Chapter One
Carbohydrates Sugars, Starches and Fiber
Learning Objectives After reading this chapter, students should be able to:
1 Discuss the difference between refined and unrefined carbohydrates.
2 Compare the structures of simple and complex carbohydrates.
3 Distinguish between soluble and insoluble fiber and name food sources of each.
4 Define lactose intolerance and explain why it causes gas and bloating when milk is consumed.
5 Discuss the effects of dietary fiber and other indigestible carbohydrates on gastrointestinal function and health.
6 Describe the steps involved in metabolizing glucose to produce energy.
7 Explain how insulin and glucagon are involved in regulating blood glucose.
8 Compare the causes and consequences of type 1 and type 2 diabetes.
9 Discuss the health risks and benefits of diets high in unrefined carbohydrates and diets high in refined carbohydrates.
10 Modify a diet so it meets current recommendations for the types and amounts of carbohydrate.
11 Discuss the role of artificial sweeteners in a weight-loss plan.
As the low-carbohydrate diet craze faded, the 15 pounds Shamara had lost while consuming this diet slowly crept back onto her 5'' 6" frame. She has always been heavier than she liked, and was delighted to lose 15 pounds. Now she has regained 10 pounds to bring her weight up to 153 pounds. Since this weight is still in the healthy range, she decided to forget the weight-loss diet and try to just focus on eating healthy foods. To do this, Shamara looked up her MyPyramid recommendations. She was surprised to find that not all carbs are bad-MyPyramid recommended that she eat the equivalent of about six servings of grain products and that at least half of these be whole grains. She also saw that she needed to increase her intake of fruits and vegetables. These foods contain carbohydrate but are also high in fiber.
The first step Shamara took to improve her diet was to boost her fruit and vegetable intake by keeping a bag of cut-up raw vegetables and a bowl of fruit salad in her refrigerator, ready to be snacked on or added to a meal. To increase her intake of whole grains she started by making smarter choices at the grocery store. She switched from white rice to brown rice and chose multi-grain breads and healthy sounding cereals. A few weeks later, she had her diet analyzed at a health fair. She was surprised to see that her fiber intake was still below the recommended 25 grams per day. She started reading the labels on the foods in her cupboard and found that her seven-grain bread didn''t contain any whole grains. Her breakfast cereal did contain whole wheat but only provided 2 grams of fiber in a cup-it also contained 18 grams of sugars. Choosing healthy carbohydrates was turning out to be almost as difficult as eliminating them from her diet had been when she was consuming the low-carb diet.
Carbohydrates in the Modern Diet
Carbohydrates are the basis of our diet. They are found in foods as diverse as whole-wheat bread, chocolate cake, fresh fruit, milk, and carbonated soft drinks. The carbohydrates in these foods are a readily available source of energy; they supply 4 kcalories per gram. However, the additional nutritional impact they deliver varies depending on whether the carbohydrate is refined or in its natural state. The carbohydrates in the whole-wheat bread, the fresh fruit, and the milk are considered whole food sources of carbohydrate because they have not been altered from their natural state. These foods contain vitamins, minerals, and other health-promoting substances as well as carbohydrates. The cake and the soda provide carbohydrates that have been refined. Refining separates carbohydrates from essential nutrients and other food components present in the whole food (Table 4.1).
Over the last century the amounts and sources of carbohydrates in the American diet have changed. Our total carbohydrate intake decreased between 1909 and 1963. Most of this drop was due to a decrease in the consumption of whole grains, and with it came a 40% drop in the amount of fiber consumed. Since the 1960s our total carbohydrate intake has increased, but our fiber intake did not rise with it, suggesting an increase in the intake of refined carbohydrates. Much of the carbohydrates added back to our diet between 1960 and 2000 came from sugars; over this time period per capita sugar consumption rose by 33%. Whole-grain breads, dried peas and beans, and bulgur had been replaced by white bread, snack foods, and sugared soft drinks. The type of sweetener also changed. In the 1960s we sweetened food with cane and beet sugar, but today most of the foods we buy are sweetened with corn sweeteners.
