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Nutrition: Science and Everyday Application: Digestion and Absorption of Carbohydrates

Nutrition: Science and Everyday Application
Digestion and Absorption of Carbohydrates
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table of contents
  1. Cover
  2. Title Page
  3. Copyright
  4. Table Of Contents
  5. Introduction
  6. About the Authors
  7. Acknowledgements
  8. Updates Made to OER
  9. Unit 1 – Designing A Healthy Diet
    1. Introduction to Designing a Healthy Diet
    2. Nutrition and Health
    3. An Introduction to Molecules
    4. Classification of Nutrients
    5. Defining Nutrient Requirements: Dietary Reference Intakes
    6. Understanding Food Labels
    7. Tools for Achieving a Healthy Diet
  10. Unit 2 — Nutrition Science and Information Literacy
    1. Introduction to Nutrition Science and Information Literacy
    2. The Scientific Method
    3. Types of Research Studies and How To Interpret Them
    4. Healthy Skepticism in Nutrition Science
    5. Fact-Checking Online Health and Nutrition Information
    6. Who Are the Experts in Nutrition Science?
    7. Finding Accurate Sources of Health and Nutrition Information
  11. Unit 3 – Molecules of Life: Photosynthesis, Digestion, and Metabolism
    1. Introduction to Molecules of Life
    2. Organization of Life
    3. Photosynthesis and Metabolism
    4. The Digestive System
    5. Disorders of the GI Tract
    6. Food Intolerances, Allergies, and Celiac Disease
  12. Unit 4- Carbohydrates
    1. Introduction to Carbohydrates
    2. Types of Carbohydrates
    3. Carbohydrate Food Sources and Guidelines for Intake
    4. Digestion and Absorption of Carbohydrates
    5. Glucose Regulation and Utilization in the Body
    6. Fiber – Types, Food Sources, Health Benefits, and Whole Versus Refined Grains
    7. Sugar: Food Sources, Health Implications, and Label-Reading
    8. Sugar Substitutes
  13. Unit 5- Lipids
    1. Introduction to Lipids
    2. The Functions of Fats
    3. Lipid Types and Structures
    4. Fatty Acid Types and Food Sources
    5. Digestion and Absorption of Lipids
    6. Lipid Transport, Storage, and Utilization
    7. Lipid Recommendations and Heart Health
  14. Unit 6- Protein
    1. Introduction to Protein
    2. Protein Structure
    3. Protein Functions
    4. Protein in Foods and Dietary Recommendations
    5. Protein Digestion and Absorption
    6. Health Consequences of Too Little and Too Much Dietary Protein
    7. Protein Food Choices and Sustainability
  15. Unit 7 — Body Weight and Health
    1. Introduction to Body Weight and Health
    2. Energy Balance: Energy In, Energy Out—Yet Not As Simple As It Seems
    3. Measures of Body Size, Body Composition, and Metabolic Health
    4. Risks of Too Little and Too Much Body Fat
    5. Eating Disorders
    6. Complex Causes of Excess Weight
    7. Solutions for Improving Health
  16. Unit 8 – Vitamins and Minerals Part 1
    1. Introduction to Vitamins and Minerals
    2. Vitamins Important for Vision
    3. Vitamins and Minerals as Antioxidants
    4. Dietary Supplements
    5. Sources of Vitamins and Minerals
    6. Classification of Vitamins and Minerals
    7. Vitamins and Minerals Involved In Fluid And Electrolyte Balance
  17. Unit 9 – Vitamins and Minerals Part 2
    1. Introduction to Vitamins and Minerals Part 2
    2. Vitamins and Minerals Involved in Energy Metabolism
    3. Introduction to Bone Health
    4. Other Minerals Important to Bone Health
    5. Vitamin D: Important to Bone Health and Beyond
    6. Vitamins and Minerals Involved in Blood Health
    7. Calcium: Critical for Bones and Throughout the Body
  18. Unit 10 — Nutrition and Physical Activity
    1. Introduction to Nutrition and Physical Activity
    2. Essential Elements and Benefits of Physical Fitness
    3. Fuel Sources for Exercise
    4. Nutrient Needs of Athletes
  19. Unit 11 — Nutrition Throughout the Lifespan
    1. Introduction to Nutrition Throughout the Lifecycle
    2. Nutrition in Pregnancy and Lactation
    3. Nutrition in Early Infancy
    4. Nutrition in Later Infancy and Toddlerhood
    5. Raising Healthy Eaters
    6. Nutrition in Adolescence
    7. Nutrition in Older Adults
  20. Glossary

Digestion and Absorption of Carbohydrates

Girl eating pizza

Imagine taking a bite of pizza. It tastes amazing, but it’s also full of fuel for your body, much of it in the form of carbohydrates.

