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  • 1. CHAPTER 3 CarbohydratesEleanor D. Schlenker Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
  • 2. The Nature of Carbohydrates Two forms of carbohydrate occurring naturally in plant foods: 1. Starches 2. Sugars Photosynthesis: Plants transform the sun’s energy into the stored fuel form of carbohydrate Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 2
  • 3. The Nature of Carbohydrates – Cont’d Starch that plants store for their own energy needs becomes a source of fuel for humans who eat those plants Carbohydrates are often referred to as “quick energy” foods They are our primary source of energy Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 3
  • 4. Dietary Importance In some countries, carbohydrate-containing foods (fruits, vegetables, grains, cereals) make up 85% of the diet Rice feeds 3 billion people in developing countries Half of the total kcalories come from carbohydrates in the typical American diet Carbohydrate foods can be easily stored for long periods of time without spoiling Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 4
  • 5. Classification of CarbohydratesCarbohydrate Name comes from the chemical nature of these molecules  Carbon, hydrogen, and oxygen with the hydrogen/oxygen ratio usually that of water— CH2O Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 5
  • 6. Classification of Carbohydrates – Cont’dCarbohydrate – cont’d Classified according to the number of basic sugar or saccharide units that make up their structure  Simple carbohydrates: monosaccharides and disaccharides  Complex carbohydrates: polysaccharides Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 6
  • 7. Classification of Carbohydrates – Cont’dMonosaccharides Three important sugars in human nutrition: 1. Glucose 2. Fructose 3. Galactose Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 7
  • 8. Classification of Carbohydrates – Cont’dMonosaccharides – cont’d Glucose  Moderately sweet sugar found naturally in only a few foods, one being corn syrup  The common body fuel oxidized by cells to provide energy  Supplied from digestion of starch and conversion of other simple sugars  Form of sugar that circulates in the blood (called dextrose in intravenous solutions) Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 8
  • 9. Classification of Carbohydrates – Cont’dMonosaccharides – cont’d Fructose  The sweetest of the simple sugars  Found in fruits and other natural substances such as honey  Fructose is absorbed less efficiently than glucose, 25 to 50 g can cause GI distress  Accounts for 10% of the total energy intake of Americans 2 years of age and older, mostly because of high-fructose corn syrup Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 9
  • 10. Classification of Carbohydrates – Cont’dMonosaccharides – cont’d Galactose  Not found free in foods  Released through the digestion of lactose (milk sugar) and then converted to glucose in the liver  This reaction is reversible: in lactation glucose is reconverted to galactose for use in milk production Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 10
  • 11. Classification of Carbohydrates – Cont’dDisaccharides Double sugars made up of two monosaccharides linked together Three of physiologic importance: 1. Sucrose 2. Lactose 3. Maltose Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 11
  • 12. Classification of Carbohydrates – Cont’dDisaccharides – cont’d Sucrose = one glucose + one fructose  Common “table sugar,” made commercially from sugar cane and sugar beets  Found naturally in some fruits and vegetables Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 12
  • 13. Classification of Carbohydrates – Cont’dDisaccharides – cont’d Lactose = one glucose + one galactose  The sugar found in milk  It is the least sweet of the disaccharides, one sixth as sweet as sucrose  Cheese contains little lactose Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 13
  • 14. Classification of Carbohydrates – Cont’dDisaccharides – cont’d Maltose = one glucose + one glucose  Arises from the breakdown of starch  Found in commercial malt products and germinating cereal grains Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 14
  • 15. Classification of Carbohydrates – Cont’dSugar Intake Most sugar is added in food preparation or processing Intake of added sugar is about 24 tsp/day in children and about 20 tsp/day in adults Nearly 37% of added sugars come from soft drinks and another 11% from fruit drinks Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 15
  • 16. Classification of Carbohydrates – Cont’dSugar Alcohols Are other forms of carbohydrate with sweetening power  Also referred to as polyols Three types that occur in nature but are also used by food industry: 1. Sorbitol 2. Mannitol 3. Xylitol Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 16
  • 17. Classification of Carbohydrates – Cont’dSugar Alcohols – cont’d Foods made with sugar alcohols may be labeled “sugar free” Only 1.6 to 2.6 kcal/g Do not require insulin for their metabolism Do not promote tooth decay High intakes can cause abdominal distress or exert a laxative effect Foods with these sweeteners are best used in moderation Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 17
