Unformatted text preview: Chapter 4 – The Carbohydrates: Sugars, Starches, and Fibers Learning Objectives After completing Chapter 4, the student will be able to: 1. Classify carbohydrates as mono‐, di‐ or polysaccharides. 2. Explain the two major types of dietary fibers and their uses in the body. 3. Trace the digestion and absorption of carbohydrates. 4. List the enzymes used in the digestion of carbohydrates. 5. Describe lactose intolerance symptoms, prevalence and treatment. 6. List the major uses of glucose in the body. 7. Discuss the hormones used by the body to maintain normal blood glucose concentration. 8. List the major types of diabetes. 9. Explain glycemic load and the pros and cons of its use in meal planning. 10. Identify the major sources of added sugars in the diet. 11. Describe the effect of added sugars on dental health. 12. Identify the health benefits of a diet rich in starch and fiber. 13. Identify the Daily Value and DRI for fiber and methods to increase fiber in the diet. 14. List the common alternatives to sugar in the diet. I. The Chemist’s View of Carbohydrates Carbohydrates are made of carbon, hydrogen and oxygen atoms. These atoms form chemical bonds that follow the laws of nature. II. The Simple Carbohydrates The six simple carbohydrates include the monosaccharides (glucose, fructose and galactose) and the disaccharides (sucrose, maltose and lactose). Monosaccharides are similar in numbers and kinds of atoms but differ in their arrangement and sweetness. Disaccharides are formed by condensation reactions and are broken down by hydrolysis reactions. A. Monosaccharrides are single sugars (most are hexoses). 1. Glucose serves as the essential energy source, and is commonly known as blood sugar or dextrose. 2. Fructose is the sweetest, occurs naturally in honey and fruits, and is added to many foods in the form of high‐fructose corn syrup. 3. Galactose rarely occurs naturally as a single sugar. B. Disaccharides are pairs of monosaccharides, one of which is always glucose 1. Condensation reactions link monosaccharides together. 2. Hydrolysis reactions split molecules and commonly occur during digestion. 3. Maltose consists of two glucose units. It is produced during the germination of seeds and fermentation. 4. Sucrose is fructose and glucose combined. It is refined from sugarcane and sugar beets, tastes sweet, and is readily available. 5. Lactose is galactose and glucose combined. It is found in milk and milk products. III. The Complex Carbohydrates are few (oligosaccharides) or many (polysaccharides) glucose units bound/linked together in straight or branched chains. A. Glycogen 1. Storage form of glucose in the body HUN2201‐ Carbohydrate Study Outline‐ Chapter 4 2 2. Provides a rapid release of energy when needed B. Starches 1. Storage form of glucose in plants 2. Found in grains, tubers, and legumes C. Dietary fibers provide structure in plants, are very diverse, and cannot be broken down by human enzymes. 1. Soluble fibers are viscous and can be digested by intestinal bacteria (this property is also known as fermentability). These fibers are found in fruits and vegetables. 2. Insoluble fibers are nonviscous and are not digested by intestinal bacteria. These fibers are found in grains and vegetables. 3. Fiber Sources a. Dietary fibers are found in plant foods. b. Functional fibers are health‐benefiting fibers that are added to foods or supplements. c. Total fiber considers both dietary and functional fibers. 4. Resistant starches escape digestion and are found in legumes, raw potatoes and unripe bananas. 5. Phytic acid or phytate has a close association with fiber and binds some minerals. IV. Digestion and Absorption of Carbohydrates The body breaks down starches and sugars to short glucose chains called dextrins. They are then broken down into monosaccharides and the glucose is used for energy. Fiber slows glucose absorption and regulates the passage of food through the GI tract. A. Carbohydrate Digestion 1. In the mouth, the salivary enzyme amylase begins to hydrolyze starch into short polysaccharides and maltose. 2. In the stomach, acid continues to hydrolyze starch while fiber delays gastric emptying and provides a feeling of fullness (satiety). 3. In the small intestine, pancreatic amylase among other enzymes (maltase, sucrase, and lactase) hydrolyzes starches to disaccharides and monosaccharides. 4. In the large intestine, fibers remain and attract water, soften stools and ferment. B. Carbohydrate Absorption 1. Primarily takes place in the small intestine 2. Glucose and galactose are absorbed by active transport. 3. Fructose is absorbed by facilitated diffusion. C. Lactose Intolerance 1. Symptoms include bloating, abdominal discomfort, and diarrhea. 