The carbohydrates in today''s diet are more refined than they were earlier in the century. Recommendations for a healthy diet tell us to choose more of the carbohydrates we used to eat: more whole sources such as whole grains, vegetables, legumes, and fruits, and fewer foods high in refined carbohydrates and added sugars such as baked goods and soft drinks (Figure 4.1).
Refined Carbohydrates
Whole food sources of carbohydrate such as grains, legumes, vegetables, fruit, and milk contain a variety of nutrients in addition to carbohydrates. Grains, legumes, and vegetables provide B vitamins, some minerals, and fiber. Fruits provide vitamins A and C along with fiber. Milk is a good source of the B vitamin riboflavin and the mineral calcium. In contrast, refined sources of carbohydrate such as the corn flakes you may have had for breakfast, are made from corn that has been ground, sieved, washed, cooked, extruded, and dried. During these refining steps, many of the nutrients and other healthful components of the corn kernel are lost. When we eat the entire kernel or seed of a grain, such as corn or wheat, we are eating an unrefined or whole-grain product. The whole-grain kernel includes three parts (Figure 4.2). The outermost bran layers contain most of the fiber and are a good source of vitamins. The germ, which lies at the base of the kernel, is the plant embryo where sprouting occurs. It is the source of vegetable oils such as corn or safflower oil, and is rich in vitamin E. It also contains protein, fiber, and the B vitamins riboflavin, thiamin, and vitamin [B.sub.6]. The remainder of the kernel is the endosperm, which is primarily starch but also contains most of the protein and some vitamins and minerals. During the milling of grain into flour, the grinding detaches the germ and bran from the endosperm. Whole-grain flours such as whole-wheat flour include most of the bran, germ, and endosperm (see Off the Shelf: Choosing Whole Grains: Not as Easy as It Sounds). White flour however is produced from just the endosperm. Fiber and some vitamins, minerals, and phytochemicals naturally found in the whole grain are therefore lost. In order to restore some of the lost nutrients, refined grains sold in the United States are fortified with some, but not all, of the nutrients lost in processing. Enriched grains contain added thiamin, riboflavin, niacin, and iron and are fortified with folate. However, they do not contain added vitamin E, magnesium, vitamin [B.sub.6], or a number of other nutrients that are also removed by milling (see Chapter 8).
Added Sugars
If you sprinkle some sugar on your corn flakes you are adding another refined source of carbohydrate. This added sugar was most likely extracted from a sugar beet, boiled, bleached, and purified. It adds kcalories without adding any nutrients thereby reducing the nutrient density of your breakfast. But the sugar you add isn''t the only source of added sugars in the diet-much of the added sugar we consume comes from desserts, beverages, and snacks that we purchase already prepared. Refined added sugars make up about 16% of the kcalories in the American diet. Added sugars are not nutritionally or chemically different from sugars occurring naturally in foods. The only difference is that they have been separated from their plant sources and therefore are not consumed with the fiber, vitamins, minerals, and other substances found in the original plant. Because added sugars provide few nutrients for the number of kcalories they contain they have a low nutrient density and are therefore thought of as empty kcalories. Unrefined or whole food sources of sugar such as fruit provide vitamins, minerals, and phytochemicals as well as kcalories. For example, a 12-ounce soda contains about 140 kcalories but almost no nutrients other than sugar. Three kiwis also have about 140 kcalories but contribute vitamin C, folate, potassium, and some calcium as well as fiber (Figure 4.3).
Simple and Complex Carbohydrates
Chemically, carbohydrates are compounds that contain carbon (carbo), as well as hydrogen and oxygen in the same proportion as in water (hydrate). They are typically divided into simple carbohydrates, also known as sugars, and complex carbohydrates, which include starches and fibers. Both can provide a source of energy to fuel the body.
Simple Carbohydrates
The basic unit of carbohydrate is a single sugar molecule, a monosaccharide (mono means one). When two sugar molecules combine, they form a disaccharide (di means two). Monosaccharides and disaccharides are known as simple sugars or simple carbohydrates. Fruits, vegetables, and milk are sources of simple carbohydrates. The sugars we add to food such as white table sugar, brown sugar, molasses, and confectioner''s sugar are also simple carbohydrates. These are produced by refining the sugar from plants such as sugar cane and sugar beets.