  • 18. Classification of Carbohydrates – Cont’dPolysaccharides Made up of many (poly) single glucose (saccharide) units Important energy-yielding polysaccharides: 1. Starch 2. Glycogen 3. Dextrins Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 18
  • 19. Classification of Carbohydrates – Cont’dPolysaccharides – cont’d Starch  Most important energy-yielding polysaccharide  Large complex substance made of many coiled and branching chains of single glucose units and yields only glucose on complete digestion  Cooking improves flavor and makes digestion easier  Thickening and gel-like qualities Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 19
  • 20. Classification of Carbohydrates – Cont’dPolysaccharides – cont’d Resistant starch  Some starch in particular foods is not digested  Undigested starch is fermented by bacteria in the colon  Produces short-chain fatty acids, the preferred energy source of the cells lining the colon  Other health effects similar to dietary fiber Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 20
  • 21. Classification of Carbohydrates – Cont’dPolysaccharides – cont’d Glycogen  Storage form of carbohydrate in animals  Synthesized in liver cells and stored in relatively small amounts in the liver and muscle Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 21
  • 22. Classification of Carbohydrates – Cont’dPolysaccharides – cont’d Dextrins  Polysaccharide compounds formed as intermediate products in the breakdown of starch  Starch breakdown is ongoing in the process of digestion Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 22
  • 23. Classification of Carbohydrates – Cont’dOligosaccharides Small fragments of partially digested starch ranging in size from 3 to 10 glucose units They are formed through digestion and produced commercially by acid hydrolysis Used in sports drinks Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 23
  • 24. Classification of Carbohydrates – Cont’dOligosaccharides – cont’d Naturally occurring oligosaccharides formed with bonds that cannot be broken by human enzymes and remain undigested: 1. Stachyose 2. Affinose  Found in legumes such as beans, peas, and soybeans Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 24
  • 25. Importance of Complex Carbohydrates Should be the major dietary source of energy Supplies 4 kcal/g; less than fat (9 kcal/g) Carbohydrate foods also supply other important nutrients Emphasize whole grain choices Dietary Guidelines for Americans recommends limiting sugars to 8 tsp or less per day Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 25
  • 26. Functions of CarbohydratesEnergy Primary function of starches and sugars is to provide energy to cells, especially brain cells that depend on glucose Amount of carbohydrates stored in the body, although small, is an important energy reserve Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 26
  • 27. Functions of Carbohydrates – Cont’dEnergy – cont’d Glycogen stores and available blood glucose can meet the energy needs for a half day of moderate activity Carbohydrate foods must be eaten regularly and at reasonably frequent intervals to meet the body’s demand for energy Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 27
  • 28. Special Functions of Carbohydrates in Body TissuesGlycogen – Carbohydrate Storage Protect cells, especially brain cells, from depressed metabolic function and injury and support urgent muscle responses as neededProtein-Sparing Action Allows protein to be used for tissue building and repair Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 28
  • 29. Special Functions of Carbohydrates in Body Tissues – Cont’dAntiketogenic Effect Ketones are intermediate products of fat metabolism Under extreme conditions when available carbohydrates are inadequate to meet energy needs, fat is oxidized at excessive rates Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 29
  • 30. Special Functions of Carbohydrates in Body Tissues – Cont’dAntiketogenic Effect – cont’d Ketones accumulate and the result is ketoacidosis Sufficient amounts of dietary carbohydrates prevent any damaging excess of ketones Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 30
  • 31. Special Functions of Carbohydrates in Body Tissues – Cont’dHeart Action Fatty acids are the preferred fuel for the heart, but glycogen stored in cardiac muscle is an important emergency source of contractile energy Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 31
  • 32. Special Functions of Carbohydrates in Body Tissues – Cont’dCentral Nervous System Depends on carbohydrates as an energy source These tissues have very low carbohydrate reserves—enough to last only 10 to 15 minutes—so they are especially dependent on a minute-to-minute supply of glucose from the blood Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 32
  • 33. Special Functions of Carbohydrates in Body Tissues – Cont’dCentral Nervous System – cont’d Sustained hypoglycemic shock causes irreversible brain damage Glucose increases the synthesis of acetylcholine  Responsible for memory and cognitive function Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 33
