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CHAPTER 3
Carbohydrates
Eleanor D. Schlenker
Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
2 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
The Nature of Carbohydrates
Two forms of carbohydrate occurring
naturally in plant foods:
1. Starches
2. Sugars
Photosynthesis: Plants transform the sun’s
energy into the stored fuel form of
carbohydrate
3 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
The Nature of Carbohydrates –
Cont’d
Starch that plants store for their own energy
needs becomes a source of fuel for humans
who eat those plants
Carbohydrates are often referred to as “quick
energy” foods
They are our primary source of energy
4 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Dietary Importance
In some countries, carbohydrate-containing
foods (fruits, vegetables, grains, cereals)
make up 85% of the diet
Rice feeds 3 billion people in developing
countries
Half of the total kcalories come from
carbohydrates in the typical American diet
Carbohydrate foods can be easily stored for
long periods of time without spoiling
5 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates
Carbohydrate
Name comes from the chemical nature of
these molecules
Carbon, hydrogen, and oxygen with the
hydrogen/oxygen ratio usually that of water—
CH2O
6 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Carbohydrate – cont’d
Classified according to the number of basic
sugar or saccharide units that make up their
structure
Simple carbohydrates: monosaccharides and
disaccharides
Complex carbohydrates: polysaccharides
7 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Monosaccharides
Three important sugars in human nutrition:
1. Glucose
2. Fructose
3. Galactose
8 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Monosaccharides – cont’d
Glucose
Moderately sweet sugar found naturally in only a
few foods, one being corn syrup
The common body fuel oxidized by cells to provide
energy
Supplied from digestion of starch and conversion
of other simple sugars
Form of sugar that circulates in the blood (called
dextrose in intravenous solutions)
9 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Monosaccharides – cont’d
Fructose
The sweetest of the simple sugars
Found in fruits and other natural substances such
as honey
Fructose is absorbed less efficiently than glucose,
25 to 50 g can cause GI distress
Accounts for 10% of the total energy intake of
Americans 2 years of age and older, mostly
because of high-fructose corn syrup
10 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Monosaccharides – cont’d
Galactose
Not found free in foods
Released through the digestion of lactose (milk
sugar) and then converted to glucose in the liver
This reaction is reversible: in lactation glucose is
reconverted to galactose for use in milk production
11 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Disaccharides
Double sugars made up of two
monosaccharides linked together
Three of physiologic importance:
1. Sucrose
2. Lactose
3. Maltose
12 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Disaccharides – cont’d
Sucrose = one glucose + one fructose
Common “table sugar,” made commercially from
sugar cane and sugar beets
Found naturally in some fruits and vegetables
13 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Disaccharides – cont’d
Lactose = one glucose + one galactose
The sugar found in milk
It is the least sweet of the disaccharides, one sixth
as sweet as sucrose
Cheese contains little lactose
14 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Disaccharides – cont’d
Maltose = one glucose + one glucose
Arises from the breakdown of starch
Found in commercial malt products and
germinating cereal grains
15 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Sugar Intake
Most sugar is added in food preparation or
processing
Intake of added sugar is about 24 tsp/day in
children and about 20 tsp/day in adults
Nearly 37% of added sugars come from soft
drinks and another 11% from fruit drinks
16 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Sugar Alcohols
Are other forms of carbohydrate with
sweetening power
Also referred to as polyols
Three types that occur in nature but are also
used by food industry:
1. Sorbitol
2. Mannitol
3. Xylitol
17 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Sugar Alcohols – cont’d
Foods made with sugar alcohols may be labeled “sugar free”
Only 1.6 to 2.6 kcal/g
Do not require insulin for their metabolism
Do not promote tooth decay
High intakes can cause abdominal distress or exert a laxative effect
Foods with these sweeteners are best used in moderation
18 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Polysaccharides
Made up of many (poly) single glucose
(saccharide) units
Important energy-yielding polysaccharides:
1. Starch
