Digestion and Absorption of Carbohydrates

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The ultimate goal of digestion and absorption of sugars and starches is to dismantle them into small molecules that the body can absorb and use—chiefly glucose. The large starch molecules require extensive breakdown; the disaccharides need only to be hydrolyzed once. The initial splitting begins in the mouth; the final splitting and absorption occur in the small intestine; and conversion to a common energy currency (glucose) takes place in the liver. The details follow.

The Processes of Digestion and Absorption

Figure 4-10 traces the digestion of carbohydrates through the GI tract. When a person eats foods containing starch, enzymes hydrolyze the long chains to shorter chains, the short chains to disaccharides, and, finally, the disaccharides to mono-saccharides. This process begins in the mouth.

  • In the Mouth • In the mouth, vigorous chewing of high-fiber foods slows eating and stimulates the flow of saliva. The salivary enzyme amylase starts to work, hydrolyzing starch to shorter polysaccharides and to maltose. Because food is in the mouth for only a short time, very little digestion takes place there.
  • In the Stomach • The swallowed bolus mixes with the stomach's acid and protein-digesting enzymes, and these digest the salivary enzyme amylase. Thus amylase is removed from the scene before its job of starch digestion is completed. To a small extent, the stomach's acid continues breaking starch down, but its juices contain no enzymes to digest carbohydrate. Fibers linger in the stomach and delay gastric emptying, thereby providing a feeling of fullness and satiety.
  • In the Small Intestine • The small intestine performs most of the work of carbohydrate digestion. A major carbohydrate-digesting enzyme, pancreatic amylase, enters the intestine via the pancreatic duct and continues breaking down the poly-saccharides to shorter glucose chains and disaccharides. The final step takes place on the outer membranes of the intestinal cells. There specific enzymes dismantle specific disaccharides:
  • Maltase breaks maltose into 2 glucose molecules.
  • Sucrase breaks sucrose into 1 glucose and 1 fructose molecule.
  • Lactase breaks lactose into 1 glucose and 1 galactose molecule.

Figure 4-10

Carbohydrate Digestion in the GI Tract


Mouth and salivary glands

The salivary glands secrete saliva into the mouth to moisten the food. The salivary enzyme amylase begins digestion:

amylase small

Starch —-—polysaccharides, maltose


Stomach acid inactivates salivary enzymes, halting starch digestion.

Small intestine and pancreas

The pancreas produces an amylase that is released through the pancreatic duct into the small intestine:

Starch pancreatic amylase

' small polysaccharides, disaccharides

Then disaccharidase enzymes on the surface of the small intestinal cells hydrolyze the disaccharides into monosaccharides:

Maltose maltase >


Lactose lactase >

glucose +

glucose fructose +

glucose galactose +


Salivary glands

Intestinal cells absorb these monosaccharides.

Salivary glands

Carbohydrate Digestion The Tract


The mechanical action of the mouth crushes and tears fiber in food and mixes it with saliva to moisten it for swallowing.


Fiber is not digested, and it delays gastric emptying.


Small intestine

Fiber is not digested, and it delays absorption of other nutrients.



The mechanical action of the mouth crushes and tears fiber in food and mixes it with saliva to moisten it for swallowing.


Fiber is not digested, and it delays gastric emptying.


Small intestine

Fiber is not digested, and it delays absorption of other nutrients.

Large intestine

Most fiber passes intact through the digestive tract to the large intestine. Here, bacterial enzymes digest fiber:

Some fiber bacterial enzymes

■fatty acids, gas Fiber holds water; regulates bowel activity; and binds substances such as bile, cholesterol, and some minerals, carrying them out of the body.

sucrase resistant starch: starch that escapes digestion and absorption in the small intestine of healthy people.

ferment: to digest in the absence of oxygen.

At this point, all disaccharides contribute at least one glucose molecule to the body. Fructose and galactose can eventually become glucose after being processed in the liver, as explained later.

Fibers delay the absorption of carbohydrates and fats in the small intestine, conferring benefits on health that a later section describes further. In addition, fibers in the intestines can bind with minerals there, as mentioned earlier.

