© 2011 pearson education, inc. 4 carbohydrates: plant-derived energy nutrients

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4Carbohydrates: Plant-Derived

Energy Nutrients

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What Are Carbohydrates?

• 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

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© 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 • 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

• 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

Carbohydrate Digestion

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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:• 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:• 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

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© 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

• Saccharin • Acesulfame-K• Aspartame• Sucralose• Truvia and PureVia (stevia plant)

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© 2011 Pearson Education, Inc.

Diabetes

• 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

• 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

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© 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