2. Causes include lactase deficiency due to a natural decrease that occurs with aging or damaged intestinal villi. 3. Prevalence a. Lowest in Scandinavians and northern Europeans b. Highest in Southeast Asians and native North Americans 4. Dietary Changes a. Increase consumption of milk products gradually. b. Mix dairy with other foods. c. Spread dairy intake throughout the day. d. Use of acidophilus milk, yogurt, and kefir (fermented products) e. Use of enzymes f. Individualization of diets g. Must be careful that vitamin and mineral deficiencies do not develop V. Glucose in the Body HUN2201‐ Carbohydrate Study Outline‐ Chapter 4 3 Through the study of sugars, known as glycobiology, researchers are uncovering many roles that glucose has in the body. These include glucose’s interaction with protein, lipid, and cancer cells. A. A Preview of Carbohydrate Metabolism 1. The body stores glucose as glycogen in liver and muscle cells. 2. The body uses glucose for energy if glycogen stores are available. 3. If glycogen stores are depleted, the body makes glucose from protein. a. Gluconeogenesis is the conversion of protein to glucose. b. Protein‐sparing action is having adequate carbohydrate in the diet to prevent the breakdown of protein for energy. 4. Making ketone bodies from fat fragments a. The accumulation of ketone bodies in the blood is called ketosis. b. Ketosis upsets the acid‐base balance in the body. 5. The body can use glucose to make body fat when carbohydrates are consumed excessively. B. The Constancy of Blood Glucose 1. Maintaining Glucose Homeostasis a. Low blood glucose may cause dizziness and weakness. b. High blood glucose may cause fatigue. c. Extreme fluctuations can be fatal. 2. The Regulating Hormones a. Insulin moves glucose into the cells and helps to lower blood sugar levels. b. Glucagon brings glucose out of storage and raises blood sugar levels. c. Epinephrine acts quickly to bring glucose out of storage during times of stress. 3. Balance glucose within the normal range by eating balanced meals regularly with adequate complex carbohydrates. 4. Blood glucose can fall outside the normal range with hypoglycemia or diabetes. 5. Diabetes a. Type 1 diabetes is the less common type with no insulin produced by the body. b. Type 2 diabetes is the more common type where fat cells resist insulin. c. Prediabetes is blood glucose that is higher than normal but below the diagnosis of diabetes. 6. Hypoglycemia is low blood glucose and can often be controlled by dietary changes. 7. Glycemic response is how quickly the blood glucose rises and elicits an insulin response. a. Glycemic index classifies foods according to their potential for raising blood glucose. b. Glycemic load refers to a food’s glycemic index and the amount of carbohydrate the food contains. c. The benefit of the glycemic index is controversial. VI. Health Effects and Recommended Intakes of Sugars Sugar poses no major health problem except dental caries. Excessive intakes may displace nutrients and contribute to obesity. Consuming foods with added sugars should be limited. Naturally occurring sugars from fruits, vegetables and milk are acceptable sources. A. Health Effects of Sugars – Foods with added sugars have sugars listed as a first ingredient. Sugars may include brown sugar, confectioners’ sugar, corn sweeteners, corn syrup, dextrose, granulated sugar, high‐fructose corn syrup, honey, invert sugar, levulose, maple sugar, molasses, raw sugar, turbinado sugar and white sugar. 1. Nutrient deficiencies may develop from the intake of empty kcalories. a. Just because a substance is natural does not mean it is nutritious. (Example: honey) 2. Dental caries may be caused by bacteria residing in dental plaque and the length of time sugars have contact with the teeth. HUN2201‐ Carbohydrate Study Outline‐ Chapter 4 4 B. Controversies Surrounding Sugars 1. Excessive sugar intake can contribute to the development of body fat. 2. Sugar may be able to alter blood lipid levels and contribute to heart disease in some. 3. There is no scientific evidence that sugar causes misbehavior in children and criminal behavior in adults. 4. There is a theory that sugar increases serotonin levels, which can lead to cravings and addictions. C. Recommended Intakes of Sugars 1. The USDA Food Guide states that added sugars can be included in the diet as part of discretionary kcalories. 2. Dietary Guidelines state to limit intake of foods and beverages that are high in added sugars. 