Chapter 4. Carbohydrates and Fibers PDF

4 The Carbohydrates: Sugar, Starch,...

4 The Carbohydrates: Sugar, Starch, Glycogen, and Fiber what do you think? Do carbohydrates provide only unneeded calories to the body? Why do nutrition authorities unanimously recommend Valentyn Volkov/Shutterstock.com whole grains? Are low-carbohydrate diets the best way to lose weight? Should people with diabetes eat sugar? Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Learning Objectives After completing this chapter, you should be able to accomplish the following: LO 4.1 Explain how plants synthesize carbohydrates. LO 4.6 Discuss hypoglycemia. LO 4.2 Describe the need for carbohydrates in the diet. LO 4.7 Identify foods that are rich in carbohydrates. LO 4.3 Explain how carbohydrates are converted to glucose LO 4.8 Recognize the effects of added sugars on in the human body. health. LO 4.4 Discuss the body’s use of glucose. LO 4.5 Summarize the causes, consequences, and management of diabetes. C arbohydrates are ideal nutrients to meet your body’s energy needs, to feed your brain and nervous system, to keep your digestive system fit, and, within calo- rie limits, to help fuel physical activity and keep your body lean. Digestible carbohy- drates, together with fats and protein, add bulk to foods and provide energy and other benefits for the body. Indigestible carbohydrates, which include most of the fibers in foods, yield little or no energy but provide other important benefits. All carbohydrates are not equal in terms of nutrition. This chapter invites you to learn the differences between foods containing complex carbohydrates (starch and fiber) and those made of simple carbohydrates (the sugars) and to consider the effects of both on the body. Controversy 4 goes on to explore current theories about how consumption of certain carbohydrates may affect human health. This chapter on the carbohydrates is the first of three on the energy-yielding nutri- ents. Chapter 5 deals with the fats and Chapter 6 with protein. Controversy 3 already addressed one other contributor of energy to the human diet, alcohol. carbohydrates compounds composed of A Close Look at Carbohydrates single or multiple sugars. The name means “carbon and water,” and a chemical shorthand for carbohydrate is CHO, signifying carbon (C), hydrogen (H), and oxygen (O). LO 4.1 Explain how plants synthesize carbohydrates. complex carbohydrates long chains of sugar Carbohydrates contain the sun’s radiant energy, captured in a form that living units arranged to form starch or fiber; also called things can use to drive the processes of life. Green plants make carbohydrate through polysaccharides. photosynthesis in the presence of chlorophyll and sunlight. In this process, water simple carbohydrates sugars, including (H2O) absorbed by the plant’s roots donates hydrogen and oxygen. Carbon dioxide gas both single sugar units and linked pairs of sugar units. The basic sugar unit is a molecule con- (CO2) absorbed into its leaves donates carbon and oxygen. Water and carbon dioxide taining six carbon atoms, together with oxygen combine to yield the most common of the sugars, the single sugar glucose. Scientists and hydrogen atoms. know the reaction in the minutest detail but have yet to fully reproduce it—green photosynthesis the process by which green plants are required to make it happen (see Figure 4–1). plants make carbohydrates from carbon dioxide Light energy from the sun drives the photosynthesis reaction. The light energy and water using the green pigment chlorophyll becomes the chemical energy of the bonds that hold six atoms of carbon together in to capture the sun’s energy (photo means the sugar glucose. Glucose provides energy for the work of all the cells of the stem, “light”; synthesis means “making”). roots, flowers, and fruits of the plant. For example, in the roots, far from the energy- chlorophyll the green pigment of plants giving rays of the sun, each cell draws upon some of the glucose made in the leaves, that captures energy from sunlight for use in breaks it down (to carbon dioxide and water), and uses the energy thus released to fuel photosynthesis. its own growth and water-gathering activities. sugars simple carbohydrates; that is, mol- Plants do not use all of the energy stored in their sugars, so it remains available ecules of either single sugar units or pairs of those sugar units bonded together. By common for use by the animal or human being that consumes the plant. Thus, carbohydrates usage, sugar most often refers to sucrose. form the first link in the food chain that supports all life on earth. Carbohydrate-rich glucose (GLOO-cose) a single sugar used in foods come almost exclusively from plants; milk is the only animal-derived food that both plant and animal tissues for energy; some- contains significant amounts of carbohydrate. The next few sections describe the times known as blood sugar or dextrose. forms assumed by carbohydrates: sugars, starch, glycogen, and fibers. 