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Jennifer Atkinson October 14, 2013 HUN 3230 Section 81944 Carbohydrates and Type 2 Diabetes Atkinson1 Carbohydrates are one of the main, essential sources of nutrients in the body and provide the most amount of energy to one’s body by means of being consumed in the production of ATP. Carbohydrates are composed of carbon, hydrogen, and oxygen molecules. There are various types of carbohydrates that occur in the body and food sources. There are simple carbohydrates that contain only one or two sugar molecules and complex carbohydrates that contain three to hundreds or thousands of monosaccharides. The simple carbohydrates, or simple sugars, are referred to as monosaccharides and disaccharides. Monosaccharides have only one sugar molecule and are the sugars known as glucose, fructose, and galactose. When these sugars combine with one another, they form the two sugar molecules known as disaccharides, which include lactose, maltose, and sucrose. Simple carbohydrates come from food sources such as soft drinks, candy, and fruit. The complex carbohydrates incorporate oligosaccharides that contain three to ten monosaccharides and polysaccharides that can consist of hundreds to thousands of glucose molecules. Starch, glycogen, and most fibers are considered to be polysaccharides. Food sources that are composed of complex carbohydrates include breads, potatoes, rice, nuts, and flour. The RDA for carbohydrates is about “125-175 grams, which equals about 55-60% of one’s total chloric intake”3. 1,2,3 Carbohydrate digestion begins right as it enters the body, in the mouth. Once food has arrived in the oral cavity, the salivary glands release an enzyme known as salivary amylase or alpha-amylase. A-amylase works as a catalyst to break down the starch and transform it into sugar. “Amylase digests starch by catalyzing hydrolysis, which is splitting by the addition of a water molecule. Therefore starch plus water becomes maltose (which is equivalent to two joined glucose molecules).”4 When the food arrives in the stomach, Atkinson 2 carbohydrate digestion ceases for that time because the “gastric acid penetrates the food bolus and lowers the pH sufficiently to inactivate the enzyme [a-amylase].”1 The digestive process picks back up when the chyme enters the duodenum. The pancreas releases pancreatic a-amylase to hydrolyze the polysaccharides into disaccharides. The small intestine then produces the enzymes, lactase, sucrase, and maltase that break down the disaccharides into monosaccharides.1,4 Now that the starch is completely broken down into monosaccharides, the small intestine is ready to begin absorbing them. The glucose, fructose, and galactose that are produced get absorbed by the enterocytes that are in the upper half of the villi in the small intestine. Only D-glucose and D-galactose are actively absorbed in the small intestine. Dfructose is not actively absorbed but has a greater rate of absorption than would be expected by passive diffusion5. Glucose and galactose are actively transported into the cytoplasm of the enterocyte by the sodium dependent glucose transporter, SGLT1, and “exit across the basolateral membrane by the glucose transporter GLUT2.”6 Once glucose, galactose, and fructose cross the intestinal wall, they enter portal circulation where they go directly to the liver. Once in the liver, the galactose and fructose are metabolized and converted into glucose.5,6 Carbohydrate metabolism is composed of three pathways: Glycolysis, Krebs Cycle, and the Electron Transport Chain (ETC). Glycolysis is the first step of carbohydrate metabolism and it occurs in the cytosol. In this process, one six-carbon glucose molecule is split into two three-carbon pyruvate molecules. The outcome of this process is two net ATP, because four are created but two are used, and two NAD are hydrogenated (Table 1). Then the pyruvate from glycolysis is converted into Acetyl CoA in the cytosol and then it is moved Atkinson 3 into the mitochondria. The mitochondria are the site for the Krebs cycle. The Krebs cycle begins with the two-carbon Acetyl CoA and the four-carbon oxaloacetic acid to make a sixcarbon citric acid