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Chapter 4 – The Carbohydrates: Sugars, Starches, and Fibers Learning Objectives 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. List the common alternatives to sugar in the diet. 13. Identify the health benefits of a diet rich in starch and fiber. 14. Identify the Daily Value and DRI for fiber and methods to increase fiber in the diet. 15. Discuss the role of carbohydrates in weight gain and in weight-loss diets. 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. 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. C. The Polysaccharides contain many glucose units bound/linked together in straight or branched chains. 1. Glycogen a. Storage form of glucose in the body. b. Provides a rapid release of energy when needed. 2. Starches a. Storage form of glucose in plants. b. Found in grains, tubers, and legumes. 3. Dietary fibers provide structure in plants, are very diverse, and cannot be broken down by human enzymes. a. 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. b. Insoluble fibers are nonviscous and are not digested by intestinal bacteria. These fibers are found in grains and vegetables. c. Fiber Sources 1. Dietary fibers are found in plant foods. 2. Functional fibers are health-benefiting fibers that are added to foods or supplements. 3. Total fiber considers both dietary and functional fibers. d. Resistant starches escape digestion and are found in legumes, raw potatoes, and unripe bananas. e. Phytic acid or phytate has a close association with fiber and binds some minerals. II. 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. III. Glucose in the Body 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 and lipid molecules. 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. 4. Blood glucose can fall outside the normal range with hypoglycemia or diabetes. 5. Diabetes PL V “Diabetes Prevention” 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. IV. Health Effects and Recommended Intakes of Sugars Excessive intake of sugar 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 often 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. B. 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. V. Alternative Sweeteners PL V “Diet Soda & Weight Gain” A. Artificial sweeteners (nonnutritive sweeteners) provide little if any energy. Large doses can have negative effects on health. B. Stevia is derived from an herb and considered “generally recognized as safe” by the FDA. C. Sugar alcohols do provide some energy and because of this are known as nutritive sweeteners. These sweeteners result in a low glycemic response, but can have a laxative effect. VI. 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 PL V “High-Fiber Foods: Benefits and Tips” 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. Promote weight 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. RDA for carbohydrate is 130 g per day, or 45%-65% of energy intake. 2. Daily Value is 300 grams per day. 3. Dietary Guidelines encourage a variety of whole grains, vegetables, fruits, and legumes daily. 5. Fiber IM CA 4-5 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. National Cancer Institute recommends no more than 35 grams 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. VII. Highlight: Carbs, kCalories, and Controversies Many popular diet plans recommend carbohydrate restriction. A. Carbohydrates’ kCalorie Contributions 1. Carbohydrates have been blamed for increases in obesity. 2. Epidemiological studies have found that as carbohydrate intake increases, body weight decreases. 3. Low-carbohydrate diets do cause weight loss for the first 6 months. At one year, there is no difference in weight loss for those on a high-carbohydrate diet. 4. Weight loss is due to kcalorie restriction. B. Sugars’ Share in the Problem 1. There has been an increase in the use of high-fructose corn syrup in manufactured food and soft drinks. 2. Cravings and Addictions - Carbohydrates may be sought out to increase the secretion of serotonin in the brain, which elevates the mood. There is no physiological or pharmacological addiction that occurs with carbohydrates. 3. Simple to Swallow - Consumption of beverages sweetened with high-fructose corn syrup correlates to higher kcal intakes and overweight. 4. Appetite Control - Fructose does not cause the release of insulin, preventing appetite suppression. 5. Energy Regulation - Sugar in liquid form cannot be detected, leading to higher energy consumption. C. Insulin’s Response 1. Insulin resistance is a major health concern. 2. The Glycemic Index and Body Weight a. In studies, a low-glycemic index diet did not show greater weight loss than for those following a high-glycemic diet. b. All low-kcalorie diets support weight loss. 3. The Individual’s Response to Foods - Metabolism impacts insulin response. D. In Summary 1. Restricting all carbohydrates is not recommended. 2. Carbohydrates contain disease protective nutrients. Case Study Frank P. is a 48-year-old landscape architect recently diagnosed with hypoglycemia from a glucose tolerance blood test. He is 69 inches tall and weighs 202 pounds. His usual weight is 190 pounds; he reports an unexpected weight gain of 12 pounds over the past 6 months. Frank reports he often skips breakfast or stops for a donut and coffee on his way to work in the morning. He often eats out with clients for lunch or he grabs a fruit smoothie on his way to a job site. He eats dinner at home with his wife most evenings. He reports feeling hungry, weak, and “anxious” on many occasions during his work day, which he says is relieved somewhat by eating a candy bar or other high-sugar snack food. His diet history reveals no food intolerances although he states he avoids products which contain high-fructose corn syrup and prefers to use products sweetened with sugar. 1. Which of Frank’s symptoms could be due to hypoglycemia? What dietary habits may be aggravating these symptoms? 2. What are the similarities and differences between glucose and fructose that would be important for Frank to understand when making decisions about his diet? 3. In general and based on the glycemic index of foods, what types of carbohydrate-containing foods would you recommend Frank include in his diet to reduce his symptoms? Which foods would you recommend he avoid? 4. Why is it important for Frank to choose high-fiber carbohydrate foods in his diet? 5. Explain to Frank the reasons he should eat a balanced diet at regular intervals. 6. Based on his lifestyle patterns, devise a one-day meal plan for Frank that will help alleviate his symptoms of hypoglycemia and that includes adequate protein and at least 130 grams of carbohydrates. Include 3 meals and at least 2 snacks. Answer Key 1. Weakness, anxiety, and hunger. Skipping meals, not eating at regular times, and eating highsugar, low-protein meals and snacks. 2. Glucose and fructose have the same chemical formula but vary in structure. Glucose and fructose together form sucrose (table sugar). Glucose is the primary fuel and energy source for the brain, muscles, nerves, and other body cells. Fructose is digested more slowly and causes a smaller rise in blood sugar than glucose. Foods that contain both types of sugar should be eaten in moderation. 3. Recommend legumes, low-fat milk products, whole grains, fruits, and vegetables. Avoid processed foods made from refined flour such as snack foods, breads, and refined ready-toeat cereals. 4. Carbohydrates from whole foods and grains are packaged in fiber and water and contain essential vitamins, minerals, and phytochemicals to maintain health and regulate blood glucose. 5. Eating balanced meals at regular intervals helps the body maintain a happy medium between the extremes of blood sugars. Balanced meals that provide protein and carbohydrates, including fibers and a little fat, help to slow down the digestion and absorption of carbohydrate so that glucose enters the blood more gradually. Smaller meals eaten more frequently may also help ease the symptoms of hypoglycemia. 6. Breakfast: Whole-grain bagel (60 grams CHO) with peanut butter, coffee without non-calorie sweetener. Snack: High-protein whole-grain cereal bar (30 grams CHO). Lunch: Tuna sandwich on whole-grain bread, salad with low-fat dressing, low-fat milk (45 grams CHO). Snack: Nuts