ch 3 ppt

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


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

ch 3 ppt

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