114 Chapter 4 The Carbohydrates: Sugar, Starch, Glycogen, and Fiber Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Figure 4–1 Carbohydrate Is Made by Photosynthesis The sun’s energy becomes part of the glucose molecule—its calories, in a sense. In the molecule of glucose on the leaf here, black dots represent the carbon atoms; bars represent the chemical bonds that contain energy. Carbon dioxide Sun Energy Energy Oxygen Chlorophyll Glucose Water KEY POINTS monosaccharides (mon-oh-SACK-ah- ▪ Through photosynthesis, plants combine carbon dioxide, water, and the sun’s rides) single sugar units (mono means “one”; energy to form glucose. saccharide means “sugar unit”). ▪ Carbohydrates are made of carbon, hydrogen, and oxygen held together by disaccharides pairs of single sugars linked energy-containing bonds: carbo means “carbon”; hydrate means “water.” together (di means “two”). fructose (FROOK-tose) a monosaccharide; Sugars sometimes known as fruit sugar (fruct means “fruit”; ose means “sugar”). Six sugar molecules are important in nutrition. Three of these are single sugars, or monosaccharides. The other three are double sugars, or disaccharides. All of galactose (ga-LACK-tose) a monosaccharide; part of the disaccharide lactose (milk sugar). their chemical names end in ose, which means “sugar.” Although they all sound alike at first, they exhibit distinct characteristics once you get to know them as individuals. high-fructose corn syrup (HFCS) a widely Figure 4–2 (p. 116) shows the relationships among the sugars. used commercial caloric sweetener made by adding enzymes to cornstarch to convert a Monosaccharides The three monosaccharides are glucose, fructose, and galac- portion of its glucose molecules into sweet- tasting fructose. tose. Fructose or fruit sugar, the intensely sweet sugar of fruit, is made by rearranging the atoms in glucose molecules. Fructose occurs naturally in fruits, in honey, and as added sugars sugars and syrups added to a part of table sugar. However, most fructose is consumed in sweet beverages, desserts, food for any purpose, such as to add sweetness or bulk or to aid in browning (baked goods). Also and other foods sweetened with high-fructose corn syrup (HFCS) or other added called carbohydrate sweeteners, they include sugars.1* Glucose and fructose are the most common monosaccharides in nature. concentrated fruit juice, glucose, fructose, high- fructose corn syrup, sucrose, and other sweet 1* Reference notes are found in Appendix F. carbohydrates. Also defined in Chapter 2. A Close Look at Carbohydrates 115 Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Figure 4–2 How Monosaccharides Join to Form Disaccharides Single sugars are monosaccharides, while pairs of sugars are disaccharides. Three types of Fructose Glucose Galactosea monosaccharides … A note on the glucose symbol: The glucose molecule is really a ring of 5 carbons and 1 oxygen plus a carbon “flag.” Carbons Oxygen For convenience, glucose is symbolized as or... join together to Sucrose Maltose Lactoseb make three types of (fructose—glucose) (glucose—glucose) (glucose—galactose) disaccharides. aGalactose does not occur in foods singly but only as part of lactose. bThe chemical bond that joins the monosaccharides of lactose differs from those of other sugars and makes lactose hard for some people to digest—lactose intolerance (see later section, p. 134). The other monosaccharide, galactose, has the same number and kind of atoms as glucose and fructose but in another arrangement. Galactose is one of two single sug- ars that are bound together to make up the sugar of milk. Galactose rarely occurs free in nature but is tied up in milk sugar until it is freed during digestion. Disaccharides The three other sugars important in nutrition are disaccharides, which are linked pairs of single sugars. The disaccharides are lactose, maltose, and sucrose. All three contain glucose. In lactose, the milk sugar just mentioned, glucose is linked to galactose. Malt sugar, or maltose, has two glucose units. Maltose appears wherever starch is being broken down. It occurs in germinating seeds and arises dur- ing the digestion of starch in the human body. The last of the six sugars, sucrose, is familiar table sugar, the product most peo- ple think of when they refer to sugar. In sucrose, fructose and glucose are bonded together. Table sugar is obtained by refining the juice from sugar beets or sugar cane, but sucrose also occurs naturally in many vegetables and fruits. It tastes sweet because it contains the sweetest of the monosaccharides, fructose. When you eat a food containing monosaccharides, you can absorb them directly into your blood. When you eat disaccharides, though, you must digest them first. Enzymes in your intestinal cells must split the disaccharides into separate monosac- charides so that they can enter the bloodstream. The blood delivers all products of digestion first to the liver, which possesses enzymes to modify nutrients, making them useful to the body. Glucose is the monosaccharide used for energy by all the body’s tissues, so the liver releases abundant glucose into the bloodstream for delivery inside the body. Galactose can be converted into glucose by the liver, adding to the body’s supply. Fructose, however, is normally used for fuel by the liver or broken down to building blocks for fat or other needed molecules. lactose a disaccharide composed of glucose Although it is true that the energy of fruits and many vegetables comes from sug- and galactose; sometimes known as milk sugar ars, this doesn’t mean that eating them is the same as eating concentrated sweets (lact means “milk”; ose means “sugar”). such as candy or drinking cola beverages. From the body’s point of view, fruits are maltose a disaccharide composed of two vastly different from purified sugars (as later sections make clear) except that both glucose units; sometimes known as malt sugar. provide glucose in abundance. sucrose (SOO-crose) a disaccharide com- KEY POINTS posed of glucose and fructose; sometimes known as table, beet, or cane sugar and, often, ▪ Glucose is the most important monosaccharide in the human body. as simply sugar. ▪ Monosaccharides can be converted by the liver to other needed molecules. 