Download Chapter 5: Carbohydrates

Carbohydrates:
Simple Sugars and
Complex Chains
Chapter 5
Carbohydrates
• all plant food
• milk
• carbohydrates are not equal
– simple carbohydrates
– complex carbohydrates
Carbohydrates
• Sugars, starches, and fibers
• Major food sources: plants
– Produced during photosynthesis
• Two main carbohydrate types
– Simple and complex
Photo © PhotoDisc
Simple Carbohydrates
• sugars
– monosaccharides – single sugars
– disaccharides – 2 monosaccharides
Simple Sugars:
Monosaccharides and Disaccharides
• Simple carbohydrates
– Monosaccharides
• Glucose
• Fructose
• Galactose
– Disaccharides
• Sucrose
• Lactose
• Maltose
Simple Carbs
• monosaccharides
– all are 6 carbon hexes
•
•
•
•
6 carbons
12 hydrogens
6 oxygens
arrangement differs
– accounts for varying sweetness
– glucose, fructose, galactose
Monosaccharides
• Glucose
– Is most abundant
– Also is called dextrose
– Gives food sweet flavor
– Provides energy to
body cells
• Blood sugar
– Found in fruits,
vegetables, honey
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Monosaccharides
• Fructose
– Also called levulose or fruit sugar
– Tastes the sweetest
– Occurs naturally in fruits
and vegetables
– Found in fruits, honey,
and corn syrup
© Photodisc
Monosaccharides
• Galactose
– Primary sugar in milk and
dairy products
– Rarely found naturally
As a single sugar
- Hardly even taste sweet
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Disaccharides: The Double Sugars
• Two monosaccharides linked
together
• Sucrose, lactose, and maltose
Disaccharides
• pairs of the monosaccharides
– glucose is always present
– 2nd of the pair could be fructose, galactose or
another glucose
– taken apart by hydrolysis
– put together by condensation
– hydrolysis and condensation occur with all
energy nutrients
– maltose, sucrose, lactose
Condensation
• making a disaccharide
– chemical reaction linking 2
monosaccharides
Hydrolysis
• breaking a disaccharide
– water molecule splits
– occurs during digestion
Disaccharides: The Double Sugars
• 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
Sucrose
• fructose and
glucose
• tastes sweet
– fruit, vegetables,
grains
• table sugar is
refined sugarcane
and sugar beets
• brown, white,
powdered
Lactose
• glucose and
galactose
• main carbohydrate
in milk
– known as milk sugar
Lactose Intolerance
• age, damage, medication, diarrhea,
malnutrition
• management requires dietary change
– 6 grams (1/2 cup) usually tolerable
– take in gradually
– hard cheeses & cottage cheese
– enzyme drops or tablets
• lactose free diet is extremely difficult to
accomplish
Disaccharides: The Double Sugars
• Maltose: glucose + glucose
– Product of starch breakdown
– Found in germinating cereal grains
– Not abundant
Complex Carbohydrates
• starches and fibers
• polysaccharides
– chains of monosaccharides
Complex Carbohydrates
• Chains of two or more sugar molecules
– Oligosaccharides
– Polysaccharides
• Structural differences
• Digestible or nondigestable
Complex Carbohydrates
• Starch
– How plants store energy
– Found in grains, legumes, and
tubers
– Long chains of glucose units
• Amylose—straight chains
• Amylopectin—branched chains
– Resistant starch
Starches
• stored in plant cells
• body hydrolyzes plant starch to glucose
Complex Carbohydrates
• Glycogen
– Body’s storage form of carbohydrate
– Provides glucose when blood glucose
levels get low
– Highly branched chains of
glucose units
– Most stored in our skeletal
muscle and liver
– Carbohydrate “loading”
Glycogen
• limited in meat and not found in plants
– not an important dietary source of
carbohydrate
• BUT
– all glucose is stored as glycogen
– long chains allow for
hydrolysis and release
of energy
Complex Carbohydrates
• Fiber
– Indigestible chains of monosaccharides
– Dietary fiber: found in plants
• Fruits, vegetables, legumes, and whole
grains
– Functional fiber: isolated and added to
foods
Fiber
• structural parts of plants
– found in all plant derived food
• bonds of fibers cannot be broken down
during the digestive process
– minimal or no energy available
