5.4.1: Digestion and Absorption of Carbohydrates

The Taste of Carbohydrates

Part of the reason we eat food is because it tastes good. Some people have a sweet tooth, which means they love sweet foods with fast-releasing carbohydrates. Sweetness is one of the five basic taste sensations and is sensed by protein receptors in cells of the taste buds. Fast-releasing carbohydrates stimulate the sweetness taste sensation, which is the most sensitive of all taste sensations. Even extremely low concentrations of sugars in foods will stimulate the sweetness taste sensation. Sweetness varies between the different carbohydrate types—some are much sweeter than others. Fructose is the top naturally occurring sugar in sweetness value. See Table \(\PageIndex{1}\) for sweetness comparisons among different naturally-occurring carbohydrates.

Less-sweet whole grains containing more starches and fiber are actually more satisfying for a longer time. Whole grains take longer to chew and get sweeter the more you chew them. Additionally, once in the stomach, whole-grain foods take longer to digest and keep you full longer. Remember, too, that they contain fiber, which makes elimination much smoother. Whole-grain foods satisfy the body the entire way through the digestive tract and provide nutrients that better satisfy the body’s functional needs. In this section, you will learn more about the digestion of carbohydrates.

Sweet Simple Sugars

Figure \(\PageIndex{1}\): Blueberry pancakes.¹ Whole grains provide satisfaction from the beginning to the end of the digestion process, but simple sugars taste sweeter.

Table \(\PageIndex{1}\): Sweetness Comparison of Carbohydrates²

Carbohydrate	Sweetness (percentage of sucrose)
Sucrose	100
Glucose	50-75
Galactose	54
Fructose	80-180
Maltose	30-50
Lactose	15-40
Xylitol	90-100
Fiber	0

From the Mouth to the Stomach

As discussed ing Chapter 4, 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 oral cavity secrete saliva that coats the food particles. Saliva contains the enzyme salivary amylase, which breaks the bonds between the simple sugar subunits of disaccharides and polysaccharides, such as starch. The salivary amylase breaks down amylose and amylopectin into smaller chains of glucose, called dextrins and maltose (Figure \(\PageIndex{2}\)). 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 5% 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 the mechanical breakdown is ongoing—the strong peristaltic contractions of the stomach mix the carbohydrates into the more uniform mixture of chyme.

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 pancreatic juice 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 are sucrase, maltase, and lactase (Figure \(\PageIndex{2}\)). 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 galactose 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, lactose is not sufficiently broken down, resulting in a condition called lactose intolerance. The undigested lactose moves to the large intestine, where bacteria can 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, Native Americans, and Asian Americans have much higher incidences of lactose intolerance, whereas those of northern European descent have the least.³ Most people with lactose intolerance can tolerate some amount of dairy in their diet. The severity of the symptoms depends on how much lactose is consumed and the degree of lactase deficiency.

Chemical Digestion of Carbohydrates

Figure \(\PageIndex{2}\): Enzymes involved in polysaccharide digestion.⁴ Salivary amylase starts breaking down starch and glycogen in the mouth. Maltase, sucrase, and lactase continue breaking down disaccharides in the small intestines.

Absorption: Going to the Blood Stream

The cells in the small intestine have membranes that contain many transport proteins 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 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.

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 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, and two hormones are key for regulating blood glucose levels: insulin and glucagon. These two hormones keep blood glucose levels within the target range, also called homeostasis.

After you eat a meal containing carbohydrates, glucose levels rise in the blood. Insulin-secreting cells in the pancreas sense the increase in blood glucose and release the hormonal message, 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 to make energy or for building macromolecules. 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 it has just been fed and to use the fuel. Insulin has an opposing hormone called glucagon. As the time after a meal increases, glucose levels decrease in the blood. 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.

Negative Feedback in Glucose Homeostasis

Figure \(\PageIndex{3}\): Glucose homeostasis negative feedback loop. Two hormones, insulin and glucagon, regulate blood glucose levels. When glucose increases in the bloodstream after a meal, insulin increases glucose uptake by cells throughout the body and stimulates glycogen production in the liver. If glucose levels decrease in the bloodstream, glucagon is released, and the liver starts to convert glycogen to glucose to maintain homeostasis. Source: "Glucose homeostasis negative feedback loop" by Erin Shanle is licensed under CC BY 4.0.

Leftover Carbohydrates: The Large Intestine

Almost all carbohydrates, except for dietary fiber 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 the cells of the colon, with a small amount being transported to the liver. Colonic cells use 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. Because 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 smaller 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.

Thanksgiving Dinner: A Carbohydrate Feast

Figure \(\PageIndex{4}\):Thanksgiving dinner.⁵ 

It’s Thanksgiving, and you have just consumed turkey with mashed potatoes, stuffing smothered in gravy, green beans topped with crispy fried onions, a hot roll dripping with butter, and cranberry sauce. Less than an hour later, you top it all off with a slice of pumpkin pie and then lie down on the couch to watch the football game. What happens in your body after digesting and absorbing the whopping amount of nutrients in this Thanksgiving feast? The “hormone of plenty,” insulin, answers the nutrient call. Insulin sends out the physiological message that glucose and everything else is in abundant supply in the blood, so cells absorb and then use or store it. The result of this hormone message is the maximization of glycogen stores, and all the excess glucose, protein, and lipids are stored as fat. 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. In contrast, foods containing high amounts of fiber are like time-release sugar capsules, and insulin levels don't spike so rapidly.