116 Chapter 4 The Carbohydrates: Sugar, Starch, Glycogen, and Fiber Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Starch In addition to occurring in sugars, the glucose in food occurs in long strands of thousands of glucose units. These are the polysaccharides (see Figure 4–3, p. 118). Starch is a polysaccharide, as are glycogen and most of the fibers. Starch is a plant’s storage form of glucose. As a plant matures, it not only pro- vides energy for its own needs but also stores energy in its seeds for the next gener- ation. For example, after a corn plant reaches its full growth and has many leaves manufacturing glucose, it links glucose together to form starch, stores packed clus- ters of starch molecules in granules, and packs the granules into its seeds. These giant starch clusters are packed side by side in the kernels of corn. For the plant, starch is useful because it is an insoluble substance that will stay with the seed in the ground and nourish it until it forms shoots with leaves that can catch the sun’s rays. Glucose, in contrast, is soluble in water and would be washed away by the rains while the seed lay in the soil. The starch of corn and other plant foods is nutri- tive for people, too, because they can digest the starch to glucose and extract the sun’s energy stored in its chemical bonds. A later section describes starch digestion in detail. KEY POINT ▪ Starch is the storage form of glucose in plants and is also nutritive for human beings. Glycogen Just as plant tissues store glucose in long chains of starch, animal liver and mus- cle tissues store glucose in long chains of glycogen. Glycogen resembles starch in that it consists of glucose molecules linked together to form chains, but its chains are longer and more highly branched (see Figure 4–3). Unlike starch, which is abundant in grains, potatoes, and other foods from plants, glycogen is nearly undetectable in meats because it breaks down rapidly when the animal is slaugh- tered. A later section describes how the human body handles its own packages of stored glucose. KEY POINT ▪ Glycogen is the storage form of glucose in animals and human beings. polysaccharides another term for complex Fibers carbohydrates; compounds composed of long strands of glucose units linked together Some of the fibers of a plant form the supporting structures of its leaves, stems, and (poly means “many”). Also called complex seeds. Other fibers play other roles; for example, they retain water and thus pro- carbohydrates. tect seeds from drying out. Like starch, most fibers are polysaccharides—chains of starch a plant polysaccharide composed of glucose. After cooking, starch is highly digestible by human beings; raw starch often resists digestion. granules small grains. Starch granules are packages of starch molecules. Various plant species make starch granules of varying shapes. glycogen (GLY-co-gen) a highly branched polysaccharide that is made and stored by liver and muscle tissues of human beings and animals as a storage form of glucose. Glycogen is not a significant food source of carbohydrate and is not counted as one of the complex carbohydrates in foods. A glycogen molecule stores tens fibers the indigestible parts of plant foods, Jon Lomberg/Science Source of thousands of glucose units largely nonstarch polysaccharides that are not nested in an easy-to-retrieve digested by human digestive enzymes, although form. In this photo, individual some are digested by resident bacteria of the glucose molecules are depicted colon. Fibers include cellulose, hemicelluloses, as black balls linked together pectins, gums, mucilages, and a few non- with white sticks. polysaccharides such as lignin. A Close Look at Carbohydrates 117 Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Figure 4–3 How Glucose Molecules Join to Form Polysaccharides Glucose Starch (unbranched) Starch (branched) Glycogen Cellulose Starch Glucose units are linked in long, occasionally Glycogen Glycogen resembles Cellulose (fiber) The bonds branched chains to make starch. Human digestive starch in that the bonds between that link glucose units together enzymes can digest these bonds, retrieving glucose. its glucose units can be broken in cellulose are different from Real glucose units are so tiny that you can’t see them, by human enzymes, but the the bonds in starch or glyco- even with the highest-power light microscope. chains of glycogen are more gen. Human enzymes cannot highly branched. digest them. sugars—but they differ from starch in that the sugar units are held together by bonds