Transcript Notes

4
Carbohydrates:
Plant-Derived
Energy Nutrients
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What Are Carbohydrates?
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One of the three macronutrients
Important source of energy for all cells
Preferred energy source for nerve cells
Composed of carbon, hydrogen, oxygen
Good sources: fruits, vegetables, grains
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What Are Carbohydrates?
• Glucose
• The most abundant carbohydrate
• Produced by plants through photosynthesis
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© 2011 Pearson Education, Inc.
What Are Carbohydrates?
• Simple carbohydrates
• Contain one or two molecules
• Commonly referred to as sugars
• Monosaccharides contain one molecule
• Glucose, fructose, and galactose
• Disaccharides contain two molecules
• Lactose, maltose, and sucrose
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
What Are Carbohydrates?
• Complex carbohydrates
• Oligosaccharides contain 3 to 10
monosaccharides
• Most polysaccharides consist of hundreds to
thousands of glucose molecules
• Starch, glycogen, most fibers
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© 2011 Pearson Education, Inc.
Complex Carbohydrates
• Starch
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Plants store carbohydrates as starch
Amylose—straight chain of glucose
Amylopectin—branched chain of glucose
Resistant starch (fiber)—glucose molecules
linked by beta bonds are largely indigestible
• Sources: grains, legumes, fruits, vegetables
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Complex Carbohydrates
• Glycogen
• Storage form of glucose for animals (humans)
• Not found in food and therefore not a source of
dietary carbohydrate
• Stored in the liver and muscles
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Complex Carbohydrates
• Fiber
• Composed of long polysaccharide chains
• Dietary fibers are non-digestible parts of plants
• Functional fibers are non-digestible forms of
carbohydrates extracted from plants or
manufactured in a laboratory and have known
health benefits
• Total fiber = Dietary fiber + Functional fiber
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Complex Carbohydrates
• Soluble fibers
• Dissolve in water; viscous and gel-forming
• Fermentable, digested by intestinal bacteria
• Associated with risk reduction of cardiovascular
disease and type 2 diabetes
• Examples: pectin, gum, mucilage
• Found in citrus fruits, berries, oats, beans
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Complex Carbohydrates
• Insoluble fibers
• Do not dissolve in water, nonviscous
• Cannot be fermented by bacteria in the colon
• Promote regular bowel movements, alleviate
constipation, and reduce diverticulosis
• Examples: lignins, cellulose, hemicelluloses
• Good sources: whole grains, seeds, legumes,
fruits, and vegetables
ABC Video Whole Grains
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Carbohydrate Digestion
Carbohydrate Digestion
• Salivary amylase
• Enzyme that begins carbohydrate digestion in the mouth
• Breaks carbohydrates down to maltose
• Carbohydrate digestion does not occur in the
stomach
• Stomach acids inactivate salivary amylase
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Carbohydrate Digestion
• Most chemical digestion of carbohydrates
occurs in the small intestine
• Pancreatic amylase
• Enzyme produced in the pancreas and secreted
into the small intestine
• Digests carbohydrates to maltose
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Carbohydrate Digestion
• Additional enzymes in the microvilli digest
disaccharides to monosaccharides
• Maltase
• Sucrase
• Lactase
• Monosaccharides are absorbed into the cells
lining the small intestine and then enter the
bloodstream
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© 2011 Pearson Education, Inc.
Carbohydrate Digestion
• All monosaccharides are converted to glucose
by the liver
• Glucose circulating in the blood is our primary
energy source
• Excess glucose is stored as glycogen in the
liver and muscles
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© 2011 Pearson Education, Inc.
Carbohydrate Digestion
• We do not have the enzymes necessary to digest
fiber
• Bacteria in the large intestine can break down some
fiber
• Most fiber remains undigested and is excreted in the
feces
Carbohydrate Absorption
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Blood Glucose Regulation
• Blood glucose must be closely regulated
• Hormones control blood glucose levels:
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Insulin
Glucagon
Epinephrine
Norepinephrine
Cortisol
Growth hormone
Hormonal Control of Blood
Glucose
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Blood Glucose Regulation: Insulin
• Secreted by beta cells of the pancreas
• Stimulates glucose transporters (carrier
proteins) to help take glucose from the blood
across the cell membrane
• Stimulates the liver to take up glucose and
convert it to glycogen
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© 2011 Pearson Education, Inc.
Blood Glucose Regulation: Glucagon
• Secreted by alpha cells of the pancreas
• Stimulates the liver to convert glycogen to
glucose
• Stimulates gluconeogenesis, the production
of glucose from amino acids
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© 2011 Pearson Education, Inc.
