Vitamins are nutrients that must be obtained in some amount in the human diet. The primary function of a vitamin is to initiate or speed chemical reactions in the body. The 13 vitamins are categorized into two major classes; those that can dissolve in water, called water-soluble vitamins, and those that can dissolve in fat or oil, called fat-soluble vitamins.
The eight B complex vitamins and vitamin C make up the water-soluble vitamin class. The water soluble vitamins, found in fruits and vegetables, cannot be stored in the body so they must be obtained regularly in the diet. The fat-soluble vitamins, A, D, E and K, can be stored in body tissues so they do not need to be eaten as frequently as water-soluble vitamins. Fat-soluble vitamins are obtained from vegetable and animal sources.
The function of every vitamin is different, and vitamins in the same class are not related to one another by function. In general, vitamins are not a source of energy, nor are they part of any body tissue. Vitamins are, however, vital to the growth, maintenance and function of the human body. The B vitamins, for example, function as co-enzymes--substances that help initiate or regulate a chemical reaction, while vitamin K contributes to the manufacture of blood-clotting substances.
Vitamins are needed in the human body for growth, development, repair and daily function. The chemical reactions that process the food we eat in order to make energy and maintain healthy cells are dependent on vitamins. The lack of any of the 13 vitamins in an individual's diet will result in a form of illness called a deficiency disease.
All vitamins have an established daily amount, called the Recommended Dietary Allowance or RDA, that is recommended for preventing deficiency illnesses. The RDA for each vitamin has been determined by a government committee based on a healthy individual's needs. The RDA is different for each vitamin, and in general most fat-soluble vitamins are needed in microgram quantities each day. Water-soluble vitamins are required in higher quantities.
If vitamins are not obtained in sufficient quantities in the diet, deficiency diseases will eventually occur. Since water-soluble vitamins are not stored in the body, deficiency diseases such as scurvy (from a lack of vitamin C) and beriberi (from a lack of B1) occur much more quickly than illness from a lack of fat-soluble vitamins. Likewise, too much of any vitamin, particularly the fat-soluble vitamins D and A, can result in illness from toxicity.