Digestion of carbohydrates Alongside fat and protein, carbohydrates are one of the three macronutrients in our diet with their main function being to provide energy to the body. They occur in many different forms, like sugars and dietary fibre, and in many different foods, such as whole grains, fruit and vegetables. Digesting or metabolizing carbohydrates breaks foods down into sugars, which are also called saccharides. These molecules begin digesting in the mouth and continue through the body to be used for anything from normal cell functioning to cell growth and repair. It is also directly used as energy source in muscle, brain and other cells. Dietary carbohydrate principally consists of …. Polysaccharides: - Starch, glycogen and cellulose Disaccharides: - Sucrose and maltose Monosaccharide:-Glucose and fructose Out of these three types of carbohydrates, monosaccharide does not need digestion. Digestion of carbohydrates During digestion, carbohydrates that consist of more than one sugar get broken down into their monosaccharides by digestive enzymes, and then get directly absorbed causing a glycaemic response. Some of the carbohydrates cannot be broken down and they get either fermented by our gut bacteria or they transit through the gut without being changed. Digestive enzymes and their source Mouth: - Salivary amylase (α-amylase, Ptyalin) Stomach: - No enzymes Pancreas: - Pancreatic α-amylase Intestine: - Dextrinase, Maltase, Isomaltase, Sucrase, Lactase Digestion in mouth Digestion of carbohydrates starts at the mouth. In mouth, food undergoes mastication during which it comes in contact with saliva (secreted by salivary gland), Saliva releases an enzyme called amylase, which begins the breakdown process of the sugars Action of salivary amylase It required Cl- ion and pH 6.7 for activation The enzymes hydrolyses α-(1-4) glycosidic bond at random deep inside the polysaccharides producing dextrins, maltose, maltotriose, glucose α-amylase Glycogen molecules/Starch Dextrin (oligosaccharides) + Maltotriose + Isomaltose + Maltose Digestion in stomach After the digestion in mouth, carbohydrates travel to stomach through esophagus. At this stage, the food is referred to as chyme where stomach makes acid to kill bacteria in the chyme before it makes its next step in the digestion journey. Here, digestion of carbohydrates also temporarily stops due to high acidity of stomach and unavailability of digestive enzymes in gastric juice. Digestion in intestine The chyme then goes from the stomach into the first part of the small intestine, called the duodenum. This causes the pancreas to release pancreatic amylase. This enzyme breaks down the chyme into limit dextrin, maltotriose, Isomaltose and maltose. From there, the wall of the small intestine begins to make lactase, sucrase, and maltase. These enzymes break down the sugars even further into monosaccharides or single sugars. So, there are two phase of intestinal digestion:- 1. Digestion due to pancreatic amylase and 2. Digestion due to intestinal brush border enzymes Digestion due to pancreatic amylase α-amylase Dextrin Limit Dextrin + Maltotriose + Isomaltose + Maltose (Oligosaccharide) Digestion due to intestinal brush border enzymes These enzymes are responsible for final digestion of carbohydrate. The enzymes and their reactions are as follow….. Dextrinase Limit dextrin Glucose + Maltose + Maltotriose Isomaltase Isomaltose Glucose + Glucose Maltase Maltose Glucose + Glucose Sucrase Sucrose Glucose + Fructose Lactase Lactose Glucose + Galactose These sugars are the ones that are finally absorbed into the small intestine. Once they’re absorbed, they’re processed even more by the liver and stored as glycogen. Other glucose is moved through the body by the bloodstream. Digestion of cellulose Cellulose is a polysaccharide which is mainly found in plants. It contain β-(1-4) glycosidic bond in its stricture. Human cannot synthesize the enzyme which can break β glycosidic bond. So, cellulose is not digested by human but it is one of the important components in the diet. Undigested cellulose provides bulk as fiber in the diet which helps in intestinal motility and as a stool softer. Dr. Praveen Kumar Assistant Professor Bihar National College Patna University, Patna .