What types of carbohydrates would you find in that bite?

  • Lactose from the cheese
  • Sucrose, glucose, and fructose from the naturally-occurring sugars in the tomatoes, as well as sugar that may have been added to the sauce
  • Starch in the flour used to make the crust
  • Fiber in the flour, tomatoes, and basil

In order to use these food carbohydrates in your body, you first need to digest them. Last unit, we explored the gastrointestinal system and the basic process of digestion. Now that you know about the different types of carbohydrates, we’ll take a closer look at how these molecules are digested as they travel through the GI system.

Carbohydrate Digestion

In the image below, follow the numbers to see what happens to carbohydrates at each site of digestion.

Illustration of the digestive system, with major sites of digestion numbered: oral cavity (1); stomach (2); small intestine (3); and large intestine or colon (4).

Figure 4.9. The digestive system

1 – Mouth or Oral Cavity

As you chew your bite of pizza, you’re using mechanical digestion to begin to break it into smaller pieces and mix it with saliva, produced by several salivary glands in the oral cavity.

Some enzymatic digestion of starch occurs in the mouth, due to the action of the enzyme salivary amylase. This enzyme starts to break the long glucose chains of starch into shorter chains, some as small as maltose. (The other carbohydrates in the bread don’t undergo any enzymatic digestion in the mouth.)

Illustration showing that the enzyme salivary amylase breaks starch into smaller polysaccharides and maltose. The image shows a long chain of starch (shown as green hexagons) that is then broken into shorter lengths, including maltose, by salivary amylase.

Fig. 4.10. The enzyme salivary amylase breaks starch into smaller polysaccharides and maltose.

2 – Stomach

The low pH in the stomach inactivates salivary amylase, so it no longer works once it arrives at the stomach. Although there’s more mechanical digestion in the stomach, there’s little chemical digestion of carbohydrates here.

3 – Small intestine

Most carbohydrate digestion occurs in the small intestine, thanks to a suite of enzymes. Pancreatic amylase is secreted from the pancreas into the small intestine, and like salivary amylase, it breaks starch down to small oligosaccharides (containing 3 to 10 glucose molecules) and maltose.

Illustration showing that the enzyme pancreatic amylase breaks starch into smaller polysaccharides and maltose. The image shows a long chain of starch (shown as green hexagons) that is then broken into shorter lengths, including maltose, by pancreatic amylase.

Figure 4.11. The enzyme pancreatic amylase breaks starch into smaller polysaccharides and maltose.

The rest of the work of carbohydrate digestion is done by enzymes produced by the enterocytes, the cells lining the small intestine. When it comes to digesting your slice of pizza, these enzymes will break down the maltose formed in the process of starch digestion, the lactose from the cheese, and the sucrose present in the sauce.

Maltose is digested by maltase, forming 2 glucose molecules.

Illustration showing maltose (represented by two green hexagons linked together) being broken into two glucose molecules by the enzyme maltase.

Lactose is digested by lactase, forming glucose and galactose.

Illustration showing lactose (represented by a green hexagon linked to a blue hexagon) being broken into one glucose molecule and one galactose molecule by the enzyme lactase.

Sucrose is digested by sucrase, forming glucose and fructose.

Illustration showing sucrose (represented by a green hexagon linked to a purple pentagon) being broken into one glucose molecule and one fructose molecule by the enzyme sucrase.

Figure 4.12. Action of the enzymes maltase, lactase, and sucrase.

(Recall that if a person is lactose intolerant, they don’t make enough lactase enzyme to digest lactose adequately. Therefore, lactose passes to the large intestine. There it draws water in by osmosis and is fermented by bacteria, causing symptoms such as flatulence, bloating, and diarrhea.)

By the end of this process of enzymatic digestion, we’re left with three monosaccharides: glucose, fructose, and galactose. These can now be absorbed across the enterocytes of the small intestine and into the bloodstream to be transported to the liver.

Digestion and absorption of carbohydrates in the small intestine are depicted in a very simplified schematic below. (Remember that the inner wall of the small intestine is actually composed of large circular folds, lined with many villi, the surface of which are made up of microvilli. All of this gives the small intestine a huge surface area for absorption.)

Cartoon illustration showing major processes involved in digestion and absorption of carbohydrates in the small intestine. The figure shows starch and polysaccharides being digested down to maltose by pancreatic amylase; maltose digested to two glucose molecules; sucrose digested to one glucose and one fructose; and lactose digested to one glucose and one galactose. Monosaccharides are then absorbed into the bloodstream and travel to the liver.

Figure 4.13. Digestion and absorption of carbohydrates in the small intestine.