2. Glycogen
3. Dextrins
19 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Polysaccharides – cont’d
Starch
Most important energy-yielding polysaccharide
Large complex substance made of many coiled
and branching chains of single glucose units and
yields only glucose on complete digestion
Cooking improves flavor and makes digestion
easier
Thickening and gel-like qualities
20 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Polysaccharides – cont’d
Resistant starch
Some starch in particular foods is not digested
Undigested starch is fermented by bacteria in the
colon
Produces short-chain fatty acids, the preferred
energy source of the cells lining the colon
Other health effects similar to dietary fiber
21 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Polysaccharides – cont’d
Glycogen
Storage form of carbohydrate in animals
Synthesized in liver cells and stored in relatively
small amounts in the liver and muscle
22 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Polysaccharides – cont’d
Dextrins
Polysaccharide compounds formed as
intermediate products in the breakdown of starch
Starch breakdown is ongoing in the process of
digestion
23 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Oligosaccharides
Small fragments of partially digested starch
ranging in size from 3 to 10 glucose units
They are formed through digestion and
produced commercially by acid hydrolysis
Used in sports drinks
24 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Classification of Carbohydrates –
Cont’d
Oligosaccharides – cont’d
Naturally occurring oligosaccharides formed with bonds that cannot be broken by human enzymes and remain undigested: 1. Stachyose
2. Affinose
Found in legumes such as beans, peas, and soybeans
25 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Importance of Complex
Carbohydrates
Should be the major dietary source of energy
Supplies 4 kcal/g; less than fat (9 kcal/g)
Carbohydrate foods also supply other
important nutrients
Emphasize whole grain choices
Dietary Guidelines for Americans
recommends limiting sugars to 8 tsp or less
per day
26 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Functions of Carbohydrates
Energy
Primary function of starches and sugars is to
provide energy to cells, especially brain cells
that depend on glucose
Amount of carbohydrates stored in the body,
although small, is an important energy
reserve
27 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Functions of Carbohydrates –
Cont’d
Energy – cont’d
Glycogen stores and available blood glucose
can meet the energy needs for a half day of
moderate activity
Carbohydrate foods must be eaten regularly
and at reasonably frequent intervals to meet
the body’s demand for energy
28 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Special Functions
of Carbohydrates in Body
Tissues Glycogen – Carbohydrate Storage
Protect cells, especially brain cells, from
depressed metabolic function and injury and
support urgent muscle responses as needed
Protein-Sparing Action
Allows protein to be used for tissue building
and repair
29 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Special Functions
of Carbohydrates in Body Tissues
– Cont’d
Antiketogenic Effect
Ketones are intermediate products of fat
metabolism
Under extreme conditions when available
carbohydrates are inadequate to meet energy
needs, fat is oxidized at excessive rates
30 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Special Functions
of Carbohydrates in Body Tissues
– Cont’d
Antiketogenic Effect – cont’d
Ketones accumulate and the result is
ketoacidosis
Sufficient amounts of dietary carbohydrates
prevent any damaging excess of ketones
31 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Special Functions
of Carbohydrates in Body Tissues
– Cont’d
Heart Action
Fatty acids are the preferred fuel for the heart,
but glycogen stored in cardiac muscle is an
important emergency source of contractile
energy
32 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Special Functions of
Carbohydrates in Body Tissues –
Cont’d
Central Nervous System
Depends on carbohydrates as an energy source
These tissues have very low carbohydrate
reserves—enough to last only 10 to 15
minutes—so they are especially dependent on a
minute-to-minute supply of glucose from the
blood
33 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Special Functions of
Carbohydrates in Body Tissues –
Cont’d
Central Nervous System – cont’d
Sustained hypoglycemic shock causes
irreversible brain damage
Glucose increases the synthesis of acetylcholine
Responsible for memory and cognitive function
34 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Recommended Intake
of Carbohydrates
Recommended Dietary Allowance (RDA)
Same for all persons over 1 year of age
Children, adolescents, and adults should take
