4.3: Digestion and Absorption of Carbohydrates

Learning Objectives

• Discuss how carbohydrates are digested and absorbed in the human body.
• Describe blood glucose regulation.

The primary goal of carbohydrate digestion is to break down polysaccharides and disaccharides into monosaccharides that can then be converted to glucose.

From the Mouth to the Stomach

The mechanical and chemical digestion of carbohydrates begins in the mouth. Chewing, also known as mastication, crumbles the carbohydrate foods into smaller and smaller pieces. The salivary glands in the mouth secrete saliva that coats the food particles. Saliva contains the enzyme, salivary amylase. This enzyme begins carbohydrate digestion by breaking some of the bonds between individual units of disaccharides, oligosaccharides, and starches. The salivary amylase breaks down amylose and amylopectin into smaller chains of glucose, called dextrins and maltose. Only about five percent of starches are broken down in the mouth. When carbohydrates reach the stomach, no further chemical breakdown occurs because the amylase enzyme does not function in the acidic conditions of the stomach. But mechanical breakdown is ongoing—the strong peristaltic contractions of the stomach mix the carbohydrates into a semi-fluid mass of partly digested food known as chyme.

From the Stomach to the Small Intestine

Most chemical digestion of carbohydrates occurs in the small intestine. Chyme from the stomach is gradually released into the upper part of the small intestine. Upon entry of the chyme into the small intestine, the pancreas releases pancreatic juice through a duct into the small intestine. This pancreatic juice contains the enzyme, pancreatic amylase, which starts again the breakdown of dextrins into shorter and shorter carbohydrate chains. Additionally, enzymes are secreted by the intestinal cells that line the villi. These enzymes, known collectively as disaccharidases, are sucrase, maltase, and lactase. Sucrase breaks sucrose into glucose and fructose molecules. Maltase breaks the bond between the two glucose units of maltose, and lactase breaks the bond between the galactose and glucose units of lactose. Once carbohydrates are chemically broken down into single sugar units they are then transported into the inside of intestinal cells.

When people do not have enough of the enzyme lactase, lactose is not sufficiently broken down resulting in a condition called lactose intolerance. The undigested lactose moves to the large intestine where bacteria are able to digest it. The bacterial digestion of lactose produces gases leading to symptoms of diarrhea, bloating, and abdominal cramps. Lactose intolerance usually occurs in adults and is associated with a gene that is inherited and associated with ancestry from certain parts of the world. African Americans, Hispanic Americans, American Indians, and Asian Americans have much higher incidences of lactose intolerance while those of northern European descent have the least.¹ Some people with lactose intolerance can tolerate a small amount of dairy products in their diet. The severity of the symptoms depends on how much lactose is consumed and the degree of lactase deficiency.

Absorption: Going to the Blood Stream

The cells in the small intestine have membranes that contain many transport proteins in order to get the monosaccharides and other nutrients into the blood where they can be distributed to the rest of the body (see Figure \(\PageIndex{1}\)). The first organ to receive glucose, fructose, and galactose is the liver. The liver takes them up and converts galactose to glucose, breaks fructose into even smaller carbon-containing units, and either stores glucose as glycogen or exports it back to the blood. How much glucose the liver exports to the blood is under hormonal control and you will soon discover that even the glucose itself regulates its concentrations in the blood.

If needed for energy, glucose is released from the liver to the bloodstream, and on to cells that need it. Excess glucose is converted to glycogen in the liver and muscles and stored in those organs. The glycogen stored in the liver maintains blood glucose between meals; muscle glycogen provides immediate energy to the muscle during exercise. Enzymes in the liver and muscles combine glucose molecules to form glycogen through a process known as glycogenesis. Stored glycogen can be broken down into glucose when needed through glycogenolysis ("-lysis" = break down). Once the storage capacity of the liver and muscles is reached, excess glucose is stored as fat.

