Vitamin | Definition, Types, & Facts

Inadequate intake of specific vitamins can result in a characteristic deficiency disease (hypovitaminosis), and the severity depends upon the degree of vitamin deprivation. Symptoms may be specific (e.g., functional night blindness with vitamin A deficiency), nonspecific (e.g., loss of appetite, failure to grow), or irreversible (e.g., damage to the cornea of the eye, nerve tissue, calcified bone).

Vitamins are any of several organic substances that are necessary in small quantities for normal health and growth in higher forms of animal life. They are usually designated by selected letters of the alphabet, as in vitamin C, though they are also designated by chemical names.

vitamin, any of several organic substances that are necessary in small quantities for normal health and growth in higher forms of animal life. Vitamins are distinct in several ways from other biologically important compounds such as proteins, carbohydrates, and lipids. Although these latter substances also are indispensable for proper bodily functions, almost all of them can be synthesized by animals in adequate quantities. Vitamins, on the other hand, generally cannot be synthesized in amounts sufficient to meet bodily needs and therefore must be obtained from the diet or from some synthetic source. For this reason, vitamins are called essential nutrients. Vitamins also differ from the other biological compounds in that relatively small quantities are needed to complete their functions. In general these functions are of a catalytic or regulatory nature, facilitating or controlling vital chemical reactions in the body’s cells. If a vitamin is absent from the diet or is not properly absorbed by the body, a specific deficiency disease may develop.

Vitamins are usually designated by selected letters of the alphabet, as in vitamin D or vitamin C, though they are also designated by chemical names, such as niacin and folic acid. Biochemists traditionally separate them into two groups, the water-soluble vitamins and the fat-soluble vitamins. The common and chemical names of vitamins of both groups, along with their main biological functions and deficiency symptoms, are listed in the table.

The vitamins

vitamin

alternative names/forms

biological function

symptoms of deficiency

Water-soluble

thiamin

vitamin B1

component of a coenzyme in carbohydrate metabolism; supports normal nerve function

impairment of the nerves and heart muscle wasting

riboflavin

vitamin B2

component of coenzymes required for energy production and lipid, vitamin, mineral, and drug metabolism; antioxidant

inflammation of the skin, tongue, and lips; ocular disturbances; nervous symptoms

niacin

nicotinic acid, nicotinamide

component of coenzymes used broadly in cellular metabolism, oxidation of fuel molecules, and fatty acid and steroid synthesis

skin lesions, gastrointestinal disturbances, nervous symptoms

vitamin B6

pyridoxine, pyridoxal, pyridoxamine

component of coenzymes in metabolism of amino acids and other nitrogen-containing compounds; synthesis of hemoglobin, neurotransmitters; regulation of blood glucose levels

dermatitis, mental depression, confusion, convulsions, anemia

folic acid

folate, folacin, pteroylglutamic acid

component of coenzymes in DNA synthesis, metabolism of amino acids; required for cell division, maturation of red blood cells

impaired formation of red blood cells, weakness, irritability, headache, palpitations, inflammation of mouth, neural tube defects in fetus

vitamin B12

cobalamin, cyanocobalamin

cofactor for enzymes in metabolism of amino acids (including folic acid) and fatty acids; required for new cell synthesis, normal blood formation, and neurological function

smoothness of the tongue, gastrointestinal disturbances, nervous symptoms

pantothenic acid

as component of coenzyme A, essential for metabolism of carbohydrate, protein, and fat; cofactor for elongation of fatty acids

weakness, gastrointestinal disturbances, nervous symptoms, fatigue, sleep disturbances, restlessness, nausea

biotin

cofactor in carbohydrate, fatty acid, and amino acid metabolism

dermatitis, hair loss, conjunctivitis, neurological symptoms

vitamin C

ascorbic acid

antioxidant; synthesis of collagen, carnitine, amino acids, and hormones; immune function; enhances absorption of non-heme iron (from plant foods)

swollen and bleeding gums, soreness and stiffness of the joints and lower extremities, bleeding under the skin and in deep tissues, slow wound healing, anemia

Fat-soluble

vitamin A

retinol, retinal, retinoic acid, beta-carotene (plant version)

normal vision, integrity of epithelial cells (mucous membranes and skin), reproduction, embryonic development, growth, immune response

ocular disturbances leading to blindness, growth retardation, dry skin, diarrhea, vulnerability to infection

vitamin D

calciferol, calatriol (1,25-dihydroxy vitamin D1 or vitamin D hormone), cholecalciferol (D3; plant version), ergocalciferol (D2; animal version)

maintenance of blood calcium and phosphorus levels, proper mineralization of bones

defective bone growth in children, soft bones in adults

vitamin E

alpha-tocopherol, tocopherol, tocotrienol

antioxidant; interruption of free radical chain reactions; protection of polyunsaturated fatty acids, cell membranes

peripheral neuropathy, breakdown of red blood cells

vitamin K

phylloquinone, menaquinone, menadione, naphthoquinone

synthesis of proteins involved in blood coagulation and bone metabolism

impaired clotting of the blood and internal bleeding