, IX, and X) and proteins C and S, critical for blood coagulation.
- It activates prothrombin into thrombin, forming blood clots essential for wound healing.
- Bone Health:
- Vitamin K activates osteocalcin, a protein that binds calcium to bone, enhancing bone mineralization.
- Adequate Vitamin K intake reduces the risk of fractures and is associated with increased bone density.
- Cardiovascular Health:
- Helps prevent arterial calcification by regulating proteins that inhibit calcification in blood vessels, reducing the risk of atherosclerosis.
Deficiency in Detail:
- Hemorrhagic Disease of the Newborn: Newborns are given Vitamin K injections to prevent excessive bleeding due to their low stores of Vitamin K.
- Increased Bleeding: In adults, deficiency can lead to excessive bruising, bleeding, and slow wound healing.
Toxicity:
- Vitamin K toxicity is rare, but excessive intake of synthetic forms like menadione (K3) can lead to hemolytic anaemia and liver toxicity.
Water-Soluble Vitamins
- Water-soluble vitamins are a group of vitamins that dissolve in water and are not stored in the body in significant amounts.
- Since these vitamins are quickly excreted in urine, they must be consumed more frequently than fat-soluble vitamins.
- The primary water-soluble vitamins include the B-complex vitamins and Vitamin C.
General Properties of Water-Soluble Vitamins
- Solubility: They dissolve in water, making them easily absorbed in the intestines.
- Absorption: Water-soluble vitamins are absorbed directly into the bloodstream, primarily through the small intestine.
- Storage: Unlike fat-soluble vitamins, water-soluble vitamins are not stored in large quantities in the body. Excess is typically excreted in urine, so toxicity is rare but possible with excessive supplementation.
- Frequency of Intake: Because they are not stored, regular intake through diet is necessary to maintain adequate levels.
- Toxicity: Generally lower risk of toxicity due to their excretion in urine, although megadoses of certain vitamins can still cause adverse effects.
1. Vitamin B1 (Thiamine)
Chemical Structure:
- Thiamine is a water-soluble vitamin with a chemical formula of C₁₂H₁₇N₄OS.
Functions:
- Energy Metabolism: Thiamine is a cofactor for enzymes involved in the metabolism of carbohydrates, specifically pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase.
- Nerve Function: It aids in the conduction of nerve impulses by synthesising neurotransmitters.
Deficiency:
- Symptoms: Fatigue, irritability, reduced reflexes, muscle weakness, and in severe cases, Beriberi.
- Beriberi Types:
- Wet Beriberi: Affects the cardiovascular system, leading to symptoms such as oedema and heart failure.
- Dry Beriberi: Affects the nervous system, leading to peripheral neuropathy and muscle wasting.
Food Sources:
- Whole grains, legumes, nuts, seeds, and pork.
Recommended Daily Intake:
- Adult men: 1.2 mg; adult women: 1.1 mg.
2. Vitamin B2 (Riboflavin)
Chemical Structure:
- Riboflavin has a molecular formula of C₁₃H₁₄N₄O₆.
Functions:
- Energy Production: Riboflavin is a precursor to the coenzymes FMN and FAD, essential for the electron transport chain.
- Antioxidant Function: Helps regenerate glutathione, a key antioxidant in the body.
Deficiency:
- Symptoms: Cracked lips (cheilosis), sore throat, swollen tongue (glossitis), and dermatitis. It can also lead to anaemia.
Food Sources:
- Dairy products, eggs, lean meats, green leafy vegetables, and fortified cereals.
Recommended Daily Intake:
- Adult men: 1.3 mg; adult women: 1.1 mg.
3. Vitamin B3 (Niacin)
Chemical Structure:
- Niacin (niacinamide form) has a formula of C₆H₅N₃O.
Functions:
- Energy Metabolism: It forms NAD and NADP, critical for redox reactions in the body.
- Cholesterol Regulation: Niacin can help lower LDL cholesterol and raise HDL cholesterol levels.
Deficiency:
- Symptoms: Pellagra, characterized by the “three D’s”: dermatitis, diarrhoea, and dementia.
Food Sources:
- Meat, fish, poultry, whole grains, and legumes.
Recommended Daily Intake:
- Adult men: 16 mg; adult women: 14 mg.
4. Vitamin B5 (Pantothenic Acid)
Chemical Structure:
- Pantothenic acid has a molecular formula of C₁₄H₂₅N₃O₆S.
