|Topic:||Dietary carbohydrate .|
|Details:||Dietary carbohydrate, from which humans gain energy, enter the body in complex forms, such as disaccharides and the polymers starch (amylose and amylopectin) and glycogen. The polymer cellulose is also consumed but not digested. The first step in the metabolism of digestible carbohydrate is the conversion of the higher polymers to the simpler, soluble monosaccharide forms that can be transported across the intestinal wall and delivered to the tissues. The breakdown of polymeric sugars begins in the mouth. Saliva has a slightly acidic pH of 6.8 and contains salivary amylase that begins the digestion of carbohydrates. The action of salivary amylase is limited to the area of the mouth and the esophagus as it is virtually inactivated by the much stronger acid pH of the stomach. Once the food has arrived in the stomach, acid hydrolysis contributes to its degradation; specific gastric and pancreatic proteases and lipases aid this process for proteins and fats, respectively. The mixture of gastric secretions, saliva, and food, known collectively as chyme, moves to the small intestine.
The main polymeric-carbohydrate digesting enzyme of the small intestine is α-amylase. This enzyme is secreted by the pancreas and has the same activity as salivary amylase, producing disaccharides and trisaccharides. The latter are converted to monosaccharides by intestinal saccharidases, including maltase that hydrolyzes di- and trisaccharides composed of glucose, and the more specific disaccharidases, sucrase-isomaltase, lactase (β-galactosidase), and trehalase. The net result is the almost complete conversion of digestible carbohydrate to its constituent monosaccharides. The resultant glucose, fructose, and galactose are transported into the intestinal enterocytes via the actions of various carbohydrate transporters. Glucose is transported into enterocytes via the action of two transporters. One of these transporters is the Na+-dependent glucose transporter 1 (SGLT1) while the other is the Na+-independent glucose transporter 2 (GLUT2). SGLT1 is the major transporter of glucose from the lumen of the small intestine. Although GLUT2 does indeed transport glucose into intestinal enterocytes, this only occurs in response glucose-mediated translocation of intracellular vesicle-associated GLUT2, thus even in the absence of GLUT2 (such as is the case in individuals with Fanconi-Bickel disease), intestinal uptake of dietary glucose is unimpaired. Galactose is also absorbed from the gut via the action of SGLT1. Fructose is absorbed from the intestine via GLUT5 uptake. Indeed, GLUT5 has a much higher affinity for fructose than for glucose. These monosaccharides are then transported into the circulation via the action of enterocyte GLUT2 present in the basolateral membrane. Following entry into the duodenal superior mesenteric vein the dietary sugars travel to the hepatic portal vein and then to liver parenchymal cells and other tissues of the body. Within cells, the sugars are oxidized by the various catabolic pathways of cells or they can be used as precursors for biomass production or stored as glycogen.
Oxidation of glucose is known as glycolysis. Glucose is oxidized to pyruvate or lactate. Under aerobic conditions, the dominant product in most tissues is pyruvate and the pathway is known as aerobic glycolysis. When oxygen is depleted, as for instance during prolonged vigorous exercise, the dominant glycolytic product in many tissues is lactate and the process is known as anaerobic glycolysis. Given that erythrocytes lack mitochondria, they cannot completely oxidize glucose-derived pyruvate and instead reduce the pyruvate to lactate which enters the blood for delivery to the liver where it is used for glucose synthesis via gluconeogenesis.
COPYRIGHT ©2019 . All Rights Reserved.