Monosaccharides The three most common monosaccharides in the diet are glucose, galactose, and fructose. Each contains 6 carbon, 12 hydrogen, and 6 oxygen atoms but differ in their arrangement (Figure 4.4). Glucose, commonly referred to as blood sugar, is the most important carbohydrate fuel for the body. It is produced in plants by the process of photosynthesis, which uses energy from the sun to combine carbon dioxide and water (Figure 4.5). Glucose rarely occurs as a monosaccharide in food. It is most often found as part of a disaccharide or starch. Galactose is also rarely present as a monosaccharide in the food supply. It occurs most often as a part of lactose, the disaccharide in milk.
Fructose is a monosaccharide that tastes sweeter than glucose. It is found in fruits and vegetables and makes up more than half the sugar in honey. Because fructose does not cause as great a rise in blood glucose as other sugars, it is sometimes used in products for people with diabetes. However, because fructose causes an increase in blood lipids, its use should be limited. Fructose consumed in fruits or juices can also cause diarrhea in children. Most of the fructose in our diet comes from high-fructose corn syrup. This sweetener is produced by modifying starch extracted from corn to produce a syrup that is approximately half glucose and half fructose. High-fructose corn syrup is sweeter and less expensive than table sugar. It is now the most common kcaloric sweetener added to foods and beverages and is the kcaloric sweetener used in most soft drinks in the United States. The dramatic increase in the use of high-fructose corn sweeteners that has occurred in the last few decades has been suggested to be related to the increased incidence of diabetes and obesity.
Disaccharides Disaccharides are simple carbohydrates made up of two monosaccharides linked together (Figure 4.6). Sucrose, or common white table sugar, is the disaccharide formed by linking glucose to fructose. It is found in sugar cane, sugar beets, honey, and maple syrup. Sucrose is the only sweetener that can be called "sugar" in the ingredient list on food labels in the United States. Lactose, or milk sugar, is glucose linked to galactose. Lactose is the only sugar found naturally in animal foods. It contributes about 30% of the energy in whole cow''s milk and about 40% of the energy in human milk. Maltose is a disaccharide consisting of two molecules of glucose. This sugar is made whenever starch is broken down. For example, it is responsible for the slightly sweet taste experienced when bread is held in the mouth for a few minutes. As salivary amylase begins digesting the starch, some sweeter-tasting maltose is formed.
Making and Breaking Sugar Chains The chemical reaction that breaks the bonds between sugar molecules is called a hydrolysis reaction (Figure 4.7). Hydrolysis reactions use water to add a hydroxyl group (OH) to one sugar and a hydrogen atom (H) to the other. The reaction that links two sugars together is called a condensation reaction. Condensation reactions release a molecule of water by taking a hydroxyl group from one sugar and a hydrogen atom from the other.
Complex Carbohydrates
Complex carbohydrates are made up of many monosaccharides linked together in chains. They are generally not sweet to the taste like simple carbohydrates. Short chains of three to ten monosaccharides are called oligosaccharides, and longer chains are called polysaccharides (poly means many). The polysaccharides include glycogen in animals and starch and fiber in plants (Figure 4.8).
Oligosaccharides Some oligosaccharides are formed in the gut during the breakdown of polysaccharides. These are then further digested to simple sugars. Other oligosaccharides are found naturally in foods such as beans and other legumes, onions, bananas, garlic, and artichokes. Many of these are not digested by human enzymes in the digestive tract and pass into the colon where they are broken down by the intestinal microflora. Therefore they can affect the types of bacteria that grow in the colon and have beneficial effects on gastrointestinal (GI) health.
Glycogen Glycogen is the storage form of carbohydrate in animals. It is a polysaccharide made up of highly branched chains of glucose molecules (see Figure 4.8). The branched structure allows it to be broken down quickly when glucose is needed. In humans, glycogen is stored in the muscles and in the liver. Muscle glycogen provides glucose to the muscle as a source of energy during activity; liver glycogen releases glucose into the bloodstream for delivery to cells throughout the body. We consume only minimal amounts of glycogen in our food because glycogen present in animal muscles is broken down soon after slaughter so is not present when the meat is consumed.
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Excerpted from Nutritionby Lori A. Smolin Mary B. Grosvenor Copyright © 2007 by Lori A. Smolin. Excerpted by permission.
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