All living things, plant or animal, need vitamins for health, growth, and reproduction. Yet vitamins are not a source of calories and do not contribute significantly to body mass. The plant or animal uses vitamins as tools in processes that regulate chemical activities in the organism and that use basic food elements--carbohydrates, fats, and proteins--to form tissues and to produce energy.
Vitamins can be used over and over, and only tiny amounts are needed to replace those that are lost. Nevertheless, most vitamins are essential in the diet because the body does not produce enough of them or, in many cases, does not produce them at all.
Thirteen different vitamins have been identified by nutritionists: A, eight B-complex vitamins, C, D, E, and K. Some substances, such as carnitine and choline, behave like vitamins but are made in adequate amounts in the human body.
Vitamins were originally placed in categories based on their function in the body and were given letter names. Later, as their chemical structures were revealed, they were also given chemical names. Today, both naming conventions are used.