Vitamins are organic compounds that are required in small amounts for normal metabolic function, growth, and maintenance of health. Thirteen essential vitamins are classified as fat-soluble (A, D, E, K) or water-soluble (B-complex and C). With few exceptions, they cannot be synthesized by the body and must be obtained from the diet.
Fat-Soluble Vitamins
Fat-soluble vitamins are absorbed with dietary fat, transported in chylomicrons, and stored in the liver and adipose tissue. Because they are stored, toxicity is possible with excessive intake (hypervitaminosis). They are not excreted in urine.
Vitamin A (Retinol, Retinal, Retinoic Acid)
Functions. Vision (retinal is a component of rhodopsin, the light-sensitive pigment in the retina), cell differentiation and proliferation, immune function, and reproduction. Beta-carotene and other carotenoids are provitamin A precursors found in plants.
Sources. Preformed vitamin A (retinol): liver, fish oils, eggs, dairy products. Provitamin A carotenoids: orange and dark green vegetables and fruits (carrots, sweet potatoes, spinach, kale, butternut squash, cantaloupe).
RDA. 700–900 mcg RAE (retinol activity equivalents) per day for adults.
Deficiency. Night blindness (early sign), xerophthalmia (dry eyes, Bitot spots, corneal ulceration), and increased susceptibility to infections. Vitamin A deficiency is a leading cause of preventable blindness in children in low-resource settings.
Toxicity. Hypervitaminosis A: nausea, headache, fatigue, hepatotoxicity, bone pain, teratogenicity (birth defects). Chronic intake exceeding 10,000 mcg/day is toxic. Beta-carotene does not cause toxicity but can cause harmless skin yellowing (carotenemia).
Vitamin D (Calciferol)
Functions. Regulates calcium and phosphorus absorption and metabolism. Essential for bone mineralization, immune function, muscle function, and cell proliferation. Vitamin D acts as a steroid hormone — the active form (1,25-dihydroxyvitamin D) binds to vitamin D receptors throughout the body.
Sources. Sunlight exposure (UVB radiation converts 7-dehydrocholesterol in the skin to previtamin D3), fatty fish (salmon, mackerel, sardines, tuna), fish liver oils, and fortified foods (milk, plant milk alternatives, orange juice, breakfast cereals). Very few foods naturally contain vitamin D.
RDA. 600–800 IU (15–20 mcg) per day for adults. Many experts recommend higher intakes (1000–2000 IU/day) for optimal blood levels.
Deficiency. Rickets in children (soft, weak bones, skeletal deformities), osteomalacia in adults (bone pain, muscle weakness), and increased fracture risk. Low vitamin D is also associated with increased risk of falls, infections, autoimmune diseases, and possibly cancer and cardiovascular disease.
Toxicity. Hypercalcemia (nausea, vomiting, weakness, kidney stones, cardiac arrhythmias). Toxicity occurs only with high-dose supplementation (typically >10,000 IU/day for months), not from sun exposure.
Vitamin E (Alpha-Tocopherol)
Functions. Primary fat-soluble antioxidant — protects cell membranes from lipid peroxidation by free radicals. Also supports immune function and cell signaling.
Sources. Vegetable oils (wheat germ, sunflower, safflower, soybean, corn), nuts (almonds, hazelnuts), seeds (sunflower), spinach, and fortified cereals.
RDA. 15 mg (22.4 IU of natural alpha-tocopherol) per day for adults.
Deficiency. Rare — hemolytic anemia, peripheral neuropathy, ataxia, and impaired immune function. Occurs in fat malabsorption syndromes, abetalipoproteinemia, and premature infants.
Toxicity. Rare — may increase bleeding risk by antagonizing vitamin K. The upper limit is 1000 mg/day.
Vitamin K (Phylloquinone, Menaquinones)
Functions. Cofactor for the carboxylation of proteins involved in blood coagulation (factors II, VII, IX, X, protein C, protein S) and bone metabolism (osteocalcin). Vitamin K1 (phylloquinone) is from plants; vitamin K2 (menaquinones) is from bacterial fermentation.
Sources. Green leafy vegetables (kale, spinach, collards, broccoli, Brussels sprouts), vegetable oils, and natto (fermented soybeans, rich in K2). Gut bacteria also synthesize menaquinones.
RDA. 90–120 mcg per day for adults.
Deficiency. Bleeding (elevated INR, bruising, hemorrhage). Deficiency is rare in adults but occurs in fat malabsorption, chronic antibiotic use, and newborns (vitamin K prophylaxis is standard at birth).
Water-Soluble Vitamins
Water-soluble vitamins are not stored in significant amounts (except B12) and are excreted in urine. They are generally non-toxic (except B6 at high doses), and toxicity is rare.
Vitamin B1 (Thiamine)
Functions. Cofactor in energy metabolism — required for decarboxylation of alpha-keto acids in the Krebs cycle and pentose phosphate pathway.
Sources. Pork, whole grains, fortified grains, legumes, nuts, and seeds.
RDA. 1.1–1.2 mg per day for adults.
Deficiency. Beriberi: dry (peripheral neuropathy, muscle wasting) and wet (high-output heart failure, edema). Wernicke encephalopathy (confusion, ataxia, nystagmus) and Korsakoff syndrome (amnestic syndrome) are seen in chronic alcoholism due to impaired thiamine absorption and utilization.
Vitamin B2 (Riboflavin)
Functions. Component of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), cofactors in redox reactions in energy metabolism.