  • 34. Recommended Intake of CarbohydratesRecommended Dietary Allowance (RDA) Same for all persons over 1 year of age Children, adolescents, and adults should take in a minimum of 130 g/day Will ensure a sufficient amount of glucose to supply the energy needs of the brain for 1 day Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 34
  • 35. Recommended Intake of Carbohydrates – Cont’dAcceptable Macronutrient DistributionRange (AMDR) AMDR for carbohydrates is 45% to 65% of total energy intake No more than 25% from added sugar Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 35
  • 36. Nonnutritive Sweeteners Allow persons to indulge their taste for sweets while limiting their kcalorie intake Sweeteners grouped as nutritive or nonnutritive depending on the kcalories they contain Sucrose (table sugar) and other natural sweeteners contain 4 kcal/g Nonnutritive sweeteners yield little or no energy Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 36
  • 37. Nonnutritive Sweeteners – Cont’d Six nonnutritive sweeteners have been approved by the Food and Drug Administration (FDA) for use in the United States Aspartame contains phenylalanine  Should be avoided by persons with phenylketonuria Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 37
  • 38. Carbohydrates and Oral Health Synergistic relationship between oral health and nutrition Dental caries is affected by oral hygiene, diet, and specific nutrients Fluoride strengthens tooth enamel Amounts and types of carbohydrates influence dental caries  Continuous sipping of sugar-sweetened drinks increases risk of tooth decay Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 38
  • 39. Carbohydrates and Oral Health – Cont’d Whole-grain cereals are protective against dental caries Chronic disease increases vulnerability to dental caries and tooth loss Age-related osteoporosis may result in tooth loss Xerostomia (dry mouth) accelerates tooth decay Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 39
  • 40. Imbalances in Carbohydrate IntakeHigh-Carbohydrate Diets Contain 65% to 75% carbohydrates May cause low intakes of nutrients associated with high-fat foods Lead to a rise in plasma triglycerides and a drop in high-density lipoprotein (HDL) cholesterol levels Elevated blood glucose levels increase demand for insulin Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 40
  • 41. Imbalances in Carbohydrate Intake – Cont’dLow-Carbohydrate Diets Contain less than 20% carbohydrate Unlikely to contain the minimum servings of fruits, vegetables, and whole grains recommended by MyPyramid Replace carbohydrate foods with high-fat foods and may contain as much as 46% of total kcalories as fat, adding to cardiovascular risk Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 41
  • 42. Fiber Two general categories of fiber: 1. Dietary fiber 2. Functional fiber Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 42
  • 43. Dietary Fiber Includes nondigestible carbohydrates and lignin that are intact in plant foods  Cellulose  Hemicellulose  Lignin  Pectin  Gums  Beta glucans Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 43
  • 44. Functional Fiber Nondigestible polysaccharides that have been isolated from plants or commercially produced and added to foods Term developed to indicate those fibers that were separated from plants or commercially produced as compared with those that are intact in plants and eaten in that form Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 44
  • 45. Dietary and Functional Fiber A persons total fiber intake includes both dietary fiber and functional fiber Many foods high in fiber are low to moderate in kcalories Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 45
  • 46. Health BenefitsFiber Increase in fecal mass/promotes laxation Binding of bile acids and cholesterol Positive effect on colonic microflora Slows rise in blood glucose and insulin levels Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 46
  • 47. Recommended Fiber IntakeAdequate Intake Men: 38 g/day below age 51; 30 g/day age 51 and over Women: 25 g/day below age 51; 21 g/day age 51 and overActual Intakes Are Barely Half AI Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 47
  • 48. Functional FoodsPhytochemicals Fruits, vegetables, and whole grain foods rich in carbohydrates and fiber contain phytochemicals (plant chemicals) beneficial to health Those who eat fruits, vegetables, and whole grains regularly have a lower risk of heart disease, cancer, and diabetes Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 48
  • 49. Digestion Starches and sugars must be converted to glucose Mouth: salivary ptyalin Pancreatic secretions: Amylase breaks starch to maltose Intestinal secretions: Sucrase, lactase, and maltase Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 49
  • 50. Absorption and Metabolism Glucose is absorbed by an active pumping system using sodium as a carrier Enters portal blood circulation In the liver, fructose and galactose are converted to glucose Glucose is used for immediate energy or converted to glycogen or adipose tissue Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 50