Complex Carbohydrates
• Types of fiber
– Oligosaccharides: short chains of
monosaccharides
• Raffinose, stachyose
• Dried beans, peas,
lentils, human milk
– Cellulose
• In plants: grains, fruit,
nuts, vegetables
– Hemicellulose
• Variety of monosaccharides
Complex Carbohydrates
– Pectins
• All plants, especially fruits
– Gums and cilages
• Gel-forming fibers that hold plant cells
together
– Lignins
• Nondigestable substances in
vegetables and fruit
Fiber types
• cellulose
• pectins
• lignins
• resistant starches
– classified as fibers
– escape digestion and
absorption
Fiber Characteristics
• soluble fibers, viscous, fermentable
– easily digested by bacteria in colon
– associated with protection against heart disease
and diabetes
• lower cholesterol and glucose levels
– found in legumes and fruits
Fiber
• insoluble and not easily fermented
– promote bowel movements
– alleviate constipation
– found in grains and vegetables
Carbohydrate Digestion
• break down into glucose
– body is able to absorb and use
• large starch molecules
– extensive breakdown
• disaccharides
– broken once
• monosaccharides
– don’t need to be broken down
Carbohydrate Digestion
and Absorption
• Digestion: breaking down carbohydrates
into simple sugars
– Begins in the Mouth – Chewing releases
saliva
• Salivary amylase begins digestion of
carbohydrates
– Stomach
• No carbohydrate digestion
Carbohydrate Digestion and
Absorption
• Digestion
– Small intestine
• Pancreatic amylase continues starch
digestion
• The brush border enzymes digest
disaccharides
• Other digestive enzymes
–Maltose, sucrose, and lactase
• small intestine
– majority of carbohydrate digestion takes
place here
– pancreatic amylase reduces carbs to
glucose chains or disaccharides
– specific enzymes finish the job
• maltase
– maltose into 2 glucose
• sucrase
– sucrose into glucose and fructose
• lactase
– lactose into glucose and galactose
• large intestine
– 1-4 hours for sugars and
starches to be digested
– only fibers remain
• attract water, which softens stool
– bacteria ferment some fibers
• water, gas, short-chain fatty acids
(used for energy)
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
• To maintain blood glucose levels
Carbohydrate Digestion and
Absorption
• Travels with carbohydrate
Carbohydrates
• glucose provides energy for the brain
and ½ of energy for muscles and
tissues
• glycogen is stored glucose
• glucose is immediate energy
• glycogen is reserve energy
Carbohydrates in Action
• Glucose is our primary fuel
– Maintain normal blood
glucose levels
– Excess glucose is stored
• Using glucose for energy
– Stored glucose as
glycogen
• Liver
• Muscle
Carbohydrate Metabolism
•
1/3 of body’s glycogen is stored in liver
–
1.
2.
3.
4.
released as glucose to bloodstream
eat – intake glucose
liver condenses extra glucose to glycogen
blood glucose falls
liver hydrolyzes glycogen to glucose
Glycogen is bulky, so we store only so much:
short term energy supply
Fat is the long term energy supply.
Glucose for Energy
• enzymes break apart glucose – yielding
energy
• inadequate supply of carbohydrates
– ketone bodies (fat fragments) are an
alternate energy source during starvation
– excess ketones can lead to ketosis:
imbalance of acids in body
• minimum of 50 – 100 grams of
carbs/day are needed to avoid ketosis
Carbohydrates in Action
• Using glucose for energy
– Sparing body protein
• Adequate carbohydrates spare protein
– Preventing ketosis
• Ketone bodies
–Ketosis  dehydration
Glucose Homeostasis
• maintaining an even balance of glucose
is controlled by insulin and glucagon
– insulin
• moves glucose into the blood
– glucagon
• brings glucose out of storage
Carbohydrates in Action
• Regulating blood glucose
– Why? To maintain supply for cells
– Controlled by hormones
• Insulin
–Key
• Glucagon
–Breaks down glycogen
• Epinephrine
–“Fight or flight” hormone
– Glycemic index
1
Intestine
When a person eats,
blood glucose rises.
Maintaining
Blood Glucose
Homeostasis
2
Pancreas
High blood glucose stimulates
the pancreas to release insulin.
