Submitted to: Journal of Nutrition
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/21/2008
Publication Date: 2/4/2009
Citation: Nichols, B.L., Quezada-Calvillo, R., Robayo-Torres, C.C., Ao, Z., Hamaker, B.R., Butte, N.F., Marini, J., Jahoor, F., Sterchi, E.E. 2009. Mucosal maltase-glucoamylase plays a crucial role in starch digestion and prandial glucose homeostasis of mice. Journal of Nutrition. 139:684-690. Interpretive Summary: Starches are the main food source of glucose. Glucose is essential for brain function, but excess glucose leads to degenerative diseases such as diabetes and obesity. Two different genes produce four related intestinal enzyme activities needed for food starch digestion to glucose. Maltase-glucoamylase is essential for brain function and sucrase-isomaltase constrains degenerative diseases. Because of the overlapping, we removed the activities produced by the maltase-glucoamylase gene. We measured the rate of starch digestion in the enzyme deficient mice and observed that actual glucose produced from fed starch was reduced by 40%; however, total blood glucose response to feeding remained intact. Because of reduced glucose digestion from starch the liver increases new glucose production sufficient to normalize total blood glucose levels. These results show that measurement of total blood glucose response to feeding, the so-called "glycemic index", is not an accurate measure of starch digestion to glucose and that the presence of the maltase-glucoamylase gene is necessary for normal starch digestion to glucose.
Technical Abstract: Starch is the major source of food glucose, and its digestion requires small intestinal alpha-glucosidic activities provided by the 2 soluble amylases and 4 enzymes bound to the mucosal surface of enterocytes. Two of these mucosal activities are associated with sucrase-isomaltase complex, while another 2 are named maltase-glucoamylase (Mgam) in mice. Because the role of Mgam in alpha-glucogenic digestion of starch is not well understood, the Mgam gene was ablated in mice to determine its role in the digestion of diets with a high content of normal corn starch (CS) and resulting glucose homeostasis. Four days of unrestricted ingestion of CS increased intestinal alpha-glucosidic activities in wild-type (WT) mice but did not affect the activities of Mgam-null mice. The blood glucose responses to CS ingestion did not differ between null and WT mice; however, insulinemic responses elicited in WT mice by CS consumption were undetectable in null mice. Studies of the metabolic route followed by glucose derived from intestinal digestion of (13)C-labeled and amylase-predigested algal starch performed by gastric infusion showed that, in null mice, the capacity for starch digestion and its contribution to blood glucose was reduced by 40% compared with WT mice. The reduced alpha-glucogenesis of null mice was most probably compensated for by increased hepatic gluconeogenesis, maintaining prandial glucose concentration and total flux at levels comparable to those of WT mice. In conclusion, mucosal alpha-glucogenic activity of Mgam plays a crucial role in the regulation of prandial glucose homeostasis.