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4 Carbohydrates: Plant-Derived Energy Nutrients © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. What Are Carbohydrates? • Glucose • The most abundant carbohydrate • Produced by plants through photosynthesis © 2011 Pearson Education, Inc. © 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 © 2011 Pearson Education, Inc. © 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. © 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 © 2011 Pearson Education, Inc. © 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. © 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 © 2011 Pearson Education, Inc. © 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. © 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 © 2011 Pearson Education, Inc. © 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. © 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 © 2011 Pearson Education, Inc. © 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 © 2011 Pearson Education, Inc. © 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) © 2011 Pearson Education, Inc. © 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 © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Alternative Sweeteners • • • • • Saccharin Acesulfame-K Aspartame Sucralose Truvia and PureVia (stevia plant) © 2011 Pearson Education, Inc. © 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. © 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 © 2011 Pearson Education, Inc.