Vitamins have been discovered over the last century as a result of the investigations into what caused certain diseases. Vitamins are defined by two essential characteristics. A vitamin must be a vital organic dietary substance that is not carbohydrate, fat, or protein, and is necessary in very small quantities to perform a metabolic function or prevent a deficiency state. A vitamin cannot be manufactured by the body and therefore must be supplied by food. The discovery of vitamin D has lead to the belief that it is more a hormone than a vitamin, but still is discussed with vitamins.
There are two main classifications of vitamins according to their solubility in solutions: fat-soluble and water-soluble. The fat-soluble vitamins are vitamin A, vitamin E, vitamin D, and vitamin K. The presence of fat is required for the assimilation of these vitamins in the body.