Vitamins

Vitamins are nutrients required in tiny amounts for essential metabolic reactions in the body. The term vitamin does not include other essential nutrients such as dietary minerals, essential fatty acids, or essential amino acids, nor does it encompass the large number of other nutrients that promote health but that are not essential for life.
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Vitamins are bio-molecules that act both as catalysts and substrates in chemical reactions. When acting as a catalyst, vitamins are bound to enzymes and are called cofactors. For example, vitamin K is part of the proteases involved in blood clotting. Vitamins also act as coenzymes to carry chemical groups between enzymes. Folic acid, for example, carries various forms of carbon groups–methyl, formyl or methylene in the cell.
Vitamins are classified as either water-soluble, meaning that they dissolve easily in water, or fat-soluble, and are absorbed through the intestinal tract with the help of lipids. Each vitamin is typically used in multiple reactions and, therefore, most have multiple functions.In humans there are thirteen vitamins, divided into two groups; four fat-soluble vitamins (A, D, E and K), and nine water-soluble vitamins (eight B vitamins and vitamin C).