Figure \(\PageIndex{1}\): Carbohydrate digestion begins in the mouth and is most extensive in the small intestine. The resultant monosaccharides are absorbed into the bloodstream and transported to the liver. (CC BY-NC-SA 4.0; LibreTexts An Introduction to Nutrition (Zimmerman))

Maintaining Blood Glucose Levels: The Pancreas and Liver

Glucose levels in the blood are tightly controlled, as having either too much or too little glucose in the blood can have health consequences. Glucose regulates its levels in the blood via a process called negative feedback. An everyday example of negative feedback is in your oven because it contains a thermostat. When you set the temperature to bake a dessert at 375°F the thermostat senses the temperature and sends an electrical signal to turn the elements on and heat up the oven. When the temperature reaches 375°F the thermostat senses the temperature and sends a signal to turn the element off. Similarly, your body senses blood glucose levels and maintains the glucose “temperature” in the target range. The glucose thermostat is located within the cells of the pancreas.

After eating a meal containing carbohydrates, glucose levels rise in the blood. Beta cells in the pancreas sense the increase in blood glucose and release a hormone, insulin, into the blood. Insulin sends a signal to the body’s cells to remove glucose from the blood by transporting it to the insides of cells and to use it as fuel or to store any extra glucose. Glucose can be stored only in muscle and liver tissues. In these tissues it is stored as glycogen, a highly branched macromolecule consisting of thousands of glucose molecules. Glycogen levels do not take long to reach their physiological limit and when this happens excess glucose will be converted to fat.

Insulin has an opposing hormone called glucagon. As the time after a meal increases, glucose levels decrease in the blood. Alpha cells in the pancreas sense the drop in glucose and, in response, release glucagon into the blood. Glucagon communicates to the cells in the body to stop using all the glucose. More specifically, it signals the liver to begin glycogenolysis (the break down of glycogen into glucose) and release the stored glucose into the blood, so that glucose levels stay within the target range and all cells get the needed fuel to function properly. If additional glucose is needed, glucagon will stimulate the production of new glucose from amino acids (a process known as gluconeogenesis).

Watch the video below for a review of blood glucose regulation.

"Insulin and Glucagon" by khanacademymedicine

Leftover Carbohydrates: The Large Intestine

Almost all of the carbohydrates, except for dietary fiber and resistant starches, are efficiently digested and absorbed into the body. Some of these remaining indigestible carbohydrates are broken down by enzymes (released by bacteria) in the large intestine. The products of bacterial digestion of these complex carbohydrates are short-chain fatty acids and some gases. The short-chain fatty acids are either used by the bacteria to make energy and grow, are eliminated in the feces, or are absorbed into cells of the colon, with a small amount being transported to the liver. Colonic cells use the short-chain fatty acids to support some of their functions. The liver can also metabolize the short-chain fatty acids into cellular energy. The yield of energy from dietary fiber is about 2 calories per gram for humans, but is highly dependent upon the fiber type, with soluble fibers and resistant starches yielding more energy than insoluble fibers. Since dietary fiber is digested much less in the gastrointestinal tract than other carbohydrate types (simple sugars, many starches) the rise in blood glucose after eating them is less, and slower. These physiological attributes of high-fiber foods (i.e. whole grains) are linked to a decrease in weight gain and reduced risk of chronic diseases, such as Type 2 diabetes and cardiovascular disease.

Key Takeaways

• Carbohydrate digestion begins in the mouth with the mechanical action of chewing and the chemical action of salivary amylase. Carbohydrates are not chemically broken down in the stomach, but rather in the small intestine. Pancreatic amylase and the disaccharidases finish the chemical breakdown of digestible carbohydrates.
• The monosaccharides are absorbed into the bloodstream and delivered to the liver.
• Blood glucose levels are regulated by two hormones: insulin and glucagon.
• Some of the indigestible carbohydrates are digested by bacteria in the large intestine.

References

1. Definitions and Facts for Lactose Intolerance. niddk.nih.gov. https://www.niddk.nih.gov/health-information/digestive-diseases/lactose-intolerance/definition-facts. Accessed November 10, 2022.