The science of nutrition 4th edition thompson manore vaughan chapter 04
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Chapter 4 Lecture
Chapter 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?
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Glucose
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The most abundant carbohydrate
Produced by plants through photosynthesis
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What Are Carbohydrates?
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Simple carbohydrates
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Contain one or two molecules
Commonly referred to as sugars
Monosaccharides contain one molecule
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Referred to as sugars
Glucose, fructose, galactose, and ribose
Disaccharides contain two molecules
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Lactose, maltose, and sucrose
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What Are Carbohydrates?
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Complex carbohydrates
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Oligosaccharides contain 3 to 10 monosaccharides
Most polysaccharides consist of hundreds to thousands of glucose molecules
Starch, glycogen, most fibers
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© 2017 Pearson Education, Inc.
Complex Carbohydrates
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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
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Sources: grains, legumes, fruits, vegetables
Complex Carbohydrates
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Glycogen
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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
Complex Carbohydrates
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Fiber
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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 that have known health benefits
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Total fiber = Dietary fiber + Functional fiber
Complex Carbohydrates
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Soluble fibers
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Dissolve in water; viscous and gel-forming
Fermentable, digested by intestinal bacteria
Regular consumption can reduce risk for cardiovascular disease and type 2
diabetes
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Examples: pectin, gum, mucilage
Found in citrus fruits, berries, oats, beans
Complex Carbohydrates
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Insoluble fibers
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Do not dissolve in water, nonviscous
Cannot be fermented by bacteria in the colon
Promote regular bowel movements, alleviate constipation, and reduce
diverticulosis
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Examples: lignins, cellulose, hemicelluloses
Good sources: whole grains, seeds, legumes, fruits, and vegetables
Carbohydrate Digestion
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Digestion breaks down most carbohydrates into monosaccharides
Salivary amylase (amyl- starch, -ase an enzyme)
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Enzyme that begins starch digestion in the mouth
Breaks starches down to maltose
Carbohydrate digestion does not occur in the stomach
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Stomach acids inactivate salivary amylase
Carbohydrate Digestion
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Majority of carbohydrate digestion occurs in the small intestine
Pancreatic amylase
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Enzyme produced in the pancreas and secreted into the small intestine
Digests remaining starch to maltose
Carbohydrate Digestion
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Additional enzymes in the microvilli digest disaccharides to monosaccharides
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Maltase
Sucrase
Lactase
Monosaccharides are absorbed into the mucosal cells lining the small intestine and
then enter the bloodstream
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Carbohydrate Digestion
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Transport
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Glucose and galactose absorbed via active transport
Fructose absorbed via facilitated diffusion (slower absorption)
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Carbohydrate Digestion
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Fructose and galactose are converted to glucose in the liver
Glucose in the bloodstream can provide immediate energy
Excess glucose is stored as glycogen in the liver and muscles
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Liver glycogen used to maintain blood glucose and support brain, spinal cord
and red blood cells
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Muscle glycogen provides energy to muscles during exercise
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Carbohydrate Digestion
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We do not have the enzymes necessary to digest fiber
Bacteria in the large intestine can ferment some undigested carbohydrates
Fermentation produces gases and short-chain fatty acids
Fiber remaining in the colon adds bulk to stool and is excreted in feces
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Hormones Regulate Blood Glucose Levels
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Blood glucose regulated within a narrow range
Hormones assist to maintain blood glucose levels:
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Insulin
Glucagon
Epinephrine
Norepinephrine
Cortisol
Growth hormone
Blood Glucose Regulation: Insulin
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Secreted by beta cells of the pancreas
Stimulates glucose transporters (carrier proteins) to help move glucose from the
blood across the cell membrane
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Stimulates the liver and muscle cells to take up glucose and store it as glycogen
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