Digestion and absorption of glycemic mono- and disaccharides using the Caco-2 cell model

Min-Wen Cheng, Purdue University

Abstract

Glycemic carbohydrates are the primary energy source for humans. Glycemic response after their consumption varies due to several food factors such as food matrix, food additives, and food structure. Among the factors, different structures of glycemic carbohydrates and their affect on digestion and absorption rates appear to be important. The specific goal of this project was to understand digestion and absorption mechanisms of simple glycemic substrates at the level of the small intestine enterocyte, with the overall aim to tailor carbohydrates to specific glycemic responses. The first consideration in this project was to select a model to test carbohydrate digestion behavior. Caco-2 cells, which differentiate in vitro into small-intestinal enterocyte-like cells were used. Although sucrase-isomaltase (SI) is the only mucosal glucogenic enzyme expressed by Caco-2 cells (maltase-glucoamylase is absent), this enzyme complex has the potential to digest every type of glycemic carbohydrate except lactose. In this study, Caco-2 cells were incubated with the disaccharides maltose, sucrose, and isomaltose, with cells showing only minor sucrase and isomaltase activities comparing to maltase activity. Cell viability was accordingly lower in cells fed with sucrose and isomaltose. Digestion rate of disaccharides correlated to apical to basolateral transport rate of the released monosaccharides. Caco-2 cells did not maintain their viability for more than 2 days at a concentration of 12.5 mM glucose in the growth medium. Adding an additional 6.25 mM maltose to the media provided for full cell viability. In contrast, cells only utilized glucose when cells were fed with both glucose and sucrose. Quantitative real-time PCR (q-RT PCR) results demonstrated an increase in synthesis of SI and glucose transporters GLUT2 and GLUT5 when substrates were present with differing quantitative responses. For example, glucose, compared to fructose and the other three disaccharides, stimulated the highest transcription of SGLT1, and fructose caused the highest transcription of SI, GLUT5, and GLUT2. Western blot-based protein expression of SI and the three glucose transporters also showed stimulation by glucose and fructose. This suggests that protein expression of SI and sugar transporters are transcriptionally regulated. Some coordination was observed between the expression of the SI gene and sugar transporter genes, which suggest a correlative relationship between digestion and absorption. Western blot analysis showed that maltose incubation of Caco-2 cells over time resulted in synthesis of a higher molecular weight form of SI. This stimulation effect was concentration-dependent with highest expression of the variant occurring at 24 hours of feeding cells with maltose. This suggests a complexity of response to sugar sensing of the intestinal enterocytes that may be substrate specific. Taken together, Caco-2 cells sensed different types of carbohydrates and resulted in different behavior sometimes with a coordinated response of digestion and absorption. As carbohydrate structures were shown to influence cell response, for better control of glycemic response, carbohydrate structures must be taken into consideration.

Degree

Ph.D.

Advisors

Zhang, Purdue University.

Subject Area

Food Science

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