| Glucose absorption rate of the gastrointestinal tract is correlated with plasma glucose and is a helpful health indicator for patients with metabolic syndromes. Carbohydrates are the main energy source of human diet, most of which are converted to glucose by the digestion of our body. Glucose is mainly absorbed into the bloodstream in the small intestine. When refined or rapidly digested carbohydrates are included on a regular basis in our diets, glucose absorption rates are increased, and blood glucose and insulin responses are usually enhanced, resulting in dietary related chronic diseases such as diabetes and obesity. Thus, the glucose absorption rate of the gastrointestinal tract is considered to be an important parameter to evaluate the risk of highly glycemic index of daily food.To date, several well-established animal and cell-based models are used for studying intestinal absorption, including in vitro (Caco-2 cells model), ex vivo (Ussing chamber) or in situ (Small intestinal perfusion models) and in vivo animal models. For the evaluation of intestinal glucose absorption rate, the in vitro cell models have several limitations such as wide variation with passage number, variability between different laboratories. The ex vivo or in situ methods of intestinal perfusion technique are simple for estimating intestinal absorption rate. However, as all ex vivo/in situ models, tissue viability of the intestinal respiratory system have a significant effect on the results. In addition, the barrier imposed by the freshly isolated animal intestine may result in slower absorption rates than those obtained in intact animals. Starch digestion and glucose absorption in the small intestine are very complicate, including the nervous reflex from the taste receptors, induction of hormones, secretion of enzymes into the gastrointestinal tract and increase of absorption rate. As reviewed above, the in vitro cell models and ex vivo or in situ methods may not be suitable for the evaluation of small intestinal glucose absorption rate. Herein, the in vivo animal models were used in this study. The main advantage of in vivo animal models is the presence of intact system.As a classical method, the glucose absorption rate of lots of animals had been investigated in intact individuals before. However, the drawbacks of the chemical analysis of intestinal sample are obvious:1) Always need an extra addition of inert marker, which is inconvenient and hard to detect. Moreover, as a normal use inert marker, chromic oxide (Cr2O3) is toxic for the animals.2) Need large amount of content for each test. So far as we know, the smallest animal that had enough content samples from small intestine and could be investigated by the in vivo method (intact individuals) is chick. To study the glucose absorption rate in the laboratory model animals (such as murine) in large scales conveniently, the discovery of novel inert marker are necessary, and the determination method of the starch and novel inert marker are needed to be optimized for few content sample.Analysing the component of normal feed of some laboratory model animals, we found resistant starch has the potential to work as a novel inert marker without extra addition. Resistant starch (RS) is a sort of starch, which escapes digestion of hydrolytic enzyme in the small intestine. RS could be degraded in the large intestine by the fermentation of colonic microorganisms. Although there are microorganisms in the proximal intestine,99.99% of human intestinal microorganisms are located in colon Moreover, the contents in the proximal intestine are more fluid than in ileum or colon and the retention time of feed particles in the proximal intestine is shorter, which largely lower the fermentation by microorganisms, especially in duodenum and jejunum.In this study, the glucose absorption rate of the small intestine was investigated by analysing the intestinal contents in vivo. Resistant starch was considered as a novel and endogenous non-absorbable inert marker in normal feed to correct for the intestinal fluid volume. The glucose absorption rate of the small intestine of chick (White Rock) was calculated to compare with the classical method. With the modification of the standard method of determination of the resistant starch, the glucose absorption rate of some small laboratory model animals (Sprague Dawley rat, C57/BL6 mouse, Cavia porcellus and Golden hamster) were also detectable by using few small intestinal contents. The resistant starch, a novel and endogenous inert marker, was also useful in determination of other nutrients absorption rate in the small intestine, especially the duodenum and jejunum.To date, glucose absorption rate was mainly determined by ex vivo/in situ tissue models, only larger mammals or poultry were investigated with intact animals (in vivo methods), using non-absorbable compounds as inert markers. In this study, we established a novel strategy by using resistant starch as an endogenous inert marker to determine the glucose absorption of small intestine. By optimization of the classical enzymatic method of starch measurement, the demanding weight of the sample for testing was reduced by 90%. The Glucose absorption rates were similar when using different inert marker techniques (RS and the classical yttrium-91) in birds. The fermentation of resistant starch by intestinal microorganisms was demonstrated not disturbing detection of glucose absorption rate significantly, especially in the duodenum and jejunum, by comparison analysis with the classical marker. Four different species of murine showed different glucose absorption rates as expected, which indicated the in vivo model of glucose absorption was well established using a novel endogenous marker in small animals. Moreover, the resistant starch was also useful as an inert marker in determination of other nutrients absorption rate in the small intestine.The comparison of assays with different markers in chicks showed that resistant starch may be regarded as a reliable inert marker for the measurement of glucose absorption rate of small intestine, and the fermentation of carbohydrate including the RS by microorganisms in the small intestine seemed not to induce a significant deviation when using RS as a marker. The optimization for the procedure of RS measurement largely decreased the demand of sample weight, which made it possible that the small animals in laboratory could be used as models to study the glucose absorption rate of small intestine. It’s worth noting that the determination method of glucose absorption rate described in this study was designed just for small intestine, but not applicable for the rest part of gastrointestinal tract intestine where more microorganisms lived. The resistant starch was also useful as an inert marker in determination of other nutrients absorption in the small intestine. |
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