Absorption in the Gastrointestinal Tract

The ingested amounts of vitamin C are mainly absorbed in the duodenum, in the proximal jejunum, and through the buccal mucous membrane. The physiological daily intake of vitamin C of up to 180 mg is absorbed 80 to 90%. In humans and guinea pigs the absorption mechanism of L-ascorbate depends on a pH-dependent saturable active transport and relies on the presence of a carrier and sodium ions. Dehydroascorbic acid is absorbed by facilitated diffusion. For animals with the 

Analogues of Ascorbic Acid and Their Biological Activity 

FIGURE 2. Urinary excretion of ascorbic acid (AA) following a single oral dose (starting with 1000 mg) and the percentage of excreted ascorbic acid compared to ingested ascorbic acid (curves are drawn from data published by Homig et ai, 1980). 

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Methods for Reducing Toxic Effects. The extent of lead absorption in the gastrointestinal tract... 

Human food, both plant and animal, usually contains satisfactory amounts of zinc to cover the requirement for this metal, which is present within the range of few to several pg per g of product. Zinc deficiencies are usually caused by a reduction of its absorption in the gastrointestinal tract rather than by its lack. Reduction in absorption may be caused by antagonistic activity of cadmium, calcium or phytates. A decrease in assimilation of zinc is also observed among alcoholics. 

Absorption in the gastrointestinal tract of triethanolamine administered orally to Wistar rats is rapid 63% of the dose disappeared from intestines within 65 min (Kohri et al., 1982). In dermal toxicity studies, the peak blood levels of [i C]triethanolamine were observed 2 h after its application in C3H/HeJ mice (2000 mg/kg bw), whereas in Fischer 344 rats (1000 mg/kg bw), the blood levels (expressed as radioactivity) indicated that triethanolamine was absorbed less rapidly than by mice. Data from various studies in mice and rats (1000-2000 mg/kg bw) suggest that absorption of dermally administered triethanolamine is almost complete in 24 h (Waechter Rick, 1988, cited in Knaak et a/., 1997). 

However, it is important to note that the system does not actually facilitate oral transmucosal delivery per se, rather it allows rapid release of the drug in the mouth. The drug is then washed down with the saliva for subsequent absorption in the gastrointestinal tract. 

Recently, a novel convection-dispersion model for the study of drug absorption in the gastrointestinal tract, incorporating spatial heterogeneity, was presented [182]. The intestinal lumen is modeled as a tube (Figure 6.7), where the concentration of the drug is described by a system of convection-dispersion partial differential equations. The model considers ... 

The past five years or so have seen the proliferation of LC-MS instruments to the extent that they have now replaced HPLC in the majority of laboratories. These instruments complement GC-MS and with it enable the analyst for the first time to routinely generate data on both thermally labile and stable compounds of up to 1000 daltons in rubber products and food 1000 daltons is the established upper limit for chemical absorption in the gastrointestinal tract. This new application of LC-MS, together with the research work that is being carried out on rubbers (section 12.5.4), should enable more accurate conformity checks to be performed on compounds, as well as continuing to add to the understanding of the migration behaviour of rubber-related species. 

Vitamin D, apart from its availability in the diet, is produced in the body, in response to sunlight striking the skin, and really acts like a hormone. It helps to raise the blood levels of calcium and phosphorus in several ways. It stimulates calcium and phosphorus absorption in the gastrointestinal tract promotes transfer of calcium and phosphorus from bone to blood promotes calcium retention by the kidney. As a hormone it acts 1 entering the cell nucleus and interacting with DNA to regulate protein synthesis. 

Ethyl acetate is readily absorbed following oral, dermal, or inhalation exposures. However, it is rapidly hydrolyzed to ethanol and acetic acid prior to absorption in the gastrointestinal tract following oral administration, and in the upper respiratory tract following inhalation exposure. Absorbed ethyl acetate is also quickly hydrolyzed in blood to ethanol and acetate. Blood concentrations of ethanol can increase if the production of ethanol, following absorption and hydrolysis of ethyl acetate, exceeds metabolism and elimination of ethanol. 

The primary sites and effects of drug-drug interactions involving other medications and MPA are likely to be decreased absorption in the gastrointestinal tract, inhibition of enterohepatic cycling, and inhibition of transport of the primary phenolic glucuronide metabolite. Meal consumption just before oral intake of MMF delays absorption, causing a reduction in the maximal concentration by about... 

The concentration of ionized calcium is closely regulated by the interactions of parathyroid hormone (PTH), phosphorus, vitamin D, and calcitonin (Fig. 49-9). Parathyroid hormone increases serum calcium concentrations by stimulating calcium release from bone, reducing renal excretion, and enhancing absorption in the gastrointestinal tract secondary to increased renal production of 1,25-... 

Oseltamivir (GS 4104, 4a), developed by Gilead/Hoffmann-LaRoche and marketed as Tamiflu , is the first commercially available orally active, noncarbohydrate mimetic of Zanamivir (3) [49-51]. It is based on a cyclohexene framework with a lipophilic moiety replacing the glycerol side chain of Zanamivir (3). It is orally administered as an ethyl ester prodrug GS 4104 (4a), which—upon absorption in the gastrointestinal tract—undergoes rapid enzymatic conversion by the action of... 

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