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A dynamic model of digestion and absorption in pigs

Strathe, Anders Bjerring, Danfær, Allan, Chwalibog, Andre
Animal feed science and technology 2008 v.143 no.1-4 pp. 328-371
simulation models, dietary fiber, digestion, intestinal absorption, kinetics, swine
The paper describes and evaluates the construction of a mathematical model to study the kinetics of digestion and absorption in growing pigs. The core of the model is based on a compartmental structure, which divides the gastro-intestinal tract into four anatomical segments: the stomach, two parts of the small intestine and the large intestine. Within the large intestine, a microbial sub compartment is also considered. In each of these segments, the major organic nutrients are considered: dietary protein, endogenous protein, amino acids, non-amino acid and non-protein nitrogen, lipids, fatty acids, starch, sugars and dietary fibre. Besides a chemical description of the feed, the model further requires information about daily dry matter intake and feeding frequency. Flows of nutrients from one anatomical segment to the next are assumed to be mass action processes, whereas degradation and absorption are described as enzymatic processes and are therefore represented by saturation kinetics. The model also proposes a basic concept for integrating effects of anti-nutritional factors into a mechanistic framework. The total number of pools is 38 and their rates of change are described by differential equations on the principle of mass conservation. Parameterization of the equations describing secretion and absorption is derived from literature values, whereas parameters for degradation are empirically adjusted to ileal and faecal digestibility coefficients determined for 24 diets highly variable in chemical composition. A sensitivity analysis has revealed that the prediction of nutrient digestibility is insensitive to affinity constants and moderately sensitive to maximum velocities for nutrient degradation. Predictions of apparent protein digestibility and endogenous nitrogen loss are especially sensitive to a reference level of dietary fibre in the last segment of the small intestine. The reference quantity of organic matter, which is related to the transit in the large intestine, has a large impact on the predicted apparent digestibility of dietary fibre and energy. The model predicts coefficients of crude protein and energy digestibility with good accuracy across 65 diets from 11 studies. Lipid is predicted less accurately. Predictions of endogenous protein losses in response to increased dietary fibre are compared with results from eight studies including 39 diets and it is concluded that the predictions are moderately satisfactory. The model predicts the pattern of organic matter flow at the terminal ileum and the appearance of nutrients in the portal blood with fairly good accuracy. The validation of kinetics of nutrient absorption has highlighted the necessity for future models to include a representation of gut wall metabolism in order to validate digestive models at their most valuable terminal output. Full utilization of the model as a predictive tool can be expected when it is combined with a metabolism model.