Modelling nutrient absorption

MODEL OF ROOT AERATION VERSUS NUTRIENT ABSORPTION... 

In conclusion, the Caco-2 cell monolayer model has given original data on the competition effect of several nutrients on carotenoid uptake. Most of these data have been confirmed in several in vivo studies, including clinical studies, confirming that this model is a valuable tool to study competition effects on carotenoid absorption. 

The demonstration that eclectic mechanisms have been evolved by insects for coping with the potentially toxic concomitants of their ingested nutrients necessitates careful analyses of the processing of each of these compounds by each adapted herbivore. Furthermore, it is important to realize that in Itself, sequestration is nothing more than an end product of a series of reactions that may reflect selective absorption, metabolism of specific compounds, and excretion of selected allelochemics ). In the present review, these diverse processing strategies will be explored in order to illustrate the various ways in which a multitude of herbivores accommodate potential plant toxins. IVo lepidopterous species will be used as models which, hopefully, will emphasize both the elegance and complexity identified with insects as processors of plant-derived compounds. 

From the above it can be concluded that in many instances the introduction of an artificial radionuclide into the environment provides us with a natural tracer experiment. Indeed, this is the basis for the application of deterministic compartmental models, based on tracer kinetics, to radioecology (Whicker and Schultz, 1982). This approach is largely based on the assumption that radionuclide movements will exhibit first order kinetics although the existence of naturally-occurring tracees (stable isotopes) at relatively high abundance may result in more complex concentration-dependent kinetics. Furthermore, nutrient analogues may exert even more complex effects on processes such as radioion absorption across root plasma membranes this will become evident later in the chapter. 

Extensive research on the absorption of iron from various types of meals has allowed guidelines to be developed by which the amount of dietary iron available for absorption may be estimated. Iron is the first trace mineral to be thus treated and thus serves as a model for other nutrients (19). The model for estimating bioavailable iron is based on the concept that iron forms a) a pool of heme iron which is readily available to humans and is uneffected by other dietary components and b) a pool of nonheme iron which is of low bioavailability unless enhancing factors are present concommitantly (20). 

A decrease in circulating tryptophan levels may be of importance in some models of hypertension. The DS rat is an example of this. The reduction in plasma tryptophan was clearly associated with the excess intake of NaCl and concomitant elevation in blood pressure. Mechanisms that could account for the reduction in plasma tryptophan could include a decrease in gastrointestinal absorption of tryptophan, an increase in urinary tryptophan excretion, increased oxidation of tryptophan by tryptophan oxygenase, and aberrant regulation (hormonal, nutrient, plasma protein) of tryptophan... 

Pharmacokinetic models describe the biological processes of absorption, distribution, metabolism, and excretion of exogenous compounds. With the exception of absorption into and across the skin, absorption, distribution, metabolism, and elimination are essential processes that also occur with nutrients and endogenous compounds. Many of these processes can become nonlinear with increasingly higher exposure concentrations. Pulmonary absorption can be limited by respiratory rates. Metabolism can often show Michaelis-Menten or saturable kinetics. Distribution can be limited by the affinity of the chemical for a specific tissue or by blood flow. 

Chemical additives used in baking and not already present in flour in the required quantities are added to the dough in the form of a premix. Osborne [17] used NIR in a model experiment to determine three bread improvers (ascorbic acid, L-cysteine, and azodicarbonamide) in admixture in a starch diluent. Although the accuracy of each calibration was not satisfactory for their determination at the levels encountered in bakery premixes, the results were sufficient to indicate the potential of NIR for this type of application. This potential has been realized in the determination of nicotinamide in nutrient premixes where Osborne [18] obtained excellent results by NIR compared with high-performance liquid chromatography. The NIR calibration involved a ratio of second derivatives at 2138 nm, corresponding to a nicotinamide absorption band, and 2070 nm as a reference. 

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