Fructose and galactose are converted to glucose in the liver. Once absorbed carbohydrates pass through the liver, glucose is the main form of carbohydrate circulating in the bloodstream.

4 – Large Intestine or Colon

Any carbohydrates that weren’t digested in the small intestine—mainly fiber—pass into the large intestine, but there’s no enzymatic digestion of these carbohydrates here. Instead, bacteria living in the large intestine, sometimes called our gut microbiota, ferment these carbohydrates to feed themselves. Fermentation causes gas production, and that’s why we may experience bloating and flatulence after a particularly fibrous meal. Fermentation also produces short-chain fatty acids, which our large intestine cells can use as an energy source. Over the last decade or so, more and more research has shown that our gut microbiota are incredibly important to our health, playing important roles in the function of our immune response, nutrition, and risk of disease. A diet high in whole food sources of fiber helps to maintain a population of healthy gut microbes.

Summary of Carbohydrate Digestion:

The primary goal of carbohydrate digestion is to break polysaccharides and disaccharides into monosaccharides, which can be absorbed into the bloodstream.

1. After eating, nothing needs to happen in the digestive tract to the monosaccharides in a food like grapes, because they are already small enough to be absorbed as is.

2. Disaccharides in that grape or in a food like milk are broken down (enzymatically digested) in the digestive tract to monosaccharides (glucose, galactose, and fructose).

3. Starch in food is broken down (enzymatically digested) in the digestive tract to glucose molecules.

4. Fiber in food is not enzymatically digested in the digestive tract, because humans don’t have enzymes to do this. However, some dietary fiber is fermented in the large intestine by gut microbes.

Carbohydrates in food

Is this carbohydrate enzymatically digested? (enzyme name)

What is absorbed into the villi after digestion?

Monosaccharides

Glucose

No

Glucose

Fructose

No

Fructose. It is then transported to the liver where it is converted to glucose.

Galactose

No

Galactose. It is then transported to the liver where it is converted to glucose.

Disaccharides

Maltose

Yes (maltase)

Glucose

Sucrose

Yes (sucrase)

Glucose, Fructose

Lactose

Yes (lactase)

Glucose, Galactose

Polysaccharides

Starch

Yes

(amylase, maltase)

Glucose

Fiber

No (Humans don’t have the digestive enzymes to break down fiber, but some is fermented by gut microbes in the large intestine.)

N/A

Table 4.3. Summary of enzymatic digestion of carbohydrates

VIDEO: “Digestion and Absorption of Carbohydrates” by How It Works.

One or more interactive elements has been excluded from this version of the text. You can view them online here: https://openoregon.pressbooks.pub/nutritionscience2e/?p=182#oembed-1

VIDEO: “Carbohydrates in Foods, Digestion and Absorption” by Tamberly Powell, YouTube (September 26, 2018), 7:31 minutes. This video will help you identify carbohydrates in foods, what carbohydrates need to be enzymatically digested, and what is absorbed.

Self-Check:

An interactive H5P element has been excluded from this version of the text. You can view it online here:
https://openoregon.pressbooks.pub/nutritionscience2e/?p=182#h5p-26

References:

  • Klein, S., Cohn, S. M., & Alpers, D. H. (1999). The Alimentary Tract in Nutrition. In Modern Nutrition in Health and Disease (9th ed.). Baltimore: Lippincott Williams and Wilkins.
  • Harvard T.H. Chan School of Public Health. (n.d.). The Microbiome. Retrieved November 15, 2019, from The Nutrition Source website: https://www.hsph.harvard.edu/nutritionsource/microbiome/

Image Credits:

  • “Pizza” photo by Kate Voytsutskaya on Unsplash
  • Figure 4.9. “The digestive system” by Alice Callahan is licensed under CC BY 4.0 / A derivative from the original work
  • Figure 4.10. “Carbohydrate digestion schematics” by Alice Callahan is licensed under CC BY-NC-SA 4.0
  • Figure 4.11. “Starch digestion” by Alice Callahan is licensed under CC BY-NC-SA 4.0
  • Figure 4.12. “Disaccharide digestion” by Alice Callahan is licensed under CC BY-NC-SA 4.0
  • Figure 4.13. “Carbohydrate absorption” by Alice Callahan is licensed under CC BY-NC-SA 4.0
  • Table 4.3. “Carbohydrate and digestion summary chart” by Tamberly Powell is licensed under CC BY-NC-SA 4.0

Annotate

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Glucose Regulation and Utilization in the Body
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Kinesiology, Nutrition, and Fitness

Copyright © 2020

            Nutrition: Science and Everyday Application Copyright © 2020 by Alice Callahan, PhD; Heather Leonard, MEd, RDN; and Tamberly Powell, MS, RDN is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.
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