Biotin deficiency and requirements
Biotin is widely distributed in foods and deficiency is unknown, except among people maintained for many months on total parenteral nutrition, and a very small number of people who eat large amounts of uncooked egg. Avidin, a protein in egg white, binds biotin extremely tightly and renders it unavailable for absorption. Avidin is denatured by cooking and then loses its ability to bind biotin. The amount of avidin in uncooked egg white is relatively small, and prob-lems of biotin deficiency have only occurred in people eating a dozen or more raw eggs a day, for some years.
The few early reports of human biotin deficiency are all of people who consumed large amounts of uncooked eggs. They developed a fine scaly dermatitis and hair loss (alopecia). Histology of the skin showed an absence of sebaceous glands and atrophy of the hair follicles. Provision of biotin supplements of 200–1000 μg/day resulted in cure of the skin lesions and regrowth of hair, despite continuing the abnormal diet providing large amounts of avidin. There have been no studies of providing modest doses of biotin to such patients, and none in which their high intake of uncooked eggs was not either replaced by an equivalent intake of cooked eggs (in which avidin has been denatured by heat, and the yolks of which are a good source of biotin) or continued unchanged, so there is no information from these case reports of the amounts of biotin required for normal health. More recently, similar signs of biotin deficiency have been observed in patients receiving total parenteral nutrition for pro-longed periods after major resection of the gut. The signs resolve following the provision of biotin, but again there have been no studies of the amounts of biotin required; intakes have ranged between 60 μg/ day and 200 μg/day.
Biotin is the coenzyme for one of the key enzymes of gluconeogenesis, pyruvate carboxylase, and deficiency can lead to fasting hypoglycemia. In addition, biotin acts via cell surface receptors to induce the synthesis of phosphofructokinase and pyruvate kinase (key enzymes of glycolysis), phospho-enolpyruvate carboxykinase (a key enzyme of gluconeogenesis) and glucokinase.
Rather than the expected hypoglycemia, biotin deficiency may sometimes be associated with hyper-glycemia as a result of the reduced synthesis of gluco-kinase. Glucokinase is the high Km isoenzyme of hexokinase that is responsible for uptake of glucose into the liver for glycogen synthesis when blood con-centrations are high. It also acts as the sensor for hyperglycemia in the β-islet cells of the pancreas; metabolism of the increased glucose 6-phosphate formed by glucokinase leads to the secretion of insulin. There is some evidence that biotin supple-ments can improve glucose tolerance in diabetes.
The skin lesions of biotin deficiency are similar to those seen in deficiency of essential fatty acids, and serum linoleic acid is lower than normal in biotin-deficient patients owing to impairment of the elongation of PUFAs as a result of reduced activity of acetyl-CoA carboxylase.
The impairment of lipogenesis also affects the tissue fatty acid composition, with an increase in the proportion of palmitoleic acid, mainly at the expense of stearic acid, apparently as a result of increased fatty acid desaturase activity in biotin deficiency. Although dietary protein and fat intake also affect tissue fatty acid composition, the ratio of palmitoleic to stearic acid may provide a useful index of biotin nutritional status in some circumstances.
Biotin deficiency also results in an increase in the normally small amounts of odd-chain fatty acids (mainly C15:0 and C17:0) in triacylglycerols, phos-pholipids, and cholesterol esters. This is a result of impaired activity of propionyl-CoA carboxylase, leading to an accumulation of propionyl-CoA, which can be incorporated into lipids in competition with acetyl-CoA.
There is no evidence on which to estimate require-ments for biotin. Average intakes are between 10 μg/ day and 200 μg/day. Since dietary deficiency does not occur, such intakes are obviously more than adequate to meet requirements.