• In the Large Intestine • Within one to four hours after a meal, all the sugars and most of the starches have been digested. Only a small fraction of the starches and the indigestible fibers remain in the digestive tract.3

The small fraction of starches that escapes digestion and absorption in the small intestine is known as resistant starch. Starch may resist digestion for several reasons, reflecting both the individual's efficiency in digesting starches and the food's physical properties.4 Resistant starch is common in whole lentil beans, raw potatoes, and unripe bananas. Because resistant starches remain in the large intestine, they promote bowel movements as fibers do, but unlike fibers, they do not lower blood cholesterol.5

Like resistant starches, fibers in the large intestine attract water, which softens the stools for passage without straining. Also, bacteria in the human digestive tract ferment both fibers and resistant starches. This process generates water, gas, and short-chain fatty acids (described in Chapter 5).* The short-chain fatty acids are absorbed in the colon and yield energy when metabolized. Metabolism of short-chain fatty acids occurs in both the intestinal mucosa and the liver. Food fibers and resistant starches, therefore, do contribute some energy (about 2 kcalories per gram), depending on the extent to which they are broken down and absorbed.6

• Absorption into the Bloodstream • Glucose is unique in that it can be absorbed to some extent through the lining of the mouth, but for the most part, all nutrient absorption takes place in the small intestine. The monosaccharides traverse the cells lining the small intestine by active transport and are washed away in the circulating blood.+

The blood then circulates through the liver, whose cells take up fructose and galactose and convert them to other compounds, most often to glucose, as shown in Figure 4-11. Thus all disaccharides not only provide at least one glucose molecule directly, but they can also provide another one indirectly—through the conversion of fructose and galactose to glucose. (The body does not use fructose and glucose in exactly the same ways, but for purposes of this book, they are treated as being metabolically identical.)

This description of the way the body receives carbohydrate should help explode a myth perpetrated by advertisers of high-sugar foods and beverages. They describe sugar as "quick energy" and imply that when you need a pick-me-up, you should reach for a candy bar and a cola beverage. Concentrated sugars do offer energy, but clearly, the best pick-me-ups are carbohydrate-containing foods that deliver vitamins, minerals, and fiber along with their energy. Next time you need an energy boost, why not have a delicious peanut butter and banana sandwich, a tall, cool glass of milk, and a fresh, juicy orange?


In the digestion and absorption of carbohydrates, the body breaks down starches into disaccharides and disaccharides into monosaccharides; it then converts monosaccharides mostly to glucose to provide energy for the cells' work. The fibers help to regulate the passage of food through the GI system, but contribute little, if any, energy.

*The short-chain fatty acids produced by GI bacteria are primarily acetic acid, propionic acid, and butyric acid.

tFructose is absorbed by facilitated diffusion.

Monosaccharides, the end products of carbohydrate digestion, enter the capillaries of the intestinal

Small intestine

Monosaccharides, the end products of carbohydrate digestion, enter the capillaries of the intestinal

Absorption Food With Capillaries

Small intestine

(3 Glucose Fructose Galactose

Monosaccharides travel to the liver via the portal vein.

Figure 4-11

Absorption of Monosaccharides

In the liver, galactose ^ and fructose can be ' converted to glucose.

(3 Glucose Fructose Galactose

Monosaccharides travel to the liver via the portal vein.

Figure 4-11

Absorption of Monosaccharides

Lactose Intolerance

Normally, the enzyme lactase ensures that the disaccharide lactose found in milk is both digested and absorbed efficiently. Lactase activity is highest immediately after birth, as befits an infant whose first and only food for a while will be breast milk or infant formula. In the great majority of the world's populations, lactase activity declines dramatically during childhood and adolescence to about 5 to 10 percent of the activity at birth.7 Only a relatively small percentage (about 30 percent) of the people in the world retain enough lactase to digest and absorb lactose efficiently throughout adult life.

