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Examine a new type of starch that is slowly digestible.

Glucose, a starch digestion product, is the single most highly regulated nutrient in the human body. To generate dietary glucose from starch, salivary and pancreatic alpha-amylase as well as gut mucosal alpha-glucosidases must be involved in the digestive process.

To date, controlling the digestion rate has focused primarily on pancreatic alpha-amylase, which quickly breaks down starch molecules. However, actual glucose generation, or glucogenesis, occurs at the level of gut mucosal alpha-glucosidases. These glucosidases are comprised of two enterocyte membrane bond complexes of N- and C-terminals each, as well as of maltase-glucoamylase (MGAM) and sucrase-isomaltase, making them the ultimate step in producing dietary glucose.

Purdue University research has shown how four alpha-glucosidase subunits can affect starch digestion. All four subunits are capable of attacking raw starch granules and digesting native gelatinized starch molecules and alpha-limiting dextrin (LDx) in a different manner.

These four subunits have demonstrated their differential roles in the digestive process. Nt-MGAM only digests short linear malto-oligomers. The other three subunits have additional activity against branch structures. At the gelatinized starch level, notably Ct-MGAM has a high digestion activity of about 80% without alpha-amylase participation. The other three subunits digested 20% to 30% of starch molecules.

The large size of starch molecules does not appear to be a hindrance to mucosal alphaglucosidase digestion. However, the role of the individual subunits depends on the structure of the substrate. Starch and alpha-LDx from different botanical sources have different susceptibilities to the four alpha-glucosidase subunits and showed that substrate structure determines the starch digestion rate of the cells in the digestive system.

There is a certain fraction of starch molecules that is very slowly digested or is resistant to both the alpha-amylase and mucosal alpha-glucosidases. These represent a new level of resistant starch or slowly digestible starch. The mucosal alpha-glucosidases can control dietary glucose generation from starchy food in the human body, further affecting any physiological responses by the body.

Further information. Hui Mei (Amy) Lin, Whistler Center for Carbohydrate Research, 745 Agriculture Mall Dr., Purdue University, West Lafayette, IN 47907; phone: 765-496-7849; email:
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Publication:Emerging Food R&D Report
Date:Apr 1, 2013
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