that human digestive enzymes cannot break. Most fibers therefore pass through the human body intact, without providing energy for its use. A little energy arises, however, when certain fibers encounter the colon’s bacterial colonies, which do pos- sess fiber-digesting enzymes. This digestion involves fermentation, a form of break- down that produces tiny products, mainly fat fragments, which the human colon absorbs. Many animals, such as cattle, depend heavily on their digestive system’s bac- teria to make the energy of glucose available from the abundant cellulose, a form of fiber, in their fodder. Thus, when we eat beef, we indirectly receive some of the sun’s energy that was originally stored in the fiber of the plants. Beef itself, like other ani- mal products, contains no fiber. In summary, plants combine carbon dioxide, water, and the sun’s energy to form glucose, which can be stored as the polysaccharide starch. Then animals or people eat the plants and retrieve the glucose. In the body, the liver and muscles may store the glucose as the polysaccharide glycogen, but ultimately it becomes glucose again. The glucose delivers the sun’s energy to fuel the body’s activities. In the process, glucose breaks down to the waste products carbon dioxide and water, which are excreted. Later, plants use these compounds again as raw materials to make carbohydrate. fermentation the anaerobic (without oxygen) breakdown of carbohydrates by microorganisms Fibers are plant constituents that are not digested directly by human enzymes, but that releases small organic compounds along intestinal bacteria ferment some fibers, and dietary fiber contributes to the health of with carbon dioxide and energy. the body. 118 Chapter 4 The Carbohydrates: Sugar, Starch, Glycogen, and Fiber Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  KEY POINTS ▪ Human digestive enzymes cannot break the chemical bonds of fiber. ▪ Some fiber is susceptible to fermentation by bacteria in the colon. The Need for Carbohydrates LO 4.2 Describe the need for carbohydrates in the diet. Glucose from carbohydrate is an important fuel for most body functions. Only two other nutrients provide energy to the body: protein and fats.† Protein-rich foods are usually expensive and, when used to make fuel for the body, provide no advantage over carbohydrates. Moreover, excess dietary protein has disadvantages, as Chapter 6 explains. Fats normally are not used as fuel by the brain and central nervous system—these tissues prefer glucose, and red blood cells use glucose exclu- sively. Thus, glucose is a critical energy source, and whole foods that supply carbohydrates—particularly JUPITERIMAGES/BananaStock/Alamy the fiber-rich ones—are the preferred source of glucose in the diet. Carbohydrates also play vital roles in the function- ing of body tissues. For example, sugars that dangle from protein molecules, once thought to be mere hitch- hikers, are now known to dramatically alter the shape and function of certain proteins. Such a sugar-protein complex is responsible for the slipperiness of mucus, The brain uses glucose as its primary fuel. the watery lubricant that coats and protects the body’s internal linings and membranes.‡ Sugars also bind to the outsides of cell membranes, where they facilitate cell-to-cell communication and nerve and brain cell functioning. Clearly, the body needs carbohydrates for more than just energy. If I Want to Lose Weight and Stay Healthy, Should I Avoid Carbohydrates? Carbohydrates have been wrongly accused of being the “fattening” ingredient of foods, thereby misleading millions of weight-conscious people into elimi- nating nutritious carbohydrate-rich foods from their diets. In truth, people who wish to lose fat, maintain lean tissue, and stay healthy can do no better than to attend closely to portion sizes and calorie intakes, and to design an eating plan around carbohydrate-rich fruit, legumes, vegetables, and whole grains. Lower in Calories Gram for gram, carbohydrates donate fewer calories than do dietary fats, and converting glucose into fat for storage is metabolically costly. Still, it is possible to consume enough calories of carbohydrate to exceed the need for energy, which reliably leads to weight gain. To lose weight, the whole grains grains or foods made from them that contain all the essential parts and naturally † Ethanol, the alcohol in alcoholic beverages, also supplies calories, but alcohol is toxic to body tissues. occurring nutrients of the entire grain seed ‡ Such combination molecules are known as glycoproteins. (except the inedible husk). The Need for Carbohydrates 119 Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  dieter must plan to consume fewer total calories from all foods and beverages each day. Empty Calories of Added Sugars Recommendations to choose carbohydrate- rich foods do not extend to refined added sugars. Purified, refined sugars (mostly sucrose or fructose) contain no other nutrients—no protein, vitamins, minerals, or fiber—and thus are low in nutrient density. A person choosing 400 calories of sugar in place of 400 calories of whole-grain bread loses the nutrients, phytochemicals, and fiber of the bread. You can afford to do this only if you have already met all of your nutrient needs