Biotin: A water-soluble B-complex vitamin involved in carbon dioxide transfer and therefore essential to the metabolism of carbohydrate and fat. A balanced diet usually contains enough biotin. Foods with high biotin levels include nuts, cereals, green leafy vegetables and milk. Biotin deficiency, which is characterized by hair loss and a scaly red rash, can occur with prolonged intravenous feeding or the frequent consumption of raw egg whites which contain a biotin antagonist called avidin. Biotin supplementation is recommended during pregnancy because the biotin requirement rises during pregnancy and a substantial number of pregnant women become biotin depleted. Extra biotin is also needed during long-term anticonvulsant treatment which depletes biotin.