Carbohydrates: Energy Sources and Classification

CHAPTER 3
Carbohydrates

Eleanor D. Schlenker

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

Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 2

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


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


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


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


Cont’d
Monosaccharides
 Three important sugars in human nutrition:

1. Glucose
2. Fructose
3. Galactose


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)


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


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


Cont’d
Disaccharides
 Double sugars made up of two

monosaccharides linked together
 Three of physiologic importance:

1. Sucrose
2. Lactose
3. Maltose


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


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


Cont’d
 Maltose = one glucose + one glucose

 Arises from the breakdown of starch
 Found in commercial malt products and
germinating cereal grains


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


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


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


Cont’d
Polysaccharides
 Made up of many (poly) single glucose

(saccharide) units
 Important energy-yielding polysaccharides:

1. Starch
2. Glycogen
3. Dextrins


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


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


Cont’d

 Glycogen

 Storage form of carbohydrate in animals
 Synthesized in liver cells and stored in relatively
small amounts in the liver and muscle


Cont’d

 Dextrins

 Polysaccharide compounds formed as
intermediate products in the breakdown of starch
 Starch breakdown is ongoing in the process of
digestion


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


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


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


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


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


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


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


Special Functions
– Cont’d
Antiketogenic Effect – cont’d
 Ketones accumulate and the result is

ketoacidosis
 Sufficient amounts of dietary carbohydrates

prevent any damaging excess of ketones


Special Functions
– 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


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


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


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


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


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


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


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


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


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


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


Fiber
 Two general categories of fiber:
1. Dietary fiber
2. Functional fiber


Dietary Fiber

 Includes nondigestible carbohydrates and
lignin that are intact in plant foods
 Cellulose
 Hemicellulose
 Lignin
 Pectin
 Gums
 Beta glucans


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


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


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


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


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


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


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


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