Functions:
- Coenzyme A Synthesis: Essential for fatty acid metabolism and energy production.
- Hormone Production: Plays a role in synthesizing steroid hormones.
Deficiency:
- Symptoms: Fatigue, depression, irritability, and impaired synthesis of hormones.
Food Sources:
- Chicken, beef, potatoes, oats, and whole grains.
Recommended Daily Intake:
- Adults: 5 mg.
5. Vitamin B6 (Pyridoxine)
Chemical Structure:
- Pyridoxine has a formula of C₈H₁₁NO₃.
Functions:
- Amino Acid Metabolism: B6 is crucial for transamination, synthesizing neurotransmitters like serotonin and dopamine.
- Hemoglobin Production: Necessary for synthesising heme, the iron-containing component of haemoglobin.
Deficiency:
- Symptoms: Microcytic anaemia, peripheral neuropathy, depression, and confusion.
Food Sources:
- Poultry, fish, potatoes, bananas, and fortified cereals.
Recommended Daily Intake:
- Adult men: 1.3 mg; adult women: 1.3 mg.
6. Vitamin B7 (Biotin)
Chemical Structure:
- Biotin has a molecular formula of C₁₄H₂₀N₄O₃S.
Functions:
- Fatty Acid Synthesis: A coenzyme for carboxylation reactions critical in lipid metabolism.
- Gene Regulation: Involved in regulating gene expression and cell signalling.
Deficiency:
- Symptoms: Dermatitis, hair loss (alopecia), and neurological symptoms like depression and lethargy.
Food Sources:
- Eggs, nuts, seeds, liver, and certain vegetables.
Recommended Daily Intake:
- Adults: 30 mcg.
7. Vitamin B9 (Folate)
Chemical Structure:
- Folate (or folic acid) has a formula of C₁₉H₁₉N₇O₆.
Functions:
- DNA Synthesis: Essential for nucleotide synthesis and DNA replication, crucial during periods of rapid growth.
- Red Blood Cell Formation: Necessary for producing healthy red blood cells.
Deficiency:
- Symptoms: Megaloblastic anaemia, neural tube defects in fetuses during pregnancy, fatigue, and weakness.
Food Sources:
- Leafy green vegetables, legumes, fortified cereals, and citrus fruits.
Recommended Daily Intake:
- Adults: 400 mcg; pregnant women: 600 mcg; breastfeeding women: 500 mcg.
8. Vitamin B12 (Cobalamin)
Chemical Structure:
- Cobalamin has a complex structure containing cobalt, with a formula of C₁₂H₁₄N₄O₁₈P.
Functions:
- Red Blood Cell Formation: Necessary for producing healthy red blood cells and preventing megaloblastic anaemia.
- DNA Synthesis: Involved in the synthesis of DNA and RNA.
- Nervous System Maintenance: Essential for maintaining myelin, which insulates nerve fibers.
Deficiency:
- Symptoms: Pernicious anaemia, neuropathy (tingling and numbness), memory loss, and cognitive decline.
Food Sources:
- Animal products such as meat, fish, dairy, and eggs. Fortified plant-based foods for vegans.
Recommended Daily Intake:
- Adults: 2.4 mcg; pregnant women: 2.6 mcg; breastfeeding women: 2.8 mcg.
9. Vitamin C (Ascorbic Acid)
Chemical Structure:
- Ascorbic acid has a formula of C₆H₈O₆.
Functions:
- Antioxidant: Protects cells from oxidative stress and regenerates other antioxidants.
- Collagen Synthesis: Essential for collagen formation in connective tissues.
- Immune Support: Enhances the function of white blood cells and supports skin health.
Deficiency:
- Symptoms: Scurvy, fatigue, swollen gums, joint pain, and bruising.
Food Sources:
- Citrus fruits, strawberries, kiwi, bell peppers, broccoli, and tomatoes.
Recommended Daily Intake:
- Adult men: 90 mg; adult women: 75 mg; higher amounts are recommended for smokers and pregnant/breastfeeding women.
General Functions of Water-Soluble Vitamins
- Coenzymes: Many B vitamins function as coenzymes, assisting in enzymatic reactions crucial for metabolism.
- Energy Production: They significantly convert carbohydrates, fats, and proteins into energy.
- Nutrient Interactions: These vitamins often work synergistically with other nutrients, such as minerals and amino acids, to maintain overall health.
- Support Cellular Function: They are involved in cellular functions like gene expression, cell signalling, and cellular repair.