Blood Glucose Regulation:
Epinephrine and Norepinephrine
• Secreted by the adrenal glands and nerve
endings when blood glucose is low
• Increase glycogen breakdown in the liver,
releasing glucose into the blood
• Responsible for our “fight-or-flight”
reactions to danger
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Blood Glucose Regulation:
Cortisol and Growth Hormone
• Secreted by the adrenal glands to act on
the liver, muscle, and adipose tissue
• Cortisol increases gluconeogenesis and
decreases muscle glucose use
• Growth hormone decreases muscle
glucose uptake, increases fatty acid
mobilization and use, and increases liver
glucose output
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Blood Glucose Regulation:
Glycemic Index
• A food’s potential to raise blood glucose
• Foods with a high glycemic index cause
a sudden surge in blood glucose,
triggering a large increase in insulin,
which may be followed by a dramatic fall
in blood glucose
• Foods with a low glycemic index cause
low to moderate fluctuations in blood
glucose
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Blood Glucose Regulation:
Glycemic Index
• Not always easy to predict
• Food’s absorption rate varies with type of
carbohydrate, preparation methods, and its
fat and fiber content
• Most foods are eaten in combination in a
meal
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Blood Glucose Regulation:
Glycemic Load
• Used to determine the effect of a food on a
person’s glucose response
• Grams of carbohydrates in a food are
multiplied by the glycemic index
• Glycemic index and glycemic load remain
controversial
• Evidence of health benefits is weak
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Blood Glucose Regulation:
Value of Lower Glycemic Index
• Less fluctuations in blood glucose
• Risk reduction for heart disease and colon
cancer
• High fiber intake helps to improve fat levels in
the blood (higher HDL and lower LDL)
• Foods with lower glycemic index:
• Beans, fresh vegetables, whole wheat
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© 2011 Pearson Education, Inc.
The Role of Carbohydrates
• Energy
• Each gram of carbohydrate: 4 kcal
• Red blood cells use only glucose for energy
• Both carbohydrates and fats supply energy for
daily activities
• Glucose is especially important for energy during
exercise
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© 2011 Pearson Education, Inc.
The Role of Carbohydrates
• Ketosis
• Fat breakdown during fasting forms ketones
• Excess ketones increase blood acidity and cause
ketoacidosis
• Sufficient energy from carbohydrates prevents
ketone production as alternate energy source
Fad Diets
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Carbohydrates Spare Protein
• Gluconeogenesis occurs when a diet is
deficient in carbohydrate
• The body will make its own glucose from
protein
• Amino acids from these proteins cannot be
used to make new cells, repair tissue
damage, support the immune system, or
perform any of their other functions
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Complex Carbohydrates Have Health Benefits
• Fiber
• May reduce the risk of colon cancer
• Helps prevent hemorrhoids, constipation, and other
intestinal problems
• May reduce the risk of diverticulosis
• May reduce the risk of heart disease
• May enhance weight loss
• May lower the risk of type 2 diabetes
Diverticulosis and Fiber
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© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
How Much Carbohydrate?
• Recommended Dietary Allowance (RDA) is
130 grams/day to supply adequate glucose to
the brain
• Acceptable Macronutrient Distribution Range
(AMDR) is 45% to 65% of daily calories
• Focus on fiber-rich carbohydrate foods
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© 2011 Pearson Education, Inc.
Simple Carbohydrates
• Diets high in simple sugars:
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Can cause tooth decay
May increase “bad cholesterol”
May decrease “good cholesterol”
May contribute to obesity
ABC Video Sugar and
Processed Food
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© 2011 Pearson Education, Inc.
Complex Carbohydrates
• Most Americans eat too little complex
carbohydrates
• Enriched foods are foods in which nutrients
that were lost during processing have been
added back so the food meets a specified
standard
• Fortified foods have nutrients added that did
not originally exist in the food (or existed in
insignificant amounts)
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© 2011 Pearson Education, Inc.
Complex Carbohydrates
• Adequate Intake (AI) for fiber
• 25 g per day for women
• 38 g per day for men, or
• 14 g of fiber for every 1,000 kcal per day
• It is best to get fiber from food (also a source
of vitamins and minerals)
• An adequate fluid intake (at least 8 oz/day)
with high-fiber diets is recommended
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Alternative Sweeteners
• Nutritive sweeteners
• Sucrose, fructose, honey, and brown sugar
contain 4 kcal energy per gram
• Slow-absorbing sugar alcohols: 2−3 kcal/gm
• Non-nutritive (alternative) sweeteners
• Provide little or no energy
• Safe for adults, children, people with diabetes
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© 2011 Pearson Education, Inc.
Alternative Sweeteners
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Saccharin
Acesulfame-K
Aspartame
Sucralose
Truvia and PureVia (stevia plant)
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© 2011 Pearson Education, Inc.
Diabetes
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Inability to regulate blood glucose levels
Type 1 diabetes
Type 2 diabetes
Gestational diabetes
Uncontrolled diabetes can cause nerve
damage, kidney damage, blindness, and can
be fatal
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Diabetes—Type 1
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Accounts for 10% of all cases
Body does not produce enough insulin
Causes hyperglycemia (high blood glucose)
Requires insulin injections
May be an autoimmune disease
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Diabetes—Type 2
• Insulin insensitivity (insulin resistance): cells
become less responsive to insulin
• Metabolic syndrome: a cluster of risk factors
that increase the risk for type 2 diabetes
• Once known as adult-onset diabetes
• Increasing in children and adolescents
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Diabetes—Type 2
• Cause is unclear, but genetics, obesity, and
physical inactivity play a role
• Treat with weight loss, healthy eating,
regular exercise, and, if necessary,
medications
• Healthy lifestyle choices may prevent or
delay the onset of type 2 diabetes
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Hypoglycemia
• Low blood glucose may cause shakiness,
sweating, anxiety, weakness
• Reactive hypoglycemia: pancreas secretes
too much insulin after a high-carbohydrate
meal
• Fasting hypoglycemia: pancreas produces
too much insulin, even when someone has
not eaten
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© 2011 Pearson Education, Inc.
Lactose Intolerance
• Insufficient enzyme lactase to digest the
lactose-containing foods
• GI symptoms: gas, cramping, diarrhea
• Variations in extent of intolerance
• Not to be confused with milk allergy
• Need alternate sources of calcium
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