molecule. The citric acid then goes through a series of reactions where NAD+ is hydrogenated to make NADH, ADP is used to make ATP, and two carbon dioxide molecules are released. Since the two carbon dioxide molecules get released, the six-carbon citric acid molecule is then made into a oxaloacetic acid molecule, which cycles back through to meet up with another Acetyl CoA molecule and the Krebs cycle continues on (Table 2). From the Krebs cycle, six NADH, two ATP, and two FADH2 are produced. The final step in carbohydrate metabolism is the Electron Transport Chain. The ETC occurs in the inner mitochondrial membrane of the cell. In this process, NADH and FADH2 are oxidized and their electrons are donated to O2, which gets reduced to H2O. The energy released from reducing oxygen to water is used to phosphorylate mitochondrial ADP to ATP (Table 3). In total, the metabolism of one glucose molecule creates 38 ATP from these three cycles.1, 2,3 Carbohydrates are essential to one’s body. The primary role that carbohydrates play is their use for short-term energy storage. Shown in tables 1-3, the metabolism of carbohydrates produces a vast amount of energy for one’s body that can be used right away. A secondary function is intermediate-term energy storage. When a lot of carbohydrate is consumed at once and one’s blood sugar level is adequate, the excess of glucose can be stored in the liver and muscles as glycogen and used if one’s blood-sugar level begins to decline. Other carbohydrates are involved in the structural components in cells.1,2 Because the amount of carbohydrates one intakes greatly affects one’s blood-glucose levels, Type 2 Diabetes and the intake of carbohydrates are very much linked together. Type 2 Diabetes is the most common form of diabetes and it occurs when one’s blood-glucose Atkinson 4 levels rises higher than normal, also called hyperglycemia, or lower than normal, which is hypoglycemia. In type 2 diabetes, either one’s body does not produce enough insulin or the cells deny the insulin. Insulin is essential for one’s body to be able to transform glucose into energy. Once polysaccharides are chemically broken all the way down into glucose, the glucose must get into the cells to be able to make ATP. Glucose from the blood is assisted into the cells by insulin. If glucose is unable to get into the cells, the body will lack in energy and could potentially shut down completely and seize or fall into a coma.7 If one’s blood-glucose level rises higher than normal, they may have frequent urination and an increase in thirst. When hypoglycemia occurs, one may become shaky, lightheaded, or weak; all indicating that one needs to check his/her blood-glucose level. Those with Type 2 Diabetes need to have a specific diet planned out for them to know how many carbohydrates they should eat to maintain healthy blood-glucose levels. To prevent hyperglycemia, one should have a regular exercising plan, one should limit the amount of food they intake, and be sure to drink a lot of water to maintain homeostasis within the blood and cells. For both high and low blood-glucose levels, one needs to be very in tune with one’s body and be able to detect any abnormal sensations within one’s body.7 Atkinson 5 Table 1 Atkinson 6 Table 2 Atkinson 7 Table 3 Atkinson 8 Reference List 1. Gropper SS, Smith JL. Advanced Nutrition and Human Metabolism. Wadsworth, Cengage Learning. Belmont, CA; 2013; 63-96 2. Thompson JL, Manore MM, Vaughan LA. The Science of Nutrition. Pearson; 2014; 118151 3. Georgia Highlands College Web site. Nutrition and Metabolism. http://www.highlands.edu/academics/divisions/scipe/biology/faculty/harnden/2122/notes/ metab.htm. Accessed October 9, 2013. 4. Vance, Nena. Effects of Salivary Amylase. http://biology.clc.uc.edu/students/114Fall99/Amylase.htm. Accessed October 7, 2013. 5. FAO Corporate Document Repository. Digestion, absorption and energy value of carbohydrates. http://www.fao.org/docrep/w8079e/w8079e0k.htm Accessed October 12, 2013. 6. Levin RJ. PubMed Web site. http://www.ncbi.nlm.nih.gov/pubmed/8116552. Accessed October 12, 2013. 7. American Diabetes Association Web site. http://www.diabetes.org/?loc=logo. Accessed October 12, 2013.