Absorption and body stores

In most foodstuffs biotin is bound to proteins from which it is released in the intestine by protein hydrolysis and a specific enzyme, biotinidase. Biotin is then absorbed unchanged in the upper part of the small intestine by an electron-neutral sodium (Na+) gradient dependent carrier-mediated process and also by slow passive diffusion. The carrier is regulated by the availability of biotin, with up-regulation of the number of transporter molecules when biotin is deficient. The colon is also able to absorb biotin via an analogue transport mechanism. Once absorbed, biotin is distributed to all tissues. The presence of a specific biotin carrier protein in plasma is not yet conclusive. The liver and retinal tissues are the main storage places. Biotin metabolites are not active as vitamins and are excreted in the urine. Remarkable amounts of biotin appear in the faeces deriving from colonic bacteria.