Editing Biotin (section)

==Physiology==
Biotin is a water-soluble B vitamin. Consumption of large amounts as a dietary supplement results in absorption, followed by excretion into urine as biotin. Consumption of biotin as part of a normal diet results in urinary excretion of biotin and biotin metabolites.<ref name=PKIN2020Biotin />

===Absorption===
Biotin in food is bound to proteins. Digestive enzymes reduce the proteins to biotin-bound peptides. The intestinal enzyme [[biotinidase]], found in pancreatic secretions and in the brush border membranes of all three parts of the [[small intestine]], frees biotin, which is then absorbed from the small intestine.<ref name=PKIN2020Biotin /> When consumed as a biotin dietary supplement, absorption is nonsaturable, meaning that even very high amounts are absorbed effectively. Transport across the [[jejunum]] is faster than across the [[ileum]].<ref name=PKIN2020Biotin />

The large intestine [[microbiota]] synthesizes amounts of biotin estimated to be similar to the amount taken in the diet, and a significant portion of this biotin exists in the free (protein-unbound) form and, thus, is available for absorption. How much is absorbed in humans is unknown, although a review did report that human colon epithelial cells in vitro demonstrated an ability to uptake biotin.<ref name=Said2013>{{cite journal |vauthors=Said HM |title=Recent advances in transport of water-soluble vitamins in organs of the digestive system: a focus on the colon and the pancreas |journal=Am J Physiol Gastrointest Liver Physiol |volume=305 |issue=9 |pages=G601–10 |date=November 2013 |pmid=23989008 |pmc=3840235 |doi=10.1152/ajpgi.00231.2013 |url=}}</ref>

Once absorbed, [[sodium-dependent multivitamin transporter]] (SMVT) mediates biotin uptake into the liver.<ref name=PKIN2020Biotin /> SMVT also binds [[pantothenic acid]], so high intakes of either of these vitamins can interfere with the transport of the other.<ref>{{cite journal | vauthors = Chirapu SR, Rotter CJ, Miller EL, Varma MV, Dow RL, Finn MG | title = High specificity in response of the sodium-dependent multivitamin transporter to derivatives of pantothenic acid | journal = Current Topics in Medicinal Chemistry | volume = 13 | issue = 7 | pages = 837–42 | date = 31 March 2013 | pmid = 23578027 | doi = 10.2174/1568026611313070006 }}</ref>

===Metabolism and excretion===
Biotin [[catabolism]] occurs via two pathways. In one, the valeric acid sidechain is cleaved, resulting in bisnorbiotin. In the other path, the sulfur is oxidized, resulting in biotin sulfoxide. Urine content is proportionally about half biotin, plus bisnorbiotin, biotin sulfoxide, and small amounts of other metabolites.<ref name=PKIN2020Biotin />

===Factors that affect biotin requirements===
Chronic alcohol use is associated with a significant reduction in plasma biotin.<ref name=Said2011>{{cite journal |vauthors=Said HM |title=Intestinal absorption of water-soluble vitamins in health and disease |journal=Biochem J |volume=437 |issue=3 |pages=357–72 |date=August 2011 |pmid=21749321 |pmc=4049159 |doi=10.1042/BJ20110326 |url=}}</ref> Intestinal biotin uptake also appears to be sensitive to the effect of the anti-[[epilepsy]] drugs [[carbamazepine]] and [[primidone]].<ref name=Said2011 /> Relatively low levels of biotin have also been reported in the urine or plasma of patients who have had a partial [[gastrectomy]] or have other causes of [[achlorhydria]], as well as burn patients, elderly individuals, and athletes.<ref name=Combs>{{cite book| author = Combs GF | title= The Vitamins: Fundamental Aspects in Nutrition and Health | year= 2008| publisher= San Diego: Elsevier, Inc| isbn = 978-0-12-183493-7}}</ref> Pregnancy and [[lactation]] may be associated with an increased demand for biotin. In pregnancy, this may be due to a possible acceleration of biotin [[catabolism]], whereas, in lactation, the higher demand has yet to be elucidated. Recent studies have shown marginal biotin deficiency can be present in [[human gestation]], as evidenced by increased urinary excretion of [[beta-Hydroxy beta-methylbutyric acid|3-hydroxyisovaleric acid]], decreased urinary excretion of biotin and bisnorbiotin, and decreased plasma concentration of biotin.<ref name=PKIN2020Biotin />