  • Symptoms • When more lactose is consumed than the available lactase can handle, lactose molecules remain in the intestine undigested, attracting water and causing bloating, abdominal discomfort, and diarrhea—the symptoms of lactose intolerance. The undigested lactose becomes food for intestinal bacteria, which multiply and produce irritating acid and gas, further contributing to the discomfort and diarrhea.
  • Causes • As mentioned, lactase activity commonly declines with age. Lactase deficiency may also develop when the intestinal villi are damaged by disease, certain medicines, prolonged diarrhea, or malnutrition; this can lead to temporary or permanent lactose malabsorption, depending on the extent of the intestinal damage. In extremely rare cases, an infant is simply born with a lactase deficiency.
  • Prevalence • The prevalence of lactose intolerance varies widely among ethnic groups, indicating that the trait is genetically determined.8 The prevalence of lactose intolerance is lowest among Scandinavians and other northern Europeans and highest among native North Americans and Southeast Asians.
  • Dietary Changes • Managing lactose intolerance requires some dietary changes, although total elimination of milk products is usually not necessary. Excluding all milk products from the diet can lead to nutrient deficiencies, for milk lactose intolerance: a condition that results from inability to digest the milk sugar lactose; characterized by bloating, gas, abdominal discomfort, and diarrhea. Lactose intolerance differs from milk allergy, which is caused by an immune reaction to the protein in milk.

lactase deficiency: a lack of the enzyme required to digest the disaccharide lactose into its component monosaccharides (glucose and galactose).

Estimated prevalence of lactose intolerance: >80% Asian Americans. 80% Native Americans. 75% African Americans. 70% Mediterranean peoples. 60% Inuits. 50% Hispanics. 20% Caucasians. <10% Northern Europeans.

Lactose in selected foods:

Whole-wheat bread, 1 slice 0.5 g

Dinner roll, 1 0.5 g

Cheese, 1 oz

Whole-wheat bread, 1 slice 0.5 g

Dinner roll, 1 0.5 g

Cheese, 1 oz

Cheddar or American

0.5 g

Parmesan or cream

0.8 g

Doughnut (cake type), 1

1.2 g

Chocolate candy, 1 oz

2.3 g

Sherbet, 1 c

4.0 g

Cottage cheese (low-fat), 1 c

7.5 g

Ice cream, 1 c

9.0 g

Milk, 1 c

12.0 g

Yogurt (low-fat), 1 c

15.0 g

  • U.S. Government
  • a href="http://www.healthfinder.gov/searchoptions/">www.healthfinder.gov/searchoptions/ topicsaz.htm

Search for Lactose intolerance

Starches and sugars are called available carbohydrates because human digestive enzymes break them down for the body's use. In contrast, fibers are called unavailable carbohydrates because human digestive enzymes cannot break their bonds.

is a major source of several nutrients, notably the mineral calcium, the B vitamin riboflavin, and vitamin D. Fortunately, many people with lactose intolerance can consume foods containing up to 6 grams of lactose (K cup milk) without symptoms.9 The most successful strategies are to increase intake of milk products gradually, take them with other foods in meals, and spread their intake throughout the day.10 A change in the GI bacteria, not the reappearance of the missing enzyme, accounts for the ability to adapt to milk products.11

In many cases, lactose-intolerant people can tolerate fermented milk products such as yogurt and acidophilus milk. The bacteria in these products digest lactose for their own use, leaving these foods relatively low in lactose. Hard cheeses and cottage cheese are often well tolerated because most of the lactose is removed with the whey during manufacturing. Lactose continues to diminish as the cheese ages.