3. DRI suggest added sugars should contribute no more than 25% of a day’s total energy intake. VII. Health Effects and Recommended Intakes of Starch and Fibers A diet rich in complex carbohydrate may help to control body weight and prevent heart disease, cancer, diabetes and GI disorders. It is recommended that people consume 45‐65% of the daily total energy intake from carbohydrate. A. Health Effects of Starch and Fibers 1. May be some protection from heart disease and stroke a. Soluble fibers bind with bile and thereby lower blood cholesterol levels. b. Fiber may also displace fat in the diet. 2. Reduce the risk of type 2 diabetes by decreasing glucose absorption 3. Enhance the health of the GI tract which can then block the absorption of unwanted particles 4. May protect against colon cancer by removing potential cancer‐causing agents from the body 5. Promoteweight control because complex carbohydrates provide less fat and added sugar. 6. Harmful effects of excessive fiber intake a. Displaces energy and nutrient‐dense foods b. Abdominal discomfort and distention c. May interfere with nutrient absorption B. Recommended Intakes of Starch and Fibers 1. RDA for carbohydrate is 130 g per day, or 45‐65% of energy intake. 2. Daily Values is 300 grams per day. 3. Dietary Guidelines encourage a variety of whole grains, vegetables, fruits and legumes daily. 4. Healthy People 2010 recommends six servings of grains and five servings of fruits and vegetables. 5. Fiber a. FDA recommends 25 grams for a 2,000‐kcalorie diet. b. DRI at 14 g per 1000 kcalorie intake (28 grams for a 2,000 kcalorie diet) c. American Dietetic Association recommends 20‐35 g per day. d. World Health Organization suggests no more than 40 g per day. C. From Guidelines to Groceries 1. Grains – encourage whole grains 2. Vegetables – starchy and nonstarchy vegetables differ in carbohydrate content 3. Fruits – vary in water, fiber and sugar content 4. Milks and Milk Products – contain carbohydrate; cheese is low 5. Meat and Meat Alternates – meats are low but nuts and legumes have some carbohydrate 6. Food labels list grams of carbohydrate, fiber and sugar; starch grams can be calculated. VIII. Highlight: Alternatives to Sugar People may find it pleasing to replace sugar with artificial sweeteners or sugar replacers. This may help to control kcalories and limit sugar. HUN2201‐ Carbohydrate Study Outline‐ Chapter 4 5 A. Artificial Sweeteners (also called nonnutritive sweeteners) 1. Saccharin a. Used primarily in soft drinks and as a tabletop sweetener b. Rapidly excreted in the urine c. Does not accumulate in the body d. Has been removed from list of cancer‐causing substances 2. Aspartame a. General purpose sweetener b. Warning about phenylalanine for those with PKU c. Controversial finding that aspartame may have caused cancer in rats d. Excessive intake should be avoided by those with epilepsy. 3. Acesulfame‐K (acesulfame potassium) a. Research confirms safety 4. Sucralose a. Made from sugar b. Passes through digestive tract 5. Neotame a. Most recent on the market b. Very sweet c. Phenylalanine not an issue 6. Tagatose a. Used for foods and beverages b. Provides less kcalories than sugar c. High doses can cause flatulence and loose stools. 7. Alitame and Cyclamate a. Pending FDA approval b. Approved in other countries 8. Acceptable Daily Intake (ADI) is the level of consumption, maintained every day and still safe by a wide margin. Moderation and variety are still recommended. 9. Artificial Sweeteners and Weight Control a. Much research still being done b. Using artificial sweeteners will not automatically reduce energy intake. B. Stevia – An Herbal Alternative 1. Lacks research 2. Classified as a dietary supplement 3. Not required to have testing and FDA approval C. Sugar Replacers 1. Also called nutritive sweeteners, sugar alcohols, and polyols 2. Maltitol, mannitol, sorbitol, xylitol, isomalt, and lactitol 3. Absorbed more slowly and metabolized differently in the body 4. Low glycemic response 5. Side effects include GI discomfort HUN2201‐ Carbohydrate Study Outline‐ Chapter 4 6 Q & As for Discussion 1. Which carbohydrates are described as simple and which are complex? Simple—monosaccharides (glucose, fructose, galactose), disaccharides (sucrose, lactose, maltose). Complex—
