Biotin

BACKGROUND

Biotin is a cofactor for four carboxylase enzymes found in mammals – pyruvate carboxylase, methylcrotonyl-CoA carboxylase, proprionyl-CoA carboxylase and acetyl-CoA carboxylase. The first three of these are mitochondrial and the fourth is both mitochondrial and cytosolic. They are involved in a range of actions including catabolising acetyl CoA, carboxylation of pyruvate, degradation of leucine and carboxylation of proprionyl-CoA. Biotin is found in free and protein-bound forms in food but little is known about its bioavailability. It is found in the protein-bound form in meats and cereals, although it seems to be less bioavailable in the latter (Mock 1996).

There are very few data about the biotin content of foods. Liver is known to be a very concentrated source, providing 100 µg/100 g compared to only 1 µg/100 g in meats and plant foods. Avidin, a protein found in raw egg white, binds biotin in the gut and prevents its absorption (Mock 1996). In the intestines, biotin is transported across the brush border membrane by a biotin carrier, against a sodium ion gradient. It can also be synthesised by intestinal microflora (Bonjour 1991) but it is not clear whether this is an additional potential source in humans. About half the biotin undergoes metabolism to bisnorbiotin and biotin sulfoxide before excretion. Urinary excretion and serum concentrations of biotin and its metabolites increase in similar proportions in response to intravenous or oral administration of large doses (Mock & Heird 1997, Zempleni et al 1997).

Although rare, biotin deficiency has been seen in people who consume raw egg white over long periods (Baugh et al 1968) and in total parenteral nutrition. Symptoms include dermatitis, conjunctivitis, alopecia and CNS abnormalities, including developmental delay in infants (Mock 1996). People with genetic biotinidase deficiency will have increased requirements.

The most useful information about requirements comes from assessment of clinical signs in patients on biotin-free intravenous nutrition, in those eating raw egg white or from the results of biotin bioavailability and pharmacokinetic experiments. The most sensitive end points are decreased biotin excretion and/or increased 3-hydroxyisovalerate excretion (Mock et al 1997a, 2002a).

Evidence about biotin requirements is not sufficient to set an EAR and RDI so AIs were set based on extrapolation from data on infants, and on some population intake data from New Zealand for people over 15 years of age (LINZ 1992).