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

© 2011 Pearson Education, Inc.

4Carbohydrates: Plant-Derived

Energy Nutrients


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



• Glucose• The most abundant carbohydrate• Produced by plants through photosynthesis



• 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



• Complex carbohydrates• Oligosaccharides contain 3 to 10

monosaccharides • Most polysaccharides consist of hundreds to

thousands of glucose molecules • Starch, glycogen, most fibers


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



• 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



• 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



• 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



• 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


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




• 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



• 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



• 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



• 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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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



• 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)



• 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


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


Alternative Sweeteners

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


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


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


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


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


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


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

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

Documents