glycogen and starch. 2. Describe the structure of a monosaccharide and name the three monosaccharides important in nutrition. Name the three disaccharides commonly found in foods and their component monosaccharides. In what foods are these sugars found? A monosaccharide is a carbohydrate with a general structure of a ring composed of carbon, hydrogen and oxygen atoms. Monosaccharides important in nutrition are glucose, fructose, and galactose; disac.0charides important in nutrition are sucrose (fructose + glucose), lactose (galactose + glucose), and maltose (2 glucoses). Nearly all plant foods contain glucose, and most plants (especially fruits and saps) contain fructose. Galactose is not found as such in foods. Sucrose occurs in many fruits and some vegetables and grains. Lactose is found in milk, and maltose is found in seeds. 3. What happens in a condensation reaction? In a hydrolysis reaction? Condensation combines two reactants to yield one product with the removal of water; hydrolysis splits one reactant into two products with the addition of water. 4. Describe the structure of polysaccharides and name the ones important in nutrition. How are starch and glycogen similar and how do they differ? How do the fibers differ from the other polysaccharides? Polysaccharides are composed of many monosaccharides strung together. Important in nutrition are: glycogen, starch and the fibers. Starch and glycogen are similar in that they are both composed of glucose; they differ in the way their glucose units are linked together. Glycogen consists of many glucose molecules linked together in highly branched chains, while starch consists of many glucose molecules linked side by side. Fibers are different in that the bonds between their monosaccharides cannot be broken by human enzymes. 5. Describe carbohydrate digestion and absorption. What role does fiber play in the process? Salivary amylase enzymes in the mouth partially break down some of the starch before it reaches the intestine, pancreatic enzymes digest the starch to disaccharides in small intestine, disaccharidase enzymes on surface of intestinal wall cells split disaccharides to monosaccharides, monosaccharides enter capillary, capillary delivers monosaccharides to liver, liver converts galactose and fructose to glucose. In the mouth, fiber slows the process of eating and stimulates the flow of saliva; in the stomach, fiber delays gastric emptying; in the small intestine, it delays absorption of carbohydrates and fats, and can bind with minerals; in the large intestine, it attracts water that softens the stools. 6. What are the possible fates of glucose in the body? What is the protein‐sparing action of carbohydrate? It can be stored as glycogen; it can be used for energy; it can be converted to fat, when carbohydrate is available. Glucose can be used for energy, leaving protein available for its special functions. 7. How does the body maintain blood glucose concentrations? What happens when blood glucose rises too high or falls too low? Hormones are secreted in response to fluctuations in blood glucose. When blood glucose is too high, the pancreas releases insulin, resulting in the storage of glucose in the cells; when blood glucose is too low, the pancreas releases glucagon, resulting in the release of glucose into the blood. HUN2201‐ Carbohydrate Study Outline‐ Chapter 4 7 8. What are the health effects of sugars? What are the dietary recommendations regarding concentrated sugar intakes? Excess sugar can cause malnutrition if sugar displaces needed nutrients from the diet. It can contribute to obesity. Excess sugar can contribute to elevated blood lipids, and it can cause dental caries. The DRI recommend that no more than 25% of kcal intake should come from added sugars, whereas the World Health Organization suggests they should not exceed 10% of kcal intake. 9. What are the health effects of starches and fibers? What are the dietary recommendations regarding these complex carbohydrates? They protect against heart disease, colon cancer, and diabetes, assist in weight control, improve large intestine function and health, lower blood cholesterol levels, and slow the rate of glucose absorption. 45 to 65% of total kcalories should come from carbohydrate; mostly from starch, some from fruits, vegetables, and milk. The DRI recommendation for fiber is 14 grams of fiber per 1000 kcalories consumed. 10. What foods provide starches and fibers? Fruits, vegetables, legumes, whole‐grain breads and cereals. HUN2201‐ Carbohydrate Study Outline‐ Chapter 4 8 Worksheet: Check Your Carbohydrate Choices Most of the energy we receive from foods comes from carbohydrates. Healthy choices provide complex carbohydrates or naturally occurring simple carbohydrates, rich in water‐soluble vitamins and dietary fiber. A diet that is consistently low in dietary fiber and high in added sugar can lead to health problems. Look at these examples of related foods and identify which are most similar to your food choices. High in fiber/ Intermediate
Low in fiber/ low in added sugar high in added sugar Apple with peel Applesauce, sweetened
Fruit drink, 10% apple juice Brown rice Cream of rice cereal
Rice crispy treat Pumpernickel bread Bagel, plain
Danish pastry Baked sweet potato Candied sweet potato casserole
Sweet potato pie Corn on the cob Creamed corn
Frosted corn flakes Oatmeal Granola
Granola breakfast bar 1.