in a minimum of 130 g/day
Will ensure a sufficient amount of glucose to
supply the energy needs of the brain for 1
day
35 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Recommended Intake
of Carbohydrates – Cont’d
Acceptable Macronutrient Distribution
Range (AMDR)
AMDR for carbohydrates is 45% to 65% of
total energy intake
No more than 25% from added sugar
36 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Nonnutritive Sweeteners
Allow persons to indulge their taste for sweets
while limiting their kcalorie intake
Sweeteners grouped as nutritive or
nonnutritive depending on the kcalories they
contain
Sucrose (table sugar) and other natural
sweeteners contain 4 kcal/g
Nonnutritive sweeteners yield little or no
energy
37 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Nonnutritive Sweeteners – Cont’d
Six nonnutritive sweeteners have been
approved by the Food and Drug
Administration (FDA) for use in the United
States
Aspartame contains phenylalanine
Should be avoided by persons with
phenylketonuria
38 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Carbohydrates and Oral Health
Synergistic relationship between oral health
and nutrition
Dental caries is affected by oral hygiene, diet,
and specific nutrients
Fluoride strengthens tooth enamel
Amounts and types of carbohydrates
influence dental caries
Continuous sipping of sugar-sweetened drinks
increases risk of tooth decay
39 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Carbohydrates and Oral Health –
Cont’d
Whole-grain cereals are protective against
dental caries
Chronic disease increases vulnerability to
dental caries and tooth loss
Age-related osteoporosis may result in tooth
loss
Xerostomia (dry mouth) accelerates tooth
decay
40 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Imbalances in Carbohydrate
Intake
High-Carbohydrate Diets
Contain 65% to 75% carbohydrates
May cause low intakes of nutrients associated
with high-fat foods
Lead to a rise in plasma triglycerides and a
drop in high-density lipoprotein (HDL)
cholesterol levels
Elevated blood glucose levels increase
demand for insulin
41 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Imbalances in Carbohydrate
Intake – Cont’d
Low-Carbohydrate Diets
Contain less than 20% carbohydrate
Unlikely to contain the minimum servings of
fruits, vegetables, and whole grains
recommended by MyPyramid
Replace carbohydrate foods with high-fat
foods and may contain as much as 46% of
total kcalories as fat, adding to cardiovascular
risk
42 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Fiber
Two general categories of fiber:
1. Dietary fiber
2. Functional fiber
43 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Dietary Fiber
Includes nondigestible carbohydrates and
lignin that are intact in plant foods
Cellulose
Hemicellulose
Lignin
Pectin
Gums
Beta glucans
44 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Functional Fiber
Nondigestible polysaccharides that have
been isolated from plants or commercially
produced and added to foods
Term developed to indicate those fibers that
were separated from plants or commercially
produced as compared with those that are
intact in plants and eaten in that form
45 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Dietary and Functional Fiber
A person's total fiber intake includes both
dietary fiber and functional fiber
Many foods high in fiber are low to moderate
in kcalories
46 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Health Benefits
Fiber
Increase in fecal mass/promotes laxation
Binding of bile acids and cholesterol
Positive effect on colonic microflora
Slows rise in blood glucose and insulin levels
47 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Recommended Fiber Intake
Adequate Intake
Men: 38 g/day below age 51; 30 g/day age 51
and over
Women: 25 g/day below age 51; 21 g/day
age 51 and over
Actual Intakes Are Barely Half AI
48 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Functional Foods
Phytochemicals
Fruits, vegetables, and whole grain foods rich
in carbohydrates and fiber contain
phytochemicals (plant chemicals) beneficial
to health
Those who eat fruits, vegetables, and whole
grains regularly have a lower risk of heart
disease, cancer, and diabetes
49 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Digestion
Starches and sugars must be converted to
glucose
Mouth: salivary ptyalin
Pancreatic secretions: Amylase breaks starch
to maltose
Intestinal secretions: Sucrase, lactase, and
maltase
50 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Absorption and Metabolism
Glucose is absorbed by an active pumping
system using sodium as a carrier
Enters portal blood circulation
In the liver, fructose and galactose are
converted to glucose
Glucose is used for immediate energy or
converted to glycogen or adipose tissue