Vitamin any of the organic compounds required by the body in small amounts for metabolism, to protect health, and for proper growth in children. Vitamins also assist in the formation of hormones, blood cells, nervous-system chemicals, and genetic material. The various vitamins are not chemically related, and most differ in their physiological actions. They generally act as catalysts, combining with proteins to create metabolically active enzymes that in turn produce hundreds of important chemical reactions throughout the body. Without vitamins, many of these reactions would slow down or cease. The intricate ways in which vitamins act on the body, however, are still far from clear.
The 13 well-identified vitamins are classified according to their ability to be absorbed in fat or water. The fat-soluble vitamins-A, D, E, and K-are generally consumed along with fat-containing foods, and because they can be stored in the body's fat, they do not have to be consumed every day. The water-soluble vitamins-the eight B vitamins and vitamin C-cannot be stored and must be consumed frequently, preferably every day.

Vitamin	functions	Deficency symptoms	Sources	Excessive
A	Formation and maintenance of skin, mucous membranes, bones, and teeth; vision; and reproduction	is night blindness (difficulty in adapting to darkness), excessive skin dryness; lack of mucous membrane secretion, and dryness of the eyes due to a malfunctioning of the tear glands	The body obtains vitamin A by manufacturing it from carotene, a vitamin precursor an by absorbing ready-made vitamin A from plant-eating organisms. In animal form, it is found in milk, butter, cheese, egg yolk, liver, and fish-liver oil.	Excess vitamin A can interfere with growth, stop menstruation, damage red blood corpuscles, and cause skin rashes, headaches, nausea, and jaundice.
Thiamine, or vitamin B1	catalyst in carbohydrate metabolism, enabling pyruvic acid to be absorbed and carbohydrates to release their energy. Also plays a role in the synthesis of nerve-regulating substances.	Beriberi: muscular weakness, swelling of the heart, and leg cramps and may, in severe cases, lead to heart failure and death	Pork, organ meats (liver, heart, and kidney), brewer's yeast, lean meats, eggs, leafy green vegetables, whole or enriched cereals, wheat germ, berries, nuts, and legumes.	unknown
Riboflavin, or vitamin B2	metabolism of carbohydrates, fats, and, especially, respiratory proteins. It also serves in the maintenance of mucous membranes	skin lesions, especially around the nose and lips, and sensitivity to light	liver, milk, meat, dark green vegetables, whole grain and enriched cereals, pasta, bread, and mushrooms.	unknown
Niacin, also known as nicotinic acid and vitamin B3	works as a coenzyme in the release of energy from nutrients	pellagra: symptoms of which is a sunburn like eruption that breaks out where the skin is exposed to sunlight, a red and swollen tongue, diarrhea, mental confusion, irritability, and, when the central nervous system is affected, depression and mental disturbances	liver poultry, meat, canned tuna and salmon, whole grain and enriched cereals, dried beans and peas, and nuts. The body also makes niacin from the amino acid tryptophan	Large doses over long periods cause liver damage.
Pyridoxine, or vitamin B6	Absorption and metabolism of amino acids. It also plays roles in the use of fats in the body and in the formation of red blood cells	skin disorders, cracks at the mouth corners, smooth tongue, convulsions, dizziness, nausea, anemia, and kidney stones.	whole (but not enriched) grains, cereals, bread, liver, avocadoes, spinach, green beans, and bananas	unknown
Cobalamin, or vitamin B12	formation of nucleoproteins, proteins, and red blood cells, and for the functioning of the nervous system.	Cobalamin deficiency is often due to the inability of the stomach to produce glycoprotein, which aids in the absorption of this vitamin. Pernicious anemia results, with symptoms of ineffective production of red blood cells, faulty myelin (nerve sheath) synthesis, and loss of epithelium (membrane lining) of the intestinal tract	only from animal sources-liver, kidneys, meat, fish, eggs, and milk. Vegetarians are advised to take vitamin B12 supplements.	unknown
Folic acid, or folacin	needed for forming body protein and hemoglobin. Folic acid is effective in the treatment of certain anemias and sprue.	folic acid deficiency may be responsible for neural tube defects, a type of birth defect that results in severe brain or neurological disorders	Dietary sources are organ meats, leafy green vegetables, legumes, nuts, whole grains, and brewer's yeast.	unknown
B5 Pantothenic acid	Metabolism of lipids, proteins and carbohydrates. contributes to the synthesis of steroids, porphyrins, acetylcholine and other substances.	Unknown	Manufactured by intestinal bacteria and widespread in foods	unknown
Biotin	formation of fatty acids and the release of energy from carbohydrates	Unknown	synthesized by intestinal bacteria and widespread in foods	unknown
Vitamin C, or Ascorbic Acid	important in the formation and maintenance of collagen, the protein that supports many body structures and plays a major role in the formation of bones and teeth. It also enhances the absorption of iron from foods of vegetable origin. ascorbic acid has been shown to prevent the formation of nitrosamines-compounds found to produce tumors in laboratory animals and possibly also in humans	. Scurvy - loss of the cementing action of collagen and include hemorrhages, loosening of teeth, and cellular changes in the long bones of children	citrus fruits, fresh strawberries, cantaloupe, pineapple, and guava. Good vegetable sources are broccoli, brussels sprouts, tomatoes, spinach, kale, green peppers, cabbage, and turnips.	prolonged doses can result in the formation of bladder and kidney stones, interference with the effects of blood-thinning drugs, destruction of B12, and the loss of calcium from bones.
Vitamin D	necessary for normal bone formation and for retention of calcium and phosphorus in the body. It also protects the teeth and bones against the effects of low calcium intake by making more effective use of calcium and phosphorus.	Rickets, is characterized by deformities of the rib cage and skull and by bowlegs, due to failure of the body to absorb calcium and phosphorus.	egg yolk, liver, tuna, and vitamin-D fortified milk. It is also manufactured in the body when sterols, which are commonly found in many foods, migrate to the skin and become irradiated	vitamin poisoning, kidney damage, lethargy, and loss of appetite.
Vitamin E	vitamin plays some role in forming red blood cells and muscle and other tissues and in preventing the oxidation of vitamin A and fats.	Unknown	vegetable oils, wheat germ, liver, and leafy green vegetables.	overdoses appear to have lower toxic effects than do overdoses of other fat-soluble vitamins
Vitamin K	mainly for the coagulation of blood. It aids in forming prothrombin, an enzyme needed to produce fibrin for blood clotting.	Digestive disturbances may lead to defective absorption of vitamin K and hence to mild disorders in blood clotting.	All leafy green vegetables, egg yolks, soybean oil, and liver. For a healthy adult, a normal diet and bacterial synthesis in the bowels usually are sufficient to supply the body with vitamin K and prothrombin	Unknown