CHAPTER 5
Carbohydrates
I.
Structures and Function
A. Background
1. Composed of carbon, hydrogen, and oxygen in a 1:2:1 ratio often
written as (CH2O)n where n=the number of times the ratio is
repeated. For example, if n=6, then (CH2O)6 = C6H12O6.
2. Provides main source of fuel for cells (at 4 kcal/gram)
3. Divided into two main classes
a. Simple sugars such as monosaccharides, disaccharides,
and oligosaccharides.
b. Complex sugars such as polysaccharides.
B. MONOSACCHARIDES (glucose, fructose, and galactose are isomers
of each other)
1. Glucose is also called dextrose or blood sugar.
a. Acts as the principle monosaccharide that acts as a building
block form any other carbohydrates.
b. In the body it forms a six carbon (hexose) ring structure.
2. Fructose is also called levulose.
a. Composed of a six carbon ring structure.
b. Commonly found in fruit, honey, and corn syrup and is
used in soft drinks and frozen desserts.
c. Accounts for 8 to 10% of our energy intake.
d. After absorption by the small intestine, fructose is
converted into glucose by the liver.
e. If fructose consumption is high, it is also converted into
glycogen, lactic acid, or fat.
3. Galactose
a. Has a six carbon ring structure.
b. This is not usually found in nature but exists mostly as a
unit of the disaccharide lactose which is found easily in
nature.
c. Easily converted into glucose in the liver or can be stored
as glycogen.
4. Ribose sugar
a. Has a five carbon ring and is not an isomer of glucose.
b. Used as part of the RNA strand, which is an important part
of a cell’s genetic material?
C. DISACCHARIDES
1. Maltose
a. Formed by condensation synthesis reaction between two
glucose molecules.
b. The glucose molecules form an alpha bond between them
which is easily broken during digestion.
c. Maltose is commonly used in the production of many
alcohols by fermentation (recall anaerobic cellular
respiration).
2. Sucrose
a. Formed by a glucose and a fructose attached by an alpha
bond during condensation synthesis.
b. Major sources are table sugar and in plants such as sugar
cane and maple syrup.
3. Lactose
a. Formed by the addition of glucose and a galactose by beta
bonds during condensation synthesis reaction.
b. Beta bonds are more difficult to digest hence the large
portion of the population that is lactose intolerant.
c. Primary sugar found in milk and milk products.
D. OLIGOSACCHARIDES
1. Raffinose and stachyose are common examples
2. These are found in beans
3. Because they contain large numbers of beta bonds they are difficult
to digest by digestive enzymes.
4. Instead bacteria in the large intestine break apart these
oligosaccharides, producing gas and other byproducts.
E. POLYSACCHARIDES
1. Starch
a. Two types of plant starch=Amylose and amylopectin.
b. Long chains of a 1000 or more glucose molecule.
c. Amylose is a straight chain polymer while amylopectin is a
branched polymer.
d. Food sources include potatoes, breads, pasta, and rice.
e. Amylopectin raises blood sugar levels quicker because of
the branched configuration which enables amylase to have
more points of attack.
2. Glycogen
a. Storage form of glucose in the human body.
b. Long branched chains of glucose that are more complex
than amylopectin.
c. Even more digestible because of its branched structure.
3. Fiber
a. Dietary fibers are chemically composed of non-starch
polysaccharides:
i. Cellulose and hemicellulose which is found in
wheat, rye, rice, and vegetables (insoluble).
ii. Pectin, gums, and mucilage which is found in citrus
fruits, oat products and beans (soluble).
b. Dietary fibers are also composed of a non-carbohydrate
material called lignin.
c. All dietary fibers come from plants and are not digested in
the stomach.
d. The water soluble fibers however are metabolized by
bacteria in the intestines.
II.
Carbohydrate Digestion and Absorption
A. Begins in the mouth
1. Salivary amylase breaks some starches into maltose during
mastication.
2. Continues digestion through the esophagus but stops in the
stomach because the high acid environment inactivates salivary
enzymes.
B. Pancreatic amylases located in the small intestine further break down
starches into maltose.
C. Enzymes in the wall of the small intestine break down disaccharides into
monosaccharides:
1. Maltose is acted on by maltase.
2. Sucrose is acted on by sucrase.
3. Lactose is acted on by lactase.
D. Glucose and galactose are actively absorbed in the small intestine (see
figure 5-6).
E. Fructose is absorbed through facilitated diffusion which although
requires a carrier does not require energy.
F. Single sugars are then transported via the portal vein that goes directly to
the liver where the liver utilizes several options:
1. Transforms the monosaccharides into glucose and releases it
directly into the bloodstream for use.
2. Produces glycogen for short term storage.
3. Produces fat for long term storage.
III.
Functions of Glucose and other sugars
A. Yields Energy (recall 4 kcal/g)
B. Spares protein from use as an energy source (i.e. no gluconeogenesis)
C. Preventing Ketosis
1. Adequate intake of carbohydrates is necessary for the complete
metabolism of fats to carbon dioxide and water.