for the day and still have calories to spend. Overuse of added sugars may have other effects as well. The Controversy section of this chapter considers evidence concerning added sugars, blood lipids, and chronic disease risks. Guidelines For health’s sake, then, most people should increase their intakes of fiber-rich whole-food sources of carbohydrates and reduce their intakes of foods high in refined white flour and added sugars. Table 4–1 presents carbohydrate rec- ommendations and guidelines from several authori- ties. This chapter’s Consumer’s Guide describes various whole-grain foods, and the Food Feature comes back to the sugars in foods. For weight loss, authorities do not recommend omitting carbohy- drates. In fact, the opposite is true. KEY POINTS ▪ The body tissues use carbohydrate for energy and other critical functions. ▪ The brain and nerve tissues prefer carbohydrate as fuel, and red blood cells can use nothing else. ▪ Intakes of refined carbohydrates should be limited. Unlike the added sugars in concentrated sweets, the iStockphoto.com/vgajic sugars in fruit are diluted with water and naturally packaged with vitamins, minerals, phytochemicals, and fiber. Why Do Nutrition Experts Recommend Fiber-Rich Foods? People who regularly eat fiber-rich fruit, legumes, vegetables, and whole grains are often reported to be healthier than those who do not, and the fiber in those foods deserves some of the credit.2 This section introduces the fibers and explores their health effects. Fibers in foods are complex and difficult to categorize. One way to group them is by whether they are soluble in water, although certain fibers possess both soluble and insoluble characteristics. Other ways are by the degree to which they form gels, add soluble fibers food components that readily viscosity, or by their susceptibility to fermentation, also imperfect systems. For conve- dissolve in water, become viscous, and often impart gummy or gel-like characteristics to nience, this chapter sorts them by solubility. foods. An example is pectin from fruit, which is used to thicken jellies. Soluble Fibers Fibers that readily dissolve in water are the soluble fibers. In viscous (VISS-cuss) having a sticky, gummy, foods, soluble fibers add a pleasing consistency, such as the pectin that puts the gel or gel-like consistency that flows relatively slowly. in jelly and the gums that make bottled salad dressings more viscous. Soluble fibers 12 0 Chapter 4 The Carbohydrates: Sugar, Starch, Glycogen, and Fiber Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Table 4–1 Recommendations for Carbohydrate Intakes 1. Total carbohydrate ▪ Strong recommendation Both adults and children should Dietary Reference Intakes (DRI) reduce the intake of added sugars to less than 10% of total ▪ At a minimum, adults and children need energy intake. 130 g/day to provide glucose to the brain. ▪ Conditional recommendationb Both children and adults should ▪ For health, most people should consume between 45 and further reduce the intake of added sugars to below 5% of total 65% of total calories from carbohydrate. energy intake. Dietary Guidelines for Americans 3. Whole grains ▪ Choose nutrient-dense grains, fruit, starchy vegetables, Dietary Guidelines for Americans legumes, and milk to meet the day’s total carbohydrate intake. ▪ A healthy eating pattern includes grains, at least half of which 2. Added sugars are whole grains. Dietary Guidelines for Americans 4. Fiber ▪ Limit intakes of added sugars to a maximum of 10% of total Dietary Reference Intakes (DRI) calories. ▪ 38 g of total fiber per day for men through age 50; 30 g for men American Heart Association 51 and older. ▪ A prudent daily upper limit is not more than 100 cal of added ▪ 25 g of total fiber per day for women through age 50; 21 g for sugars for most women or 150 cal for most men. women 51 and older. World Health Organization (WHO) ▪ Strong recommendationa Both children and adults should aim for a reduced intake of free (added) sugars throughout the lifecourse. Strong recommendations indicate that desirable effects of adherence to the recommendation outweigh undesirable consequences. The recommendation can applied in a most situations. b Conditional recommendations are made with less certainty, but with some scientific support. are naturally abundant in oats, barley, legumes, okra, and citrus fruits. In addition to food sources, extracted single soluble fiber preparations are used as medications or as food additives.