Digestion and Absorption of Carbohydrates

From the Mouth to the Stomach

The mechanical and chemical digestion of begins in the mouth. Chewing, also known as mastication, crumbles the carbohydrate foods into smaller and smaller pieces. The salivary glands in the oral cavity secrete saliva that coats the food particles. Saliva contains the enzyme, . This enzyme breaks the bonds between the monomeric sugar units of disaccharides, oligosaccharides, and starches. The salivary amylase breaks down and into smaller chains of , called dextrins and . The increased concentration of maltose in the mouth that results from the mechanical and chemical breakdown of starches in whole grains is what enhances their sweetness. Only about five percent of starches are broken down in the mouth. (This is a good thing as more glucose in the mouth would lead to more tooth decay.) 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 the more uniform mixture of .


Illustration of a human head and skull including the salivary glands: parotid gland, sublingual gland, and submandibular gland. Skeletal formula of starch "POLY-alpha-D-GLUCOSE"
Figure 4.6 Salivary Glands in the Mouth
Salivary glands secrete salivary amylase, which begins the chemical breakdown of carbohydrates by breaking the bonds between monomeric sugar units.

From the Stomach to the Small Intestine

The chyme is gradually expelled into the upper part of the small intestine. Upon entry of the chyme into the small intestine, the pancreas releases through a duct. 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 disaccharidase, are sucrase, maltase, and lactase. Sucrase breaks into glucose and molecules. Maltase breaks the bond between the two glucose units of maltose, and lactase breaks the bond between and glucose. 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, is not sufficiently broken down resulting in a condition called . 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 race. The National Digestive Diseases Information Clearing House states that 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] Most people with lactose intolerance can tolerate some 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. 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 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.


Human head showing salivary amylase, illustration of internal organs including pancreatic amlyase, then being broken down into sucrase, lactase, and maltase which then continue on to the liver.
Figure 4.7 Carbohydrate Digestion
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.

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 cook a delicious homemade noodle casserole 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.

-secreting cells in the pancreas sense the increase in blood glucose and release the hormone, insulin, into the blood. Insulin sends a signal to the body’s cells to remove glucose from the blood by transporting it into different organ cells around the body and using it to make energy. In the case of muscle tissue and the liver, insulin sends the biological message to store glucose away as glycogen. The presence of insulin in the blood signifies to the body that glucose is available for fuel. As glucose is transported into the cells around the body, the blood glucose levels decrease. Insulin has an opposing hormone called . Glucagon-secreting 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 break down glycogen 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.


Figure 4.8 The Regulation of Glucose
Image by Allison Calabrese / CC BY 4.0

Leftover Carbohydrates: The Large Intestine

Almost all of the carbohydrates, except for and resistant starches, are efficiently digested and absorbed into the body. Some of the remaining indigestible carbohydrates are broken down by enzymes released by bacteria in the large intestine. The products of bacterial digestion of these slow-releasing 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 kilocalories 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.


A human figure showing major organs involved in carbohydrate digestion. 1. Mouth: The enzyme salivary amalyse begins breaking down starch into shorter polysaccharides. 2. Stomach: Salivary amylase is inactivated and no further carbohydrate digestion occurs 3. Small intestine: Majority of starch digestion and breakdown of disaccharides occur here. The enzyme pancreatic amylase breaks down starch into monosaccharides, disaccharides, and oligosaccharides. 4. The digestion of carbohydrates is completed by the enzymes attached to the brush border of the small intestinal villi Here, the disaccharides and oligosaccharides are broken down into monosaccharides.
Figure 4.9 Overview of Carbohydrate Digestion
Image by Allison Calabrese / CC BY 4.0

A Carbohydrate Feast

You are at a your grandma’s house for family dinner and you just consumed kalua pig, white rice, sweet potatoes, mac salad, chicken long rice and a hot sweet bread roll dripping with butter. Less than an hour later you top it off with a slice of haupia pie and then lie down on the couch to watch TV. The “hormone of plenty,” insulin, answers the nutrient call. Insulin sends out the physiological message that glucose is abundant in the blood, so that cells can absorb it and either use it or store it. The result of this hormone message is maximization of glycogen stores and all the excess glucose, protein, and lipids are stored as fat.