Do you select whole‐grain products and fresh fruits and vegetables regularly? 2.
Do you choose foods that increase your intake of fiber and limit your intake of sugars? HUN2201‐ Carbohydrate Study Outline‐ Chapter 4 9 Worksheet: Chapter 4 Crossword Puzzle 1
3 5 6 7 8 9 10 Across 2. an older name for glucose 7. the monosaccharides important in nutrition; they have the formula C6H12O6 8. an animal polysaccharide composed of glucose; it is made and stored in the liver and muscles as a storage form of glucose 9. a carbohydrate of the general formula CnH2nOn that consists of a single ring 10. an older name for fructose Down 1. a monosaccharide; sometimes known as fruit sugar or levulose 3. a monosaccharide; sometimes known as blood sugar or dextrose 4. a disaccharide composed of glucose and fructose; commonly known as table sugar, beet sugar, or cane sugar 5. a disaccharide composed of glucose and galactose; commonly known as milk sugar HUN2201‐ Carbohydrate Study Outline‐ Chapter 4 1 0 Handout: Characteristics of Fiber Dietary fibers are classified according to a number of characteristics, including their solubility in water and whether they are a polysaccharide. These differences influence their physiological effect on the body. Water Soluble Water Insoluble Polysaccharides Gums Cellulose Hemicellulosea Hemicellulosea Mucilages Pectins Nonpolysaccharides Lignins Food Sources Vegetables Fruits Oats Wheat Barley Grains Legumes Health Effects Softened stools Lower blood cholesterol Lower rate of glucose Acceleration of intestinal transit time absorption a Some hemicelluloses are water soluble and others are water insoluble. HUN2201‐ Carbohydrate Study Outline‐ Chapter 4 1 1 Handout: Fibers Each of the fibers has a different structure. Most contain monosaccharides, but differ in the types they contain and in the bonds that link the monosaccharides to each other. These differences produce diverse health effects as explained in the text. Cellulose Cellulose is the primary constituent of plant cell walls and therefore occurs naturally in all vegetables, fruits, and legumes. Cellulose can also be extracted from wood pulp or cotton and added to foods as an anticaking, thickening, and texturizing agent during processing. Like starch, cellulose is composed of glucose molecules connected in long chains. Unlike starch, however, the chains do not branch, and the bonds linking the glucose molecules together cannot be broken by human enzymes. Hemicelluloses The hemicelluloses are the main constituent of cereal fibers. They are composed of various monosaccharide backbones with branching side chains of monosaccharides. a Pectins All pectins consist of a backbone of one type of monosaccharide; some are unbranched, whereas others have side chains of various monosaccharides. b Commonly found in vegetables and fruits (especially citrus fruits and apples), pectins may be isolated and used by the food industry to thicken jelly, keep salad dressings from separating, and otherwise control texture and consistency. Pectins can perform these functions because they readily form gels in water. Gums and Mucilages When cut, a plant secretes gums from the site of the injury. Like the other fibers, gums are composed of various monosaccharides and their derivatives. Gums such as guar gum and gum arabic are used as additives by the food industry to thicken processed foods. Mucilages are similar to gums in structure; they include psyllium and carrageenan, which are added to foods as stabilizers. Lignin This nonpolysaccharide fiber has a three‐dimensional structure that gives it strength. c Because of its toughness, few of the foods that people eat contain much lignin. It occurs in the woody parts of vegetables such as carrots and the small seeds of fruits such as strawberries. ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ a In hemicelluloses, the most common backbone monosaccharides are xylose, mannose, and galactose; the common side chains are arabinose, glucuronic acid, and galactose (see Appendix C for structures). b In pectins, the backbone is usually made of galacturonic acid units. c Lignins are polymers of several dozen molecules of phenol (an alcohol), with strong internal bonds that make them impervious to digestive enzymes. HUN2201‐ Carbohydrate Study Outline‐ Chapter 4 ...
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