† In the body, soluble fibers are best known for their ability to modulate blood glucose levels, lower blood cholesterol, and promote the health of the colon (details later on).3 In addition, products of fiber fermentation may: ▪ maintain the health of the colon in ways that oppose colon cancer, ▪ oppose allergies, ▪ reduce inflammation, and ▪ support immunity.4 Clearly, an eating pattern that supplies ample soluble fibers helps to maintain the body’s health. KEY POINTS ▪ Soluble fibers dissolve in water, form viscous gels, and are easily fermented by colonic bacteria. ▪ Soluble fibers and products of their fermentation play roles in maintaining the body’s health. Insoluble Fibers Other fibers are insoluble fibers that do not dissolve in water, do not form gels, are not viscous, and are poorly fermented. Insoluble fibers, such as cellulose, form structures such as the outer layers of whole grains (bran), the strings of celery, the hulls of seeds, and the skins of corn kernels. These fibers retain their shape and rough texture even after hours of cooking. In the body, they aid the diges- tive system by easing elimination, as described later. insoluble fibers the tough, fibrous structures † Examples are pills of the fibers psyllium or methylcellulose used to relieve constipation; inulin, a slightly sweet-tasting fiber, is added of fruits, vegetables, and grains; indigestible to increase fiber in foods and reduce calories. food components that do not dissolve in water. The Need for Carbohydrates 12 1 Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Figure 4–4 Characteristics, Sources, and Health Effects of Fibers People who eat these obtain these types with these actions and receive these foods... of fibers... in the body... probable health benefits. Viscous, soluble, often fermentable and gel-forming Brian Leatart/Photolibrary/Getty Images t Barley, oats, oat t
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BOE grains, extracted rectal cancer fibers used as food additives a Inulin,a soluble and fermentable but nonviscous fiber, is found naturally in a few vegetables, but is also purified from chicory root for use as a food additive. b Psyllium, a soluble fiber derived from seed husks, is used as a laxative and food additive. Sources: Information from J. W. McRorie, Evidence-based approach to fiber supplements and clinically meaningful health benefits, Part I, Nutrition Today 50 (2015): 82–89; J. W. McRorie, Evidence-based approach to fiber supplements and clinically meaningful health benefits, Part II, Nutrition Today 50 (2015): 90–97. Figure 4–4 shows the diverse effects of different fibers, and Figure 4–5 provides a brief guide to finding these fibers in foods (Appendix A lists the fiber contents of thou- sands of foods). Most unrefined plant foods contain a mix of fiber types. KEY POINTS ▪ Insoluble fibers do not dissolve in water; they form structural parts of plants and are less readily fermented by colonic bacteria. ▪ Insoluble fibers benefit digestive tract health. Heart Disease and Stroke Strong evidence suggests that diets rich in fruit, legumes, vegetables, and whole grains—and therefore rich in fibers and other complex carbohydrates—are protective against heart disease and stroke.5 Such diets are also generally low in saturated fat and trans fat and high in nutrients and phytochemicals—all factors associated with a lower risk of heart disease. Oatmeal was first to be identified among cholesterol-lowering foods.6 Apples, barley, carrots, and legumes are also rich in gel-forming fibers that can lower blood cholesterol. In contrast, diets high in refined grains and added sugars may push blood lipids toward elevate heart disease risk. Soluble, gel-forming fibers may lower blood cholesterol by binding bile, a diges- tive juice that contains cholesterol compounds. Bile is made by the liver and secreted into the intestine (see Chapter 3). Normally, much of bile’s cholesterol would be reab- sorbed from the intestine for reuse, but the fiber carries some of it out with the feces 12 2 Chapter 4 The Carbohydrates: Sugar, Starch, Glycogen, and Fiber Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Figure 4–5 Fiber Composition of Common Foods Key: Viscous, soluble fiber Nonviscous, insoluble fiber Fiber Grams per Serving Foodsa 1 2 3 4 5 6 7 8 9 10 Grains, 1 2 c Barley, whole-grain Oatmeal, instant Oat bran, dry Seeds, 1 tbs Psyllium seedsb Fruit, 1 med Apple Banana Blackberries, 1 2 c Nectarine Orange, grapefruit Peach Pear Plum, large Prunes, 1 4 c Legumes, 1 2 c Black beans Black-eyed peas Chickpeas (garbanzo beans) Kidney beans Lentils Lima beans Navy beans Northern beans Pinto beans Vegetables, 1 2 c Broccoli (and many other cooked vegetables) Brussels sprouts, chopped Carrots aValues are for cooked or ready-to-serve foods unless specified. bPsyllium is used as a fiber laxative and fiber-rich food additive. (Figure 4–6, p. 124). These bile compounds are needed in digestion, so the liver responds to their loss by drawing on the body’s cholesterol stocks to synthesize more. KEY POINT ▪ Foods rich in soluble fibers help control blood cholesterol. Blood Glucose Control High-fiber foods may play a role in reducing the risk of type 2 diabetes. The soluble fibers of foods such as oats and legumes help regulate blood glucose following a carbohydrate-rich meal.7 Soluble fibers delay the transit of nutrients through the digestive tract, slowing glucose absorption and preventing the glucose surge and rebound often associated with diabetes onset. In people with estab- lished diabetes, high-fiber foods can modulate blood glucose and insulin levels, thus helping to prevent medical complications. A later section comes back to insulin in diabetes. KEY POINT ▪ Foods rich in soluble fibers help to modulate blood glucose concentrations. The Need for Carbohydrates 12 3 Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Figure 4–6 One Way Fiber in Food May Lower Cholesterol in the Blood High-fiber diet: More cholesterol Low-fiber diet: More cholesterol (from bile) is reabsorbed (in bile) is carried out of the body. and returned to the bloodstream. 