As a water-soluble member of the B-complex family of vitamins, biotin is required for growth and metabolism in humans. Often referred to as vitamin H, biotin is found naturally in liver, kidney, soy flour, egg yolk, cereal grains, and yeast.

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In its physiologically active form, biotin is attached to the active site of four important carboxylase enzymes. The catalysis of fatty acid synthesis, gluconeogenesis, the metabolism of leucine (an essential amino acid), and the metabolism of cholesterol and odd-chain fatty acids depend on vitamin H. Important in the treatment of diabetes mellitus, biotin stimulates the secretion of insulin, increases the synthesis of glycogen (the storage form of glucose), and may increase the utilization of glucose to synthesize fats, thereby lowering blood glucose levels. It appears also that biotin plays a major role in DNA replication and transcription, as well as with the creation of antibodies and the metabolism of niacin (vitamin B-3). Interestingly, in the normal gut fauna of the human colon, the bacteria are capable of synthesizing their own biotin. However, it has not yet been determined how much biotin the host human can absorb from the commensal microorganisms.

Although biotin deficiencies are rare, prolonged intravenous feeding without biotin supplements or prolonged consumption of raw egg whites may result in hair loss, a scaly red rash, depression, and hallucinations. Avidin, an egg white protein, binds available biotin, preventing its absorption, unless the eggs are cooked and the protein is denatured. Biotin deficiencies can also be caused by two different inherited enzyme disorders, which can be countered by high-dose supplementations (40 to 100 milligrams biotin per day). Most recently, pregnant and nursing mothers have been advised to ensure proper addition of biotin (at least 30 micrograms daily) in their diets because the rapidly dividing cells of the developing fetus depletes biotin supplies, and deficiencies may cause birth complications and defects.