CHAPTER 5

Carbohydrates

  1. Structures and Function
  2. Background
  3. Composed of carbon, hydrogen, and oxygen in a 1:2:1 ratio often written as (CH2O)n where n=the number of times the ratio is repeated. For example, if n=6, then (CH2O)6 = C6H12O6.
  4. Provides main source of fuel for cells (at 4 kcal/gram)
  5. Divided into two main classes
  6. Simple sugars such as monosaccharides, disaccharides, and oligosaccharides.
  7. Complex sugars such as polysaccharides.
  8. MONOSACCHARIDES (glucose, fructose, and galactose are isomers of each other)
  9. Glucose is also called dextrose or blood sugar.
  10. Acts as the principle monosaccharide that acts as a building block form any other carbohydrates.
  11. In the body it forms a six carbon (hexose) ring structure.
  12. Fructose is also called levulose.
  13. Composed of a six carbon ring structure.
  14. Commonly found in fruit, honey, and corn syrup and is used in soft drinks and frozen desserts.
  15. Accounts for 8 to 10% of our energy intake.
  16. After absorption by the small intestine, fructose is converted into glucose by the liver.
  17. If fructose consumption is high, it is also converted into glycogen, lactic acid, or fat.
  18. Galactose
  19. Has a six carbon ring structure.
  20. This is not usually found in nature but exists mostly as a unit of the disaccharide lactose which is found easily in nature.
  21. Easily converted into glucose in the liver or can be stored as glycogen.
  22. Ribose sugar
  23. Has a five carbon ring and is not an isomer of glucose.
  24. Used as part of the RNA strand, which is an important part of a cell’s genetic material?
  25. DISACCHARIDES
  26. Maltose
  27. Formed by condensation synthesis reaction between two glucose molecules.
  28. The glucose molecules form an alpha bond between them which is easily broken during digestion.
  29. Maltose is commonly used in the production of many alcohols by fermentation (recall anaerobic cellular respiration).
  30. Sucrose
  31. Formed by a glucose and a fructose attached by an alpha bond during condensation synthesis.
  32. Major sources are table sugar and in plants such as sugar cane and maple syrup.
  33. Lactose
  34. Formed by the addition of glucose and a galactose by beta bonds during condensation synthesis reaction.
  35. Beta bonds are more difficult to digest hence the large portion of the population that is lactose intolerant.
  36. Primary sugar found in milk and milk products.
  37. OLIGOSACCHARIDES
  38. Raffinose and stachyose are common examples
  39. These are found in beans
  40. Because they contain large numbers of beta bonds they are difficult to digest by digestive enzymes.
  41. Instead bacteria in the large intestine break apart these oligosaccharides, producing gas and other byproducts.
  42. POLYSACCHARIDES
  43. Starch
  44. Two types of plant starch=Amylose and amylopectin.
  45. Long chains of a 1000 or more glucose molecule.
  46. Amylose is a straight chain polymer while amylopectin is a branched polymer.
  47. Food sources include potatoes, breads, pasta, and rice.
  48. Amylopectin raises blood sugar levels quicker because of the branched configuration which enables amylase to have more points of attack.
  49. Glycogen
  50. Storage form of glucose in the human body.
  51. Long branched chains of glucose that are more complex than amylopectin.
  52. Even more digestible because of its branched structure.
  53. Fiber
  54. Dietary fibers are chemically composed of non-starch polysaccharides:
  55. Cellulose and hemicellulose which is found in wheat, rye, rice, and vegetables (insoluble).
  56. Pectin, gums, and mucilage which is found in citrus fruits, oat products and beans (soluble).
  57. Dietary fibers are also composed of a non-carbohydrate material called lignin.
  58. All dietary fibers come from plants and are not digested in the stomach.
  59. The water soluble fibers however are metabolized by bacteria in the intestines.
  1. Carbohydrate Digestion and Absorption
  2. Begins in the mouth
  3. Salivary amylase breaks some starches into maltose during mastication.
  4. Continues digestion through the esophagus but stops in the stomach because the high acid environment inactivates salivary enzymes.
  5. Pancreatic amylases located in the small intestine further break down starches into maltose.
  6. Enzymes in the wall of the small intestine break down disaccharides into monosaccharides:
  7. Maltose is acted on by maltase.
  8. Sucrose is acted on by sucrase.
  9. Lactose is acted on by lactase.
  10. Glucose and galactose are actively absorbed in the small intestine (see figure 5-6).
  11. Fructose is absorbed through facilitated diffusion which although requires a carrier does not require energy.
  12. Single sugars are then transported via the portal vein that goes directly to the liver where the liver utilizes several options:
  13. Transforms the monosaccharides into glucose and releases it directly into the bloodstream for use.
  14. Produces glycogen for short term storage.
  15. Produces fat for long term storage.
  1. Functions of Glucose and other sugars
  2. Yields Energy (recall 4 kcal/g)
  3. Spares protein from use as an energy source (i.e. no gluconeogenesis)
  4. Preventing Ketosis