2 2 Gallbladder Gallbladder 1 The liver acts something like a stores bile stores bile vacuum cleaner, sucking up 1 1 cholesterol from the blood, using it to make bile, and discharging the bile Liver uses blood Liver uses blood into its storage bag, the gallbladder. cholesterol to 3 cholesterol to 3 make bile Intestine: bile make bile 2 The gallbladder empties its bile into aids digestion; Intestine: bile the intestine, where bile performs bound by fiber aids digestion necessary digestive tasks. 5 5 in colon Much of the 3 In the intestine, some of the A little cholesterol cholesterol from cholesterol in bile associates from bile reabsorbed bile reabsorbed with fiber. into the blood into the blood 4 4 4 Fiber carries cholesterol in bile out Fiber and bile of the digestive tract with the feces. Little bile excreted in excreted feces 5 The cholesterol that remains in the intestine is reabsorbed into the bloodstream. The microbiota of Digestive Tract Health Soluble and insoluble fibers, the digestive tract is along with an ample fluid intake, help the colon to func- described in Chapter 3. tion properly and maintain its health. Fermentable solu- ble fibers of whole foods are of special importance in this regard.8 Although human enzymes cannot digest these fibers, colonic bacteria read- ily ferment them, deriving sustenance that allows beneficial colonies to multiply and flourish. People who suffer occasional constipation often find relief by taking fiber sup- plements. Specially manufactured soluble fiber in supplements resists fermentation by the colon’s bacteria and remains intact in the digestive tract.* This fiber cannot nourish beneficial bacteria but swells with water, softening and giving weight to fecal matter, easing its passage from the system. Coarse insoluble fibers also relieve consti- pation by stimulating the colon lining to secrete mucus and water which enlarge and soften the stools. Large, soft stools ease the task of elimination. Pressure is then reduced in the lower constipation difficult, incomplete, or infrequent bowel movements associated with bowel (colon), making it less likely that rectal veins will swell (hemorrhoids). Fiber discomfort in passing dry, hardened feces from prevents compaction of the intestinal contents, which could obstruct the appendix the body. and permit bacteria to invade and infect it (appendicitis). In addition, many people hemorrhoids (HEM-or-oids) swollen, suffer from a weakness in the wall of the large intestine that leads portions of the wall hardened (varicose) veins in the rectum, to bulge out into pouches known as diverticula (illustrated in Figure 4–7). Ample usually caused by the pressure resulting from dietary fiber may help to reduce complications of diverticula, but, contrary to long- constipation. held beliefs, it may not prevent them from forming.9 appendicitis inflammation and/or infection of the appendix, a sac protruding from the KEY POINTS intestine. ▪ Soluble fibers are particularly valuable for maintaining intestinal colonies of diverticula (dye-ver-TIC-you-la) sacs or beneficial bacteria. pouches that balloon out of the intestinal wall, ▪ Both soluble and insoluble fibers ease elimination by enlarging and softening caused by weakening of the muscle layers that encase the intestine. The painful inflammation stools, and maintain digestive tract health. of one or more of the diverticula is known as diverticulitis. *The unfermentable manufactured fibers are methylcellulose (from wood pulp) and psyllium (from seed husk). 12 4 Chapter 4 The Carbohydrates: Sugar, Starch, Glycogen, and Fiber Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Digestive Tract Cancers Cancers of the colon and rectum claim tens of thou- Figure 4–7 sands of lives each year.10 The risk of these cancers is lower, however, among people Diverticula with higher dietary fiber intakes. A recent European study of almost a half-million adults confirmed a strong, linear inverse association between dietary fiber and can- Diverticula are abnormally cers of the colon and rectum.11 Subjects who ate the most fiber (28 or more grams per bulging pockets in the colon day) reduced their risk of colon and rectal cancer by 17 percent, compared with those wall. These pockets can entrap who ate the least. This study assessed fiber from grains, fruits, and vegetables but not feces and become painfully supplements. Fiber supplements lack the nutrients and phytochemicals of whole foods infected and inflamed, requiring that may also help to protect against cancers. hospitalization, antibiotic therapy, All plant foods—vegetables, fruits, and whole-grain products—have attributes or surgery. that may reduce the risks of colon and rectal cancers. Their fiber dilutes, binds, and rapidly removes potential cancer-causing agents from the colon. In addi- tion, small fat molecules arising from bacterial fermentation of fiber may activate cancer-destroying mechanisms and inhibit inflammation in the colon.12 Many other daily choices influence colon cancer risks, and you can read about them Diverticula in Chapter 11. KEY POINTS ▪ Adequate dietary fiber may reduce the risks of colon and rectal cancers. ▪ Plant foods supply fiber, nutrients and