Dishes of rice, vegetables, and other foods
Image by Allison Calabrese / CC BY 4.0

A typical American Thanksgiving meal contains many foods that are dense in carbohydrates, with the majority of those being simple sugars and starches. These types of carbohydrate foods are rapidly digested and absorbed. Blood glucose levels rise quickly causing a spike in insulin levels. Contrastingly, foods containing high amounts of fiber are like time-release capsules of sugar. A measurement of the effects of a carbohydrate-containing food on blood-glucose levels is called the glycemic response.

Glycemic Index

The glycemic responses of various foods have been measured and then ranked in comparison to a reference food, usually a slice of white bread or just straight glucose, to create a numeric value called the (GI). Foods that have a low GI do not raise blood-glucose levels neither as much nor as fast as foods that have a higher GI. A diet of low-GI foods has been shown in epidemiological and clinical trial studies to increase weight loss and reduce the risk of obesity, Type 2 diabetes, and cardiovascular disease.[2]

Table 4.1 The Glycemic Index: Foods In Comparison To Glucose – Low GI Foods (< 55)
Foods GI Value
Apple, raw 36
Orange, raw 43
Banana, raw 51
Mango, raw 51
Carrots, boiled 39
Taro, boiled 53
Corn tortilla 46
Spaghetti (whole wheat) 37
Baked beans 48
Soy milk 34
Skim milk 37
Whole milk 39
Yogurt, fruit 41
Yogurt, plain 14
Ice cream 51
Table 4.2 The Glycemic Index: Foods In Comparison To Glucose – Medium GI Foods (56–69)
Foods GI Value
Pineapple, raw 59
Cantaloupe 65
Mashed potatoes 70
Whole-wheat bread 69
Brown rice 55
Cheese pizza 60
Sweet potato, boiled 63
Macaroni and cheese 64
Popcorn 65
Table 4.3 The Glycemic Index: Foods In Comparison To Glucose – High GI Foods (70 and higher)
Foods GI Value
Banana (over-ripe) 82
Corn chips 72
Pretzels 83
White bread 70
White rice 72
Bagel 72
Rice milk 86
Cheerios 74
Raisin Bran 73
Fruit roll-up 99
Gatorade 78

For the Glycemic Index on different foods, visit http://www.mendosa.com/gilists.htm.

The type of carbohydrate within a food affects the GI along with its fat and fiber content. Increased fat and fiber in foods increases the time required for digestion and delays the rate of gastric emptying into the small intestine which, ultimately reduces the GI. Processing and cooking also affects a food’s GI by increasing their digestibility. Advancements in the technologies of food processing and the high consumer demand for convenient, precooked foods in the United States has created foods that are digested and absorbed more rapidly, independent of the fiber content. Modern breakfast cereals, breads, pastas, and many prepared foods have a high GI. In contrast, most raw foods have a lower GI. (However, the more ripened a fruit or vegetable is, the higher its GI.)

The GI can be used as a guide for choosing healthier carbohydrate choices but has some limitations. The first is GI does not take into account the amount of carbohydrates in a portion of food, only the type of carbohydrate. Another is that combining low- and high-GI foods changes the GI for the meal. Also, some nutrient-dense foods have higher GIs than less nutritious food. (For instance, oatmeal has a higher GI than chocolate because the fat content of chocolate is higher.) Lastly, meats and fats do not have a GI since they do not contain carbohydrates.

More Resources

Visit this online database to discover the glycemic indices of foods. Foods are listed by category and also by low, medium, or high glycemic index.

Learning Activities

Technology Note: The second edition of the Human Nutrition Open Educational Resource (OER) textbook features interactive learning activities.  These activities are available in the web-based textbook and not available in the downloadable versions (EPUB, Digital PDF, Print_PDF, or Open Document).

Learning activities may be used across various mobile devices, however, for the best user experience it is strongly recommended that users complete these activities using a desktop or laptop computer.



  1. Lactose Intolerance. National Digestive Diseases Information Clearing House. http://digestive.niddk.nih.gov/ddiseases/pubs/lactoseintolerance/. Updated April 23, 2012. Accessed September 22, 2017.
  2. Brand-Miller J, et al. (2009).  Dietary Glycemic Index: Health Implications. Journal of the American College of Nutrition, 28(4), 446S–49S.https://www.ncbi.nlm.nih.gov/pubmed/20234031. Accessed September 27, 2017.


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