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Chapter 4
Carbohydrates:
Simple Sugars and
Complex Chains
What Are Carbohydrates?
• Sugars, starches, and fibers
• Major food sources: Plants
– Produced during
photosynthesis
• Two main carbohydrate
types
– Simple (Sugars)
– Complex (Starches and
fibers)
Simple Sugars
• Monosaccharides
– Glucose
– Fructose
– Galactose
Simple Sugars
• Disaccharides: Consist of
two monosaccharides
linked together
– Sucrose
– Lactose
– Maltose
Monosaccharides
• Glucose
– Most abundant simple sugar in nature
– Also called dextrose
– Gives food a mildly sweet flavor
– Usually joined to another sugar in foods
– Provides energy to body cells
• Body closely regulates blood sugar levels
Monosaccharides
• Fructose
– Also called levulose or fruit sugar
– Tastes the sweetest of all the sugars
– Occurs naturally in fruits and vegetables
– Found in fruits, honey, and corn syrup
Monosaccharides
• Galactose
– Rarely occurs as a monosaccharide in food
– Usually bonds to glucose to form lactose
• Primary sugar in milk and dairy products
Disaccharides
• Sucrose: glucose + fructose
– “Table sugar”
– Made from sugar cane and sugar beets
– Listed as sugar on food labels
• Lactose: glucose + galactose
– “Milk sugar”
– Found in milk and milk products
Disaccharides
• Maltose: glucose + glucose
– “Malt sugar”
– Seldom occurs naturally in foods
– Product of starch breakdown
– Found in germinating cereal grains
Complex Carbohydrates
• Chains of two or more sugar molecules
– Oligosaccharides
• Three to ten sugar molecules
• Examples sources: dried beans, peas, and
lentils
– Polysaccharides
• Long chains of monosaccharides
• Structural differences affect how they
behave in water and with heating
• Digestible or nondigestible
Complex Carbohydrates
• Starch
– Plants store energy as starch
– Found in grains, legumes, and tubers
(potatoes and yams)
– Long chains of glucose units
• Amylose—straight chains
• Amylopectin—branched chains
– Resistant starch: A starch that is not digested
Complex Carbohydrates
• Glycogen
– Living animals store carbohydrate in the form
of glycogen
– Provides body glucose when blood glucose
levels get low
– Highly branched chains of glucose units
– Most stored in our skeletal muscle and liver
– Carbohydrate “loading”
Complex Carbohydrates
• Fiber
– Nondigestible carbohydrates and lignins
– Dietary fiber: Found in plants
• Fruits, vegetables, legumes, and whole
grains
– Functional fiber: Isolated and added to foods
– Total fiber: Sum of dietary fiber and functional
fiber
Complex Carbohydrates
• Types of fiber
– Cellulose
• Indigestible by humans and a component of
dietary fiber
– Hemicelluloses
• Variety of monosaccharides with many
branching side chains
– Pectins
• Dietary fiber found in all plants, especially
fruits
Complex Carbohydrates
• Types of fiber
– Gums and cilages
• Gel-forming fibers that help hold plant cells
together
– Lignins
• Indigestible substances that make up woody
parts of vegetables and the seeds of fruits
Complex Carbohydrates
• Types of fiber
– Beta-glucans
• Polysaccharides of branched glucose units
• Found in barley and oats
• Help decrease blood cholesterol levels
– Chitin and chitosan
• Primarily consumed in supplement form
• Marketed as weight-loss supplements
• May impair absorption of fat-soluble
vitamins and some minerals
Carbohydrate Digestion and Absorption
• Digestion breaks down carbohydrates to single
sugars
– Mouth
• Salivary amylase begins digestion of
carbohydrates
– Stomach
• Acidity of stomach juices halt action of
salivary amylase and stops carbohydrate
digestion
Carbohydrate Digestion and Absorption
• Digestion
– Small intestine
• Pancreatic amylase continues starch digestion
• Brush border enzymes digest disaccharides
• Other digestive enzymes
–Maltase, sucrase, and lactase split maltose,
sucrose, and lactose, respectively
Carbohydrate Digestion and Absorption
• Digestion
– Bonds that link glucose molecules
• Alpha bonds