phytochemicals that may oppose cancers in many ways. Colon Healthy Weight Management Foods rich in fibers tend to be low in fats, added sugars, and calories and can therefore help to prevent weight gain and pro- mote weight loss by delivering less energy per bite. In addition, fibers absorb water from the digestive juices; as they swell, they create feelings of fullness, delay hunger, Table 4–2 and reduce food intake. Fermentable fibers may be especially useful for appetite con- A Quick Method for trol. The small fat molecules formed during fiber fermentation may shift the body’s Estimating Fiber Intake hormones in ways that promote feelings of fullness.13 By whatever mechanism, as populations eat more refined low-fiber foods and concentrated sweets, body fat stores To quickly estimate fiber in a day’s creep up. meals: For fiber intakes, follow the eating patterns of the Dietary Guidelines for Americans— 1. Multiply servings (1⁄2 c cut up or choose the recommended servings of whole, nutrient-dense fruit and vegetables, 1 medium piece) of any fruit or vege- make at least half the grain choices whole grains, and choose legumes several times table (excluding juice) by 1.5 g.a per week. That way, you’ll obtain all of the benefits that plant foods have to offer. Example: 5 servings of fruits and Eating a diet of highly refined foods and adding a fiber supplement is simply not vegetables × 1.5 = 7.5 g fiber the same. 2. Multiply 1⁄2 c servings of refined grains by 1.0 g. Example: 4 servings of refined grains Fiber Intakes and Excesses × 1.0 = 4.0 g fiber Few people in the United States or Canada consume sufficient fiber. The DRI intake 3. Multiply 1⁄2 c servings of whole grains recommendation for fiber is 14 grams per 1,000 calories, or 25 grams per day for by 2.5 g. most women and 38 grams for most men—almost twice the average current intake of Example: 3 servings of whole grains × about 15 (women) and 18 (men) grams.14 Fiber recommendations (inside front cover, 2.5 = 7.5 g fiber p. A) are made in terms of total fiber with no distinction between fiber types because 4. Add fiber values for servings of most fiber-rich foods supply a mixture of fibers. legumes, nuts, seeds, and high-fiber An effective way to add fiber while lowering saturated fat is to substitute plant cereals and breads; look these up in sources of protein (legumes) for some of the animal sources of protein (meats Appendix A. and cheeses) in the diet. Another way is to focus on consuming the recommended Example: 1⁄2 c navy beans = 6.0 g fiber amounts of fruits, vegetables, legumes, and whole grains each day. You can make a 5. Add up the grams of fiber from the quick approximation of a day’s fiber intake by following the instructions in Table 4–2. previous lines. People choosing high-fiber foods are also wise to drink extra fluids to help the fiber Example: 7.5 + 4.0 + 7.5 + 6.0 = do its job. 25 g fiber Can My Diet Have Too Much Fiber? No Tolerable Upper Intake Level has been Day’s total fiber = 25 g fiber established for fiber, but consuming purified fiber added to foods or supplements can a Most cooked and canned fruits and vegetables be taken to extremes. One overly enthusiastic eater of oat bran muffins required contain about this amount, while whole raw fruits emergency surgery for a blocked intestine; too much oat bran and too little fluid and some vegetables contain more. The Need for Carbohydrates 12 5 Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Table 4–3 Usefulness of Carbohydrates Carbohydrates in the Body Carbohydrates in Foods ▪ Energy source. Sugars and starch from the diet provide energy ▪ Flavor. Sugars provide sweetness. for many body functions; they provide glucose, the preferred ▪ Browning. When exposed to heat, sugars undergo browning fuel for the brain and nerves. reactions, lending appealing color, aroma, and taste. ▪ Glucose storage. Muscle and liver glycogen store glucose. ▪ Texture. Sugars help make foods tender. Cooked starch lends a ▪ Raw material. Sugars are converted into other compounds, such smooth, pleasing texture. as amino acids (the building blocks of proteins), as needed. ▪ Gel formation. Starch molecules expand when heated and trap ▪ Structures and functions. Sugars interact with protein molecules, water molecules, forming gels. The fiber pectin forms the gel of affecting their structures and functions. jellies when cooked with sugar and acid from fruit. ▪ Digestive tract health. Fibers help to maintain healthy bowel ▪ Bulk and viscosity (thickness). Carbohydrates lend bulk and function (reduce risk of bowel diseases). increased viscosity to foods. Soluble, viscous fibers lend ▪ Blood cholesterol. Fibers promote normal blood cholesterol thickness to foods such as salad dressings. concentrations (reduce risk of heart disease). ▪ Moisture. Sugars attract water and keep foods moist. ▪ Blood glucose. Fibers modulate blood glucose concentrations ▪ Preservative. Sugar in high concentrations dehydrates bacteria (help control diabetes). and preserves the food. ▪ Satiety. Fibers and sugars contribute to feelings of fullness. ▪ Fermentation. Carbohydrates are fermented by yeast, a process ▪ Body weight. A fiber-rich diet may promote a healthy body that causes bread dough to rise and beer to brew, among other weight.