Sources. Dairy products, eggs, meat, green leafy vegetables, fortified grains, almonds.
RDA. 1.1–1.3 mg per day for adults.
Deficiency. Cheilitis (inflammation of lips), angular stomatitis, glossitis (smooth, red tongue), and seborrheic dermatitis.
Vitamin B3 (Niacin)
Functions. Component of nicotinamide adenine dinucleotide (NAD/NADH) and NADP/NADPH, essential for redox reactions in energy metabolism and biosynthetic pathways.
Sources. Meat, poultry, fish, whole grains, fortified grains, peanuts. Tryptophan (an amino acid) can be converted to niacin (60 mg tryptophan = 1 mg niacin).
RDA. 14–16 mg NE (niacin equivalents) per day for adults.
Deficiency. Pellagra: dermatitis (photosensitive rash), diarrhea, dementia, and death (the four Ds). Pellagra results from niacin deficiency and is seen in populations relying on maize (corn) treated with alkali.
Toxicity. Niacin flush (vasodilation, redness, itching) with nicotinic acid supplements. High-dose niacin (500 mg+) can cause hepatotoxicity.
Vitamin B5 (Pantothenic Acid)
Functions. Component of coenzyme A (CoA), required for fatty acid synthesis, Krebs cycle, and acetylation reactions.
Sources. Widely distributed in foods — chicken, beef, potatoes, oats, tomatoes, eggs, broccoli. Deficiency is extremely rare.
Vitamin B6 (Pyridoxine, Pyridoxal, Pyridoxamine)
Functions. Cofactor in amino acid metabolism, neurotransmitter synthesis (serotonin, dopamine, GABA), heme synthesis, and glycogen breakdown.
Sources. Poultry, fish, organ meats, potatoes, bananas, chickpeas, fortified cereals.
RDA. 1.3–1.7 mg per day for adults.
Deficiency. Microcytic anemia, dermatitis, glossitis, depression, confusion, and seizures. Elevated homocysteine.
Toxicity. Sensory neuropathy (irreversible at high doses >200 mg/day for prolonged periods).
Vitamin B7 (Biotin)
Functions. Cofactor for carboxylation reactions in fatty acid synthesis, amino acid metabolism, and gluconeogenesis.
Sources. Eggs (raw egg whites contain avidin that binds biotin — cooking denatures avidin), nuts, seeds, fish, and sweet potatoes.
Deficiency. Rare — dermatitis, alopecia, and neurological symptoms. Occurs with prolonged raw egg white consumption or parenteral nutrition without biotin.
Vitamin B9 (Folate)
Functions. Cofactor in one-carbon transfer reactions, essential for DNA synthesis, methylation, and amino acid metabolism. Required for rapid cell division during growth and development.
Sources. Dark green leafy vegetables (spinach, kale, collards), legumes, citrus fruits, avocado, and fortified grains (folic acid). The synthetic form (folic acid) is more bioavailable than natural folate.
RDA. 400 mcg DFE (dietary folate equivalents) per day for adults; 600 mcg during pregnancy; 500 mcg during lactation.
Deficiency. Megaloblastic anemia (macrocytic red cells, hypersegmented neutrophils), glossitis, elevated homocysteine. Neural tube defects (spina bifida, anencephaly) in infants of women with low folate status at conception.
Vitamin B12 (Cobalamin)
Functions. Cofactor for methionine synthase (homocysteine to methionine) and methylmalonyl-CoA mutase (odd-chain fatty acid metabolism). Essential for DNA synthesis, myelin production, and red blood cell formation.
Sources. Only from animal products: meat, poultry, fish, eggs, dairy products, and fortified foods (plant milks, nutritional yeast, breakfast cereals).
RDA. 2.4 mcg per day for adults.
Deficiency. Megaloblastic anemia, peripheral neuropathy, subacute combined degeneration of the spinal cord, cognitive decline, and elevated homocysteine and methylmalonic acid. Causes: pernicious anemia (autoimmune), dietary deficiency (vegans), malabsorption (gastrectomy, Crohn disease, pancreatic insufficiency).
Vitamin C (Ascorbic Acid)
Functions. Cofactor for collagen synthesis (essential for wound healing, blood vessel integrity), carnitine synthesis, neurotransmitter synthesis, and iron absorption. Potent antioxidant.
Sources. Citrus fruits, strawberries, kiwi, bell peppers, broccoli, Brussels sprouts, tomatoes.
RDA. 75–90 mg per day for adults (smokers require an additional 35 mg due to increased oxidative stress).
Deficiency. Scurvy: fatigue, swollen and bleeding gums, petechiae, ecchymoses, impaired wound healing, hyperkeratosis, and depression. Scurvy is rare in developed countries but occurs in severe malnutrition, alcoholism, and restrictive diets.
Toxicity. Generally low toxicity. High doses (>2000 mg/day) can cause gastrointestinal distress, diarrhea, and increased risk of kidney stones in susceptible individuals.
Summary
The 13 essential vitamins are divided into fat-soluble (A, D, E, K) and water-soluble (B-complex, C). Fat-soluble vitamins are stored and can accumulate to toxic levels. Water-soluble vitamins are excreted and generally non-toxic. Each vitamin has specific functions, deficiency syndromes, and food sources. A varied, balanced diet containing fruits, vegetables, whole grains, lean proteins, and dairy provides adequate vitamins for most healthy individuals. Targeted supplementation is appropriate for specific populations and clinical conditions.