Many lactose-intolerant people use commercially prepared milk products that have been treated with an enzyme that breaks down the lactose. Alternatively, they take enzyme tablets with meals or add enzyme drops to their milk. The enzyme hydrolyzes much of the lactose in milk to glucose and galactose, which lactose-intolerant people can absorb without ill effects. Most healthy adults with lactose intolerance can drink milk when the lactose content has been reduced by 50 percent.12

Because people's tolerance to lactose varies widely, lactose-restricted diets must be highly individualized. A completely lactose-free diet can be difficult because lactose appears not only in milk and milk products but also as an ingredient in many nondairy foods such as breads, cereals, breakfast drinks, salad dressings, and cake mixes. People on strict lactose-free diets need to read labels and avoid foods that include milk, milk solids, whey (milk liquid), and casein (milk protein, which may contain traces of lactose). They also need to check all drugs with the pharmacist because 20 percent of prescription drugs and 5 percent of over-the-counter drugs contain lactose as a filler.

People who consume few or no milk products must take care to meet riboflavin, vitamin D, and calcium needs. Later chapters on the vitamins and minerals offer help with finding good nonmilk sources of these nutrients.


Lactose intolerance is a common condition that occurs when there is insufficient lactase to digest the disaccharide lactose found in milk and milk products. Symptoms include GI distress. Because treatment requires limiting milk intake, other sources of riboflavin, vitamin D, and calcium must be included in the diet.

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  • justiina
    What would be the effect on digestion if enzymes were unavailable?
    9 years ago
  • philipp
    What percent nutrient absorption takes place int he small intestine?
    9 years ago
  • Verdiana
    What enzymes dismantle maltose ?
    9 years ago
  • kenzie
    Can amino acids converted to glucose in the liver?
    9 years ago
  • Marco
    What is unique to the digestion and absoption of carboydrates?
    9 years ago
  • denise
    How is fiber digested?
    9 years ago
  • timothy
    How is carbohydrate absorbed in small intestines?
    9 years ago
  • amanuel
    How is carbohydrate digestion completed?
    9 years ago
  • luukas
    How long does it take disaccharidase enzymes hydrolyze disaccharides into monosaccharides?
    9 years ago
  • james
    What is the ultimate goal of carbohydrate digestion?
    9 years ago
  • sara temesgen
    What are the disaccharides and what enzymes break them down for absorption?
    9 years ago
  • Peter
    What escapes digestion and absorption in the small intestine?
    9 years ago
  • Patrick
    Why is there very little absorption in the mouth and stomach?
    9 years ago
  • demi mclean
    Why is digestion takes place in small intestines?
    9 years ago
  • jana
    What is mechanical digestion?
    8 years ago
  • tanner
    Where is glucose absorbed?
    8 years ago
  • peony
    Which drugs delay digestion and absorbtion of carbs in the intestine?
    7 years ago
  • anssi rauhala
    Are fibers digested in the stomach?
    7 years ago
  • michelle
    How galactose and glucose is broken down in the small intestine?
    7 years ago
  • Mebrat
    Where glucose absorption in intestine?
    7 years ago
  • Abel Saare
    Which carbs redirect blood to intestines most dramatically?
    7 years ago
  • phillipp
    How are carbs processed by the liver after absorption?
    7 years ago
  • michelangelo esposito
    How is lactose digested and absorbed?
    7 years ago
  • Zach
    Which starch in food attracts bacteria in intestines?
    7 years ago
  • matta burrowes
    What is unique to the digestion and absoebtion of carbs?
    6 years ago
  • pimpernel
    What converts polysaccharides into disaccharides?
    3 years ago
  • palmira
    Which nutrient is good for absorption of carbohydrate?
    2 years ago
  • niko
    What is balanced diet. drscribe the digestion and absorbtion of carbohydrate?
    2 years ago
  • Jens
    How carbohydrate absorption occurs in gi track?
    2 years ago
  • Felice Onio
    Where in the body are disaccharides digested and absorbed?
    1 year ago
  • Domenica
    How many people in the world digest and absorb lactose efficiently throughout their adult life?
    6 months ago
  • isabella clark
    What undigested carbohydrate lingers in the stomach creating satiety?
    6 months ago
  • maxine
    How is the absorption of carbohydrates affected if the small and large intestine is damaged?
    4 months ago
  • zemzem
    Is carbohydrate absorption in the stomach versus small bowel?
    3 months ago
  • Kidane
    What percentage of sugar is absorbed in the mouth?
    3 months ago

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