Gastrointestinal tract viscosity

A summary of how physiological factors affect the dissolution rate is given in Table 21.2. The effective surface area will be affected by the wetting properties of the bile acids and other surface-active agents in the gastrointestinal tract. The dif-fusivity of a drug molecule in the intestinal juice will be altered by changes in viscosity that are induced, for instance, by meal components. An increased dissolution rate could be obtained at more intense intestinal motility patterns or increased... 

As noted in Section II.E, both layers decrease the absorption rate, especially of large water-soluble molecules, since the diffusion coefficient is directly proportional to the reciprocal values of both the hydrodynamic radius and the viscosity of the layers. About the influence of absorption enhancers on mucus rheology very little is known. Bile salts and phospholipids are known to reduce the viscoelasticity of bronchial mucus and probably also of mucus layers in the gastrointestinal tract [40], The results should be interpreted with caution since it is not the observed (macro)viscosity that determines the diffusion rate in polymer networks but the much lower (micro)viscosity between the polymer chains [41]. 

Despite the fact that a plethora of dietary factors could, and will, affect the absorption characteristics of phytochemicals, this area has not been systematically explored. One reason might be the complexity of dietary factors and their interactions that could affect absorption. A nonexhaustive list would include the volume and composition of the food consumed, pH, caloric density, viscosity, nutrients (carbohydrates, protein, fat, fibers), alcohol, caffeine, and the presence of other phytochemicals. Such dietary factors affect the functional status, motility, and acidity of the gastrointestinal tract in a complex manner and modify the physicochemical properties, formulation, and dissolution characteristics of the compound of interest. Calcium in dairy products, for example, has the potential to chelate tetracyclines and fluoroquinolones and, thereby, reduce their bioavailability and biological activity [31]. 

When drug molecules pass through the gastrointestinal tract, they encounter different environments with respect to the pH, enzymes, electrolytes, surface characteristics and viscosity of the gastrointestinal fluids. All these factors can influence drug absorption and interactions. Variations in the pH of various portions of the gastrointestinal tract are depicted in Fig. 5.3. 

Improves function of gastrointestinal tract, increases viscosity, and lowers glycemic index. Reduces risk of diabetes, and hypercholesterolemia. Most soluble fibers are probiotic because they are readily fermented in the hind gut, yielding short-chain fatty acids that inhibit hepatic HMG Co A reductase. 

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