  5. Adequate intake of carbohydrates is necessary for the complete metabolism of fats to carbon dioxide and water.
  6. If low carbohydrate intake leads to incomplete breakdown of fatty acids, ketone bodies form resulting in ketosis.
  7. The excess ketone bodies spill over into the urine and begin to draw sodium and potassium ions as well.
  8. This results in an ion imbalance and acidic blood that can lead to coma and death if not treated.
  9. Imparting flavor and sweetness to foods (in order of sweetness from high to low= fructose, sucrose, glucose, maltose, lactose).
  10. Functions of Dietary Fiber
  11. Dietary fiber
  12. Adds mass and water to feces
  13. Feces then becomes larger and softer and easier to eliminate
  14. Less pressure is therefore needed to expel stool.
  15. Little or no fiber in the diet can cause constipation.
  16. More pressure is needed to defecate.
  17. May cause diverticula and hemorrhoids.
  18. Diverticulosis=no symptoms
  19. Diverticulitis=pouches fill with food stuffs and eventually become inflamed.
  20. Aids in weight control because it fills you up but yields little energy.
  21. Appear to be linked to reduction of colon cancer.
  22. Higher intakes of fiber are correlated with lowering cholesterol and bile acid absorption.
  1. Recommended Carbohydrate Intakes
  2. RDA is 130 grams of carbohydrate for adults with20-35 grams of this being dietary fiber and no more than 10% of total kcal/day from sugars.
  3. Problems associated with high sugar intakes:
  4. May not contribute the right proportion of energy to the diet.
  5. Excessive/High fiber diet can produce phytobezors=pellets of dietary fiber found in the stomach.
  6. May lower nutritional value of a diet by contributing to the lack of consuming other energy yielding nutrients.
  7. May cause dental caries=cavities.
  8. Diets may contain too many carbohydrates with high glycemic indexes:
  9. Glycemic index=the blood glucose response of a given food compared to a standard (such as glucose or white bread).
  10. Influenced by starch structure, fiber content, food processing, physical structure, and macronutrients in the meal (such as fat).
  11. Glycemic load=the amount of carbohydrate in a food multiplied by the glycemic index of that carbohydrate.
  12. Lactose intolerance may occur.
  1. Food Sweeteners
  2. Nutritive Sweeteners (can be metabolized to yield energy)
  3. Sugars (mono and disaccharides)
  4. 12-48 pounds are consumed per year per person.
  5. Fructose in the form of high-fructose corn syrup is predominantly used in soft drinks, candies, jam, jelly, and desserts.
  6. Others include: brown sugar, turbinado sugar, honey, and maple syrup.
  7. Sugar alcohols (sorbitol, mannitol, xylitol)
  8. Contribute energy (3 kcal/g)
  9. Broken down more slowly than simple sugars
  10. Sorbitol and xylitol are used in sugarless gum, breath mints, and candy.
  11. These sugar alcohols aren’t readily metabolized by bacteria so they do not contribute and can reduce the development of dental caries.
  12. Alternative Sweeteners (provide no energy)
  13. Saccharine
  14. Produced in 1879
  15. 300 times sweeter than sucrose
  16. Represents half of the alternative sweetener market in the United States.
  17. Once associated with bladder cancer.
  18. Aspartame
  19. Trade names are “NutraSweet” and “Equal”.
  20. 180 to 200 times sweeter than sucrose.
  21. Because it is so sweet so little is used in foods that the energy content of 4 kcal/g is negligible.
  22. Easily damaged by high heat.
  23. Cannot be used by persons with PKU.
  24. Neotame
  25. Recently approved by FDA for general-purpose, non-nutritive sweetener.
  26. 7000 to 13000 times sweeter than sucrose.
  27. Heat stable.
  28. Can be used by any person including children, pregnant women and people with diabetes.
  29. Acesulfame-Potassium (Sunett)
  30. 200 times sweeter than sugar and calorie free.
  31. Approved in 1988 for general purpose sweetener for foods.
  32. Sucralose (Splenda)
  33. 600 times sweeter than sucrose.
  34. Approved in 1998 for use in baked goods, nonalcoholic beverages, chewing gum, frozen dairy desserts, fruit juices, and gelatins.
  35. Non-nutritive
  36. Widely used in Canada.
  1. PLEASE READ THE NUTRITION PERSPECTIVE: When Blood Glucose Regulation Fails (page 171-176).

Chapter Objectives

After reading chapter five - A Student should be able to...

  1. List the three classes of simple carbohydrates and the three complex carbohydrates
  2. List the types of monosaccharides, disaccharides, and oligosaccharides and discuss their dietary importance (if any)
  3. Describe the structural and functional difference between glycogen, starch, and fiber
  4. Discuss the process of carbohydrate digestion
  5. Discuss what is the recommended intake of carbohydrates and what problems are associated with high intakes of carbohydrates
  6. Describe the difference between a nutritive sweetener and an artificial sweetener
  7. List the various types of nutritive and artificial sweeteners and discuss their dietary importance
  8. Discuss the process of glucose regulation in the human body – include in the discussion the hormones and organs involved
  9. Discuss how carbohydrate metabolism in relationship to diabetes mellitus