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Matthew Landry HUEC 3115 – Human Nutrition and Metabolism Nutrient Mechanism of AbsorptionAmino AcidsDuodenum (primary), Jejunum (primary), and Ileum: Requires carriers for absorption; however, paracellular absorption can occur in occasional situations. Depending on the amino acid, a different amino acid transporter may be required, and some require sodium for activation. GlucoseDuodenum, Jejunum, and Ileum: by active transport. Requires energy as ATP and the involvement of a specific receptor. The glucose-galactose receptor has been designed sodium-glucose transporter 1 (SGLT1). It simultaneously transports Na+ and glucose in the same direction (symport). At high glucose concentrations, glucose is absorbed by facilitated transporter type 2 (GLUT2).. FructoseDuodenum, Jejunum, and Ileum: by a specific facilitative transporter, GLUT5. Is also independent of the active NA+-ATPase dependent transport by SGLT1. SCFADuodenum, Jejunum, and Ileum: Are able to passively diffuse through the cell membrane. Other short chains are incorporated into micelles. LCFADuodenum, Jejunum, and Ileum: incorporated into micelles for transport across the brush border. The mechanism for transporting fatty acids across the apical membrane is not fully understood. Two suggested mechanism are a protein-independent diffusion model and a protein-dependent model, which uses fatty acid transport proteins (FATP) Vitamin CDuodenum, Jejunum, and Ileum: Ascorbic acid by sodium dependent vitamin C transporters (SVCT) 1 and 2. Dehydroascorbic acid by glucose transporters (GLUT) 1 and 3. ThiaminDuodenum, Jejunum (primary), and Ileum (primary): Absorption is both passive and active (high concentrations – favors passive).Mediated by two thiamin transporters, ThTr1 and ThTr2. Active, sodium-dependent and sodium-independent carrier-mediated thiamin transport has also been suggested. RiboflavinDuodenum: by a saturable, energy dependent, sodium-independent carrier, riboflavin transporter 2, RFT2. When large amounts are consumed absorption may occur my diffusion. CholineDuodenum, Jejunum, and Ileum: by transport proteins in the enterocyte. NiacinStomach, Duodenum, Jejunum, and Ileum: Readily absorbed in the small intestines by sodium-dependent carrier-mediated, (facilitated) diffusion. At high concentrations, absorbed almost completely by passive diffusion. Pantothenic AcidJejunum: by passive diffusion when present in high concentrations and by a sodium-dependent multivitamin transporter (SMVT) when present in low concentrations. FolateDuodenum and Jejunum: by proton coupled folate transporter (PCFT). Vitamin B12Ileum: the vitamin B12 IF complex interacts with a proteinMatthew Landry HUEC 3115 – Human Nutrition and Metabolism receptor called cubilin and amnionless which facilitates attachment to the ileal cell’s plasma membrane. Binding of the complex to the receptor triggers active endocytotic internalization. BiotinDuodenum (primary) and colon: occurs by passive diffusion with consumption of pharmacological doses. With physiological intakes, absorption by high affinity, carrier mediated and sodium dependent multivitamin transporter (SMVT2). Vitamin B6Jejunum: PL, PN, and PM are absorbed by passive diffusion. At physiological doses the vitamin is absorbed rapidly in its free form. Vitamin ADuodenum and Jejunum: incorporated into micelles and are absorbed by passive diffusion across the border. Additionally, carotenoids also may be absorbed by a carotenoid transporter, called scavenger receptor class B type 1 (SR-B1)[1,2]. Vitamin DDuodenum, Jejunum, and Ileum (primary): Absorbed from a micelle, in association with fat and with aid of bile salts, by passive diffusion. Vitamin EJejunum: by passive diffusion as part of a micelle. Bile salts are required for emulsification, solubilization, and micelle formation. Little is known about tocotrienol absorption; however, the involvement of Niemann-Pick C1-like 1 (NPC1L1) transporter in ϒ-tocotrienol absorption has been suggested. Vitamin KDuodenum, Jejunum, Ileum, and Colon: Phylloquinone is absorbed primarily in the jejunum as part of micelles. Some may be absorbed by active trans[port from the proximal small intestines. Menaquinones are absorbed by passive diffusion fromthe ileum and colon. CalciumDuodenum, Jejunum, and Ileum: by a saturable, carrier-mediatedactive transport and diffusion. The main transport operates in theduodenum and proximal jejunum, requires energy, and is regulated by calcitriol, uses a vitamin D-dependent membrane channel protein called transient receptor potential (TRP) vanilloid(V)6, a vitamin D-dependent cytosolic binding protein called calbindin D9k that shuttles the calcium acorss the cell cytosol; and a vitamin D-dependent calcium-ATPase pump on the luminalside. The second route is paracellular diffusion, a passive, nonsaturable, nonregualted process in the jejunum and ileum. PhosphorusDuodenum (primary), Jejunum (primary), and Ileum: by a saturable, carrier-mediated active transport and diffusion. MagnesiumDuodenum, Jejunum (primary), Ileum (primary), and colon (mostly when a disease has interfered with small intestines absorption): by a saturable, carrier-mediated active transport and diffusion. The transient receptor potential (TRP) melastatin divalent cation-permeable channel protein (TRMP6) SodiumDuodenum, Jejunum, Ileum, and Colon: absorption by the Na+/Glucose cotransport system throughout the small intestines,the Na+ and Cl- cotransport system in the small intestines and proximal colon, and an electrogenic sodium absorptionMatthew Landry HUEC 3115 – Human Nutrition and Metabolism mechanism primarily in the colon. PotassiumDuodenum, Jejunum, and Ileum (Exact location unknown): By passive diffusion or by a K+/H+ - ATPase pump. ChlorideDuodenum, Jejunum, and Ileum: Na+/Glucose cotransport system throughout the small intestines as Cl- follows the activelyabsorbed Na+ through a paracellular pathway. Na+ creates an electrical gradient that provides the drive for the diffusion of Cl-. Additionally, the Na+ and Cl- cotransport system in the small intestinesIronDuodenum (primary), Jejunum, and Ileum: heme iron is absorbedintact across the brush border by heme carrier protein 1 (hep1). A proton-coupled folate transporter (PCFT) has also been identified. Nonheme is absorbed by the divalent cation (DCT or DMT). It may be coupled with H+ transport (symport). A membrane protein, integrin may facilitate ferric iron absorption. IodineStomach

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