Vitamins are essential organic substances that perform various metabolic functions. Vitamins cannot be synthesized in adequate amounts by the body and, therefore, inadequate dietary intake causes deficiency diseases. Vitamins are classified as fat soluble and water soluble. The fat-soluble vitamins are vitamins A, D, E, and K. Because they are stored in the body, excessive ingestion of exogenous fat-soluble vitamins may cause abnormally elevated levels. Vitamin C and the B-complex vitamins are water soluble and are not stored in the body. The B-complex vitamins include B1 (thiamine), which is involved in carbohydrate metabolism; B2 (ribo-flavin), which is involved in the transport of oxidative metabolism and fatty acids; B3 (niacin), which is involved in the transport of cellular respiration; and B6 (pyridoxine), which is involved as a cofactor of enzymes and in the conversion of tryptophan to nicotinic acid. A vitamin B6 deficiency causes beriberi, and a vitamin B3 deficiency causes pellagra.
For diagnostic purposes, blood levels of vitamins A and C and a metabolite of vitamin D are measured. Vitamin B12 and folic acid also are measured in studies pertaining to hematologic function (see Chapter 1 - Hematology and Tests of Hematopoietic Function).