Any of a group of complex organic substances that are essential in small amounts for the normal functioning of the body. There are 13 vitamins: A, C, D, E, K, B, and 7 grouped under the vitamin B complex. Apart from the B vitamin niacin and vitamin D, which the body can synthesize itself, vitamins must be obtained from the diet. A varied diet is likely to contain adequate amounts of all the vitamins, but vitamin supplements may be helpful for some people, such as young children, women who are pregnant or breast-feeding, or those taking drugs that interfere with vitamin function. Vitamins can be categorized as fatsoluble or water-soluble. The fat-soluble vitamins (A, D, E, and K) are absorbed with fats from the intestine into the bloodstream and then stored in fatty tissue (mainly in the liver). Body reserves of some of these vitamins last for several years, so a daily intake is not usually necessary. Deficiency of a fatsoluble vitamin is usually due to a disorder in which intestinal absorption of fats is impaired (see malabsorption) or to a prolonged poor diet. Vitamin C, B, and those of the B complex are water-soluble. Vitamin C and B complex vitamins can be stored in the body in only limited amounts and are excreted in the urine if taken in greater amounts than needed. A regular intake is therefore essential to prevent deficiency. However, vitamin B is stored in the liver; these stores may last for years. The role of all the vitamins in the body is not fully understood. Most vitamins have several important actions on 1 or more body systems, and many are involved in the activities of enzymes.