Insulin
3
Insulin stimulates the uptake of
glucose into cells and storage
as glycogen in the liver and
muscles. Insulin also stimulates
the conversion of excess
glucose into fat for storage.
Liver
Fat cell
Muscle
4
As the body's cells use
glucose, blood levels decline.
5
Pancreas
Glucagon
6
Glucose
Insulin
Glucagon
Glycogen
Low blood glucose stimulates
the pancreas to release
glucagon into the bloodstream.
Glucagon stimulates liver
cells to break down glycogen
and release glucose into the
blood.a
Liver
a
The stress hormone
epinephrine and other hormones
also bring glucose out of storage.
7
Blood glucose begins to
rise.
Carbohydrates in Action: Regulating
Blood Glucose
Carbohydrates in Action
• Diabetes mellitus
– Consequences
• Hyperglycemia—persistent high blood
glucose levels
–Glucose unable to enter cells
–Increased risk of high blood
pressure, heart disease, and kidney
disease
–Damage to body proteins and tissues
Carbohydrates in Action
• Diabetes mellitus
– Type 1 diabetes: lack of insulin production
– Type 2 diabetes: cells are resistant to
insulin
• Prediabetes
• Insulin resistance
– Gestational diabetes: occurs during
pregnancy
Carbohydrates in Action
Carbohydrates in Action
• Diabetes
– Best prevention
• Healthful diet
–Monitoring and well-balanced meals
–The Exchange List for Meal Planning
• Regular exercise
Sugar
• ½ comes from natural sources, ½ from
refined and added
– sucrose, corn syrup, honey
• excess can lead to nutrient deficiencies
and tooth decay
– empty calories
– sugar and starch break down in the mouth
Sugar
• recommended intake
– added sugar = no more than 10% of energy intake
Carbohydrates in Action
• Low blood glucose: hypoglycemia
– Symptoms
• Nervousness, irritability, hunger,
headache, shakiness, rapid heart rate,
and weakness
– Results from
• Too much insulin, missed meals, and
vigorous exercise
– Reactive hypoglycemia
– Fasting hypoglycemia
Carbohydrates in Your Diet
• Recommended carbohydrate intake
– RDA = 130 grams per day
– AMDR = 45–65% of calories
– Daily Value (for 2,000 kcal) = 300 grams
– Dietary Guidelines
• Choose carbohydrates wisely
• Choose fiber-rich fruits, vegetables, and
whole grains often
Carbohydrates in Your Diet
• Choosing carbohydrates
wisely
– Increase fruit, vegetables,
whole grains, low-fat milk
• Strategies
– Eat peel of fruit/vegetables
– Eat legumes
– Choose brown rice
– Choose high-fiber cereal
– Gradually increase and
drink plenty of water!
Starch and Fiber
• diet that includes starch, fiber and natural
sugars
– whole grains, vegetables, legumes, fruits
•
•
•
•
may protect against heart disease and stroke
reduces the risk of type 2 diabetes
enhances the health of the large intestine
can promote weight loss
Carbohydrates in Your Diet
• Moderating sugar intake
– Use less added sugar
– Limit soft drinks, sugary cereals, and candy
– Choose fresh fruits or those canned in water
or juice
Photo © CSquared Studios/PhotoDisc
Carbohydrates in Your Diet
• Nutritive sweeteners
– Natural vs. refined
– Sugar alcohols
• Non-nutritive
sweeteners
– Saccharin
– Aspartame
– Acesulfame K
– Sucralose
Artificial Sweeteners
• help keep sugar and energy intake down
• anything we eat has FDA approval
– saccharin
– aspartame
– acesulfame potassium
– sucralose
– neotame
Carbohydrates and Health
• Sugar
– High sugar intake
may be low in
nutrients
– High sugar intake
promotes tooth
decay
Photo @ PhotoDisc
Sugar Replacers
• sugar alcohols
– provide bulk and sweetness
• cookies, gum, candy, jelly
– do contain minimal kcal
– low glycemic response
• absorbed slowly
– do not cause dental caries
Carbohydrates and Health
• Fiber
– Possible role in weight control
– Better control of blood glucose
– Reduced risk of heart disease
– Healthier gastrointestinal functioning
– Negative effects of excess fiber
• Gradual intake and increased water
consumption
• Can bind small amounts of minerals