Glycemic Index and Glycemic Load

It can be difficult to know how a food may affect blood glucose levels based on information in the nutrition label. However, there are tools available to determine if foods will cause a rapid spike in blood glucose levels. A measurement of the effects of a carbohydrate-containing food on blood glucose levels is called the glycemic response (Figure \(\PageIndex{4}\)). The glycemic responses of various foods have been measured and then ranked in comparison with a reference food, usually a slice of white bread or just straight glucose, to create a numeric value called the glycemic index (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.⁶

The carbohydrate type within a food affects the GI, but so does its fat and fiber content (which reduce the GI). Increased fat and fiber in foods increase the time required for digestion and delay the rate of gastric emptying into the small intestine. Processing and cooking additionally affect a food’s GI by increasing their digestibility. Advancements in food processing technologies and high consumer demand for convenient, precooked foods in the United States have 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. Table \(\PageIndex{2}\) provides the GI for various foods.

The GI can be used as a guide for healthier carbohydrate choices but has some limitations. One is that the 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.) Finally, meats and fats do not have a GI because they do not contain carbohydrates. Because of these limitations, the glycemic load is more often used in practice. Glycemic load (GL) is calculated by multiplying the glycemic index by the amount of carbohydrates per serving and dividing by 100. For example, a banana has a GI of 51 and contains 26 grams of carbohydrates, so the GL would be (51x26)/100 or 13.2, which is much higher than an apple that has a GL of 6. Foods with a lower GL have lower glycemic response after they are consumed.

Glycemic Index Measures Blood Glucose Response

Figure \(\PageIndex{4}\): The glycemic index measures the effects of foods on blood glucose levels. Foods with a high glycemic index (red) cause blood glucose levels to increase much faster than foods with low glycemic index (blue).⁷ 

Table \(\PageIndex{2}\): The Glycemic Index: Foods in Comparison with Glucose⁸

Foods	GI Value
Low GI Foods (< 55)
Apple	44
Pear	38
Banana (under-ripe)	51
Grapefruit	25
Barley	25
Navy beans	38
Green peas	48
Oat bran (Quaker Oats)	50
Spaghetti (whole wheat)	37
Mashed sweet potatoes	54
Baked beans	48
Butter beans	44
Banana bread	47
Bread (sourdough)	52
Soy milk	31
Skim milk	32
Whole milk	27
Yogurt (sweetened)	33
Yogurt (plain, artificial sweetener)	14
Medium GI Foods (56–69)
Apricots	57
Cantaloupe	65
Mashed potatoes	70
Whole-wheat pita bread	57
Whole-wheat bread	69
Couscous	65
Brown rice	55
Cheese pizza	60
Rye bread	65
Hamburger bun	61
Black bean soup	64
Macaroni and cheese	64
Coca-Cola	63
High GI Foods (70 and higher)
Dates	103
Banana (over-ripe)	82
Parsnips	97
Corn chips	72
Pretzels	83
White bread	70
White rice	72
Spaghetti (durum flour)	78
White rice (instant)	87
French baguette	95
Bagel	72
Bread stuffing	74
Cheerios	74
Cream of wheat	71
Raisin Bran	73
Fruit roll-up	99
Gatorade	78

Interactive \(\PageIndex{1}\): Balancing the Thanksgiving Feast

Visit this online database of glycemic indices of foods. 

To balance the high-GI foods on the Thanksgiving table with low-GI foods, follow some of these suggestions:

• Serve a winter fruit salad.
• Leave the skins on the potatoes. The skin contains fiber and adds texture to mashed potatoes. Do not use instant potatoes.
• Instead of canned green beans with cream of mushroom soup and fried onions for a side dish, combine butter beans and green peas for a colorful, low-GI food.
• Make your stuffing with whole-grain bread and add mushrooms and extra celery and onions.
• Try a new low-sugar pumpkin pie recipe and make the crust from whole-grain flour.
• Offer homemade banana bread for dessert.

Attributions

• Zimmerman, "An Introduction to Nutrition (Zimmerman)", CC BY-NC-SA 3.0. Figures and text were updated.

References

1. Jeffreyww. Mmm...pancakes. Flickr, uploaded 8 June 2010, https://www.flickr.com/photos/jeffreyww/4256361475/. Accessed 10 Feb. 2026.
2. Starkey DE, Wang Z, Brunt K, et al. The challenge of measuring sweet taste in food ingredients and products for regulatory compliance: a scientific opinion. J AOAC Int. 2022;105(2):333-345. doi:10.1093/jaoacint/qsac005.
3. Definition & facts for lactose intolerance. National Institute of Diabetes and Digestive and Kidney Diseases. https://www.niddk.nih.gov. Updated February 2018. Accessed July 12, 2023. https://www.niddk.nih.gov/health-information/digestive-diseases/lactose-intolerance/definition-facts.
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5. Spensley, Stacy. Thanksgiving Dinner. Flickr, uploaded 28 Nov. 2011, https://www.flickr.com/photos/21001756@N06/6415627223. Licensed under CC BY 2.0. Accessed 10 Feb. 2026.
6. Glycemic Index Diet for weight loss. JACN. Accessed July 19, 2023. https://www.jacn.org/glycemic-index-diet-for-weight-loss/.
7. "File:Glycemic.png." Wikimedia Commons. 12 Oct 2024, 14:08 UTC. <https://commons.wikimedia.org/w/index.php?title=File:Glycemic.png&oldid=939949371> 10 Feb 2026, 19:22.
8. Glycemic index research and GI news. University of Sydney. Accessed July 21, 2023. https://glycemicindex.com/.