2. If low carbohydrate intake leads to incomplete breakdown of fatty
acids, ketone bodies form resulting in ketosis.
3. The excess ketone bodies spill over into the urine and begin to
draw sodium and potassium ions as well.
4. This results in an ion imbalance and acidic blood that can lead to
coma and death if not treated.
D. Imparting flavor and sweetness to foods (in order of sweetness from
high to low= fructose, sucrose, glucose, maltose, lactose).
IV.
Functions of Dietary Fiber
A. Dietary fiber
1. Adds mass and water to feces
a. Feces then becomes larger and softer and easier to
eliminate
b. Less pressure is therefore needed to expel stool.
2. Little or no fiber in the diet can cause constipation.
a. More pressure is needed to defecate.
b. May cause diverticula and hemorrhoids.
i. Diverticulosis=no symptoms
ii. Diverticulitis=pouches fill with food stuffs and
eventually become inflamed.
3. Aids in weight control because it fills you up but yields little
energy.
4. Appear to be linked to reduction of colon cancer.
5. Higher intakes of fiber are correlated with lowering cholesterol and
bile acid absorption.
V.
Recommended Carbohydrate Intakes
A. RDA is 130 grams of carbohydrate for adults with 20-35 grams of this
being dietary fiber and no more than 10% of total kcal/day from sugars.
B. Problems associated with high sugar intakes:
1. May not contribute the right proportion of energy to the diet.
2. Excessive/High fiber diet can produce phytobezors=pellets of
dietary fiber found in the stomach.
3. May lower nutritional value of a diet by contributing to the lack of
consuming other energy yielding nutrients.
4. May cause dental caries=cavities.
5. Diets may contain too many carbohydrates with high glycemic
indexes:
a. Glycemic index=the blood glucose response of a given
food compared to a standard (such as glucose or white
bread).
b. Influenced by starch structure, fiber content, food
processing, physical structure, and macronutrients in the
meal (such as fat).
c. Glycemic load=the amount of carbohydrate in a food
multiplied by the glycemic index of that carbohydrate.
6. Lactose intolerance may occur.
VI.
Food Sweeteners
A. Nutritive Sweeteners (can be metabolized to yield energy)
1. Sugars (mono and disaccharides)
a. 12-48 pounds are consumed per year per person.
b. Fructose in the form of high-fructose corn syrup is
predominantly used in soft drinks, candies, jam, jelly, and
desserts.
c. Others include: brown sugar, turbinado sugar, honey, and
maple syrup.
2. Sugar alcohols (sorbitol, mannitol, xylitol)
a. Contribute energy (3 kcal/g)
b. Broken down more slowly than simple sugars
c. Sorbitol and xylitol are used in sugarless gum, breath
mints, and candy.
d. These sugar alcohols aren’t readily metabolized by bacteria
so they do not contribute and can reduce the development
of dental caries.
B. Alternative Sweeteners (provide no energy)
1. Saccharine
a. Produced in 1879
b. 300 times sweeter than sucrose
c. Represents half of the alternative sweetener market in the
United States.
d. Once associated with bladder cancer.
2. Aspartame
a. Trade names are “NutraSweet” and “Equal”.
b. 180 to 200 times sweeter than sucrose.
c. Because it is so sweet so little is used in foods that the
energy content of 4 kcal/g is negligible.
d. Easily damaged by high heat.
e. Cannot be used by persons with PKU.
3. Neotame
a. Recently approved by FDA for general-purpose, nonnutritive sweetener.
b. 7000 to 13000 times sweeter than sucrose.
c. Heat stable.
d. Can be used by any person including children, pregnant
women and people with diabetes.
4. Acesulfame-Potassium (Sunett)
a. 200 times sweeter than sugar and calorie free.
b. Approved in 1988 for general purpose sweetener for foods.
5. Sucralose (Splenda)
a. 600 times sweeter than sucrose.
b. Approved in 1998 for use in baked goods, nonalcoholic
beverages, chewing gum, frozen dairy desserts, fruit juices,
and gelatins.
c. Non-nutritive
d. Widely used in Canada.
VII.
PLEASE READ THE NUTRITION PERSPECTIVE: When Blood Glucose
Regulation Fails (page 171-176).
Chapter Objectives
After reading chapter five - A Student should be able to...
1. List the three classes of simple carbohydrates and the three complex carbohydrates
2. List the types of monosaccharides, disaccharides, and oligosaccharides and discuss their
dietary importance (if any)
3. Describe the structural and functional difference between glycogen, starch, and fiber
4. Discuss the process of carbohydrate digestion
5. Discuss what is the recommended intake of carbohydrates and what problems are
associated with high intakes of carbohydrates
6. Describe the difference between a nutritive sweetener and an artificial sweetener
7. List the various types of nutritive and artificial sweeteners and discuss their dietary
importance
8. Discuss the process of glucose regulation in the human body – include in the discussion
the hormones and organs involved
9. Discuss how carbohydrate metabolism in relationship to diabetes mellitus