–Broken down by human enzymes
»Starch
• Beta bonds
–Bonds remain unbroken by human enzymes
»Cellulose
Carbohydrate Digestion and Absorption
• Digestion
• Enzymes
–Highly specific
»Examples include lactase and Beano
• Some carbohydrates remain intact, such as
fiber and resistant starch
Carbohydrate Digestion and Absorption
• Absorption: The small intestine swings into action
– End products of carbohydrate digestion
• Glucose
• Galactose  glucose
• Fructose  glucose
– Liver
• Stores and releases glucose as need to
maintain blood glucose levels
Carbohydrates and Glucose in the Body
• Glucose is our primary fuel
– Needed to burn fat efficiently
– When we eat food, our bodies
• Immediately use some glucose to maintain
normal blood glucose levels
• Store excess glucose as glyocgen in liver and
muscle tissue
Carbohydrates and Glucose in the Body
• Using glucose for energy
– Liver glycogen
• Maintains normal blood glucose levels
– Muscle glycogen
• Fuels muscle activity
Carbohydrates and Glucose in the Body
• Sparing body protein
– Adequate carbohydrates prevent body from
making glucose from body proteins
• Preventing ketosis
– Ketone bodies  ketosis  dehydration
– Body needs a minimum of 50 to 100 grams of
carbohydrate/day to prevent ketosis
Carbohydrates and Glucose in the Body
• Regulating blood glucose
– To maintain an adequate supply for cells
– Controlled by hormones
• Insulin
–Key “unlocking” body cells and allowing
glucose to enter and fuel them
• Glucagon
–Breaks down glycogen to glucose
• Epinephrine
–“Fight-or-flight” hormone
Carbohydrates and Glucose in the Body
• Regulating blood glucose
– Glycemic index
• Foods vary in their effect on blood glucose
levels
• Index measures the effect of food on blood
glucose levels
The Role of Carbohydrates in Our Diet
• Recommended Carbohydrate Intake
– RDA = 130 grams per day
– Dietary Guidelines
• “Reduce the intake of added sugars”
• AMDR for added sugars = < 25% of daily energy
intake
• Choose whole grains, vegetables, fruits, and
cooked dried beans and peas
• Consume > ½ of all grains as whole grains
The Role of Carbohydrates in Our Diet
• Current Consumption: How much are we eating?
– AMDR = 45–65% of kcal
– Average American adult = 49–50% of kcal
– 13% of population has added sugar intake >
25% of kcal
• High soft drink consumption
• Milk consumption and vitamin and mineral
quality of diet is declining
The Role of Carbohydrates in Our Diet
• Choosing Carbohydrates
Wisely
– Increase fruit, vegetables,
whole grains, low-fat milk
• Strategies
– Eat peel fruit/vegetables
– Eat legumes
– Choose brown rice
– Choose high-fiber cereal
– Drink plenty of water!
The Role of Carbohydrates in Our Diet
• High intake of foods rich in dietary fiber offers
many health benefits, including reduced risk of
– Obesity
– Type 2 diabetes
– Cardiovascular disease
– Gastrointestinal disorders
• Increase fiber intake gradually while drinking
plenty of fluids
Carbohydrates and Health
• Fiber and obesity
– Possible role in weight control
• Fiber and type 2 diabetes
– Better control of blood glucose
• Fiber and cardiovascular disease
– Can lower blood cholesterol levels
• Fiber and gastrointestinal disorders
– Healthier gastrointestinal functioning
Carbohydrates and Health
• Negative effects of excess fiber
– Gradual intake and increased water
consumption
– Can bind small amounts of minerals
Carbohydrates and Health
• Moderating sugar intake
– Use less added sugar
– Limit soft drinks, sugary cereals, candy, ice
cream, and sweet desserts
– Choose fresh fruits or those canned in
water or juice
Carbohydrates and Health
• Nutritive sweeteners
– Monosaccharides, disaccharides, and sugar
alcohols
– Natural vs. refined
• Non-nutritive sweeteners
– Saccharin
– Aspartame
– Acesulfame K
– Sucralose
Carbohydrates and Health
• Sugar and dental caries promoted by:
– Carbohydrate remaining in the mouth
• Food that sticks to teeth
– Frequent consumption
• Sip high sugar drinks over time