Carbohydrates
CHAPTER
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Carbohydrate literally means hydrate (water) of carbon. The name was created by early chemists who found that heating sugars for a long period in an open test tube produced droplets of water on the sides of the tube and a black substance, carbon. Later chemical analysis of sugars and other carbohydrates indicated that they all contain at least carbon, hydrogen, and oxygen atoms.
Carbohydrates are the major components of most plants, making up 60 to 90 percent of their dry weight. In contrast, animals and humans contain only a small amount of carbohy-drates. Plants are able to make their own carbohydrates from carbon dioxide in the air and water taken from the soil in a process known as photosynthesis. Photosynthesis converts energy from sunlight into energy stored in carbohydrates. The plant uses the carbohydrates to grow and be healthy. Animals are incapable of photosynthesis and therefore depend on plants as a source of carbohydrates. Plants, such as wheat and broccoli, supply most of the carbohydrates in our diets. Milk also contains some carbohydrate.
Carbohydrates are separated into two categories: simple and complex. Also called sugars, simple carbohydrates include sugars that occur naturally in foods, such as fructose in fruits and glucose in honey, as well as sugars that are added to foods, such as white or brown sugar in a chocolate chip cookie.
Carbohydrates are much more than just sugars, though, and include the complex carbo-hydrates starch and fiber. Another name for complex carbocarbo-hydrates is polysaccharides (poly- means many), a good name for starch and most fibers because both consist of long chains of many sugars.
After completing this chapter, you should be able to:
• Identify the functions of carbohydrates
• List important monosaccharides and disaccharides and give examples of foods in which each is found
• Identify foods high in natural sugars, added sugars, and fiber
• List the potential health risks of consuming too much added sugar
• Identify food sources of starch and list the uses of starch in cooking
• Distinguish between the two types of dietary fiber and list examples of food containing each one
• Describe the health benefits of a high-fiber diet
• Describe how carbohydrates are digested, absorbed, and metabolized by the body
• State the dietary recommendations for carbohydrates
• Identify foods as being made from whole grains or refined grains
• Discuss the nutritional value and use of grains and legumes on a menu
• Examine the usefulness of the glycemic index
• Recognize alternatives to sugar in foods
F U N C T I O N S O F C A R B O H Y D R A T E S
Carbohydrates are the primary source of the body’s energy, supplying about 4 kcalories per gram. Glucose, a simple carbohydrate, is the body’s number-one source of energy. Most of the carbohydrates you eat are converted to glucose in the body.
Our cells can burn protein and fat for energy, but the body uses glucose first, in part because glucose is the most efficient energy source. The brain and other nerve cells are picky about their food, and under most circumstances, they will use only glucose for energy.
82 Chapter 3 Carbohydrates CARBOHYDRATE A large class of nutrients, including sugars, starch, and
fibers, that function as the body’s primary source of Long chains of many sugars,
including starches and
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Functions of Carbohydrates 83 Some glucose is stored in your body in a form called glycogen. This way the body has a
constant, available glucose source. Glycogen is stored in two places in the body: the liver and the muscles. An active 150-pound man has about 400 kcalories stored in his liver glycogen and about 1400 kcalories stored in his muscle glycogen. When the blood sugar level starts to dip and more energy is needed, the liver converts glycogen into glucose, which then is deliv-ered by the bloodstream. Muscle glycogen does not supply glucose to the bloodstream but is used strictly to supply energy for exercise.
If you run out of glycogen and do not eat any carbohydrates, the body will break down protein in muscles to some extent. Protein can be converted to glucose to maintain glu-cose levels in the blood and supply gluglu-cose to the central nervous system. Carbohydrates spare protein from being burned for energy so that protein can be used to build and repair the body.
An inadequate supply of carbohydrates can also cause the body to convert some fat to glu-cose, but this is also not desirable. When fat is burned for energy without any carbohydrates present, the process is incomplete and results in the production of ketone bodies. Ketone bodies can be used by the brain for energy, but too many can cause the blood to become too acidic (called ketosis), a condition that interferes with the transport of oxygen in the blood.
Ketosis can cause dehydration and may even lead to a fatal coma. Carbohydrates are impor-tant to help the body use fat efficiently.
You need at least 130 grams of carbohydrates daily to prevent protein (and fat) from being burned for fuel and to provide glucose to the central nervous system (brain and spinal cord).
This amount represents what you minimally need, not what is desirable (about two times more). We obtain 50 to 60 percent of our energy intake from carbohydrates. Therefore, if you eat 2000 kcalories per day, you take in 1000 to 1200 calories of carbohydrates, which represents 250 to 300 grams.
Carbohydrates are part of various materials found in the body, such as connective tissues, some hormones and enzymes, and genetic material.
Fiber, a complex carbohydrate, promotes the normal functioning of the intestinal tract, lowers blood cholesterol, and is associated with a reduced risk of developing type 2 diabetes (a disease characterized by high blood glucose levels).
GLYCOGEN The storage form of glucose
in the body; it is stored in the liver and muscles.
KETONE BODIES A group of organic
com-pounds that cause the blood to become too acidic as a result of fat being
burned for energy without any carbohydrates present.
KETOSIS Excessive level of ketone bodies in the blood and
urine.
1.
Carbohydrates are the primary source of the body’s energy. The brain and other neuron cells rely almost exclusively on glucose for energy.2.
Glycogen is a storage form of glucose in the liver and muscles. When your blood sugar level drops, the liver converts glycogen to glucose. Muscle glycogen is used strictly to provide energy to muscles during exercise.3.
Carbohydrate spares protein from being burned for energy.4.
Carbohydrates are also important to help the body use fat efficiently. When fat is burned for energy without any carbohydrates present, the process is incomplete and can result in the pro-duction of ketone bodies and ketosis.5.
You need at least 100 to 150 grams of carbohydrate daily to spare protein (and fat) from being burned for fuel and to provide glucose to the central nervous system and red blood cells.6.
Carbohydrates are part of various materials found in the body, such as connective tissues, some hormones and enzymes, and genetic material.7.
Fiber, a complex carbohydrate, promotes the normal functioning of the intestinal tract, lowers blood cholesterol, and is associated with a reduced risk of developing type 2 diabetes.MINI-SUMMARY
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S I M P L E C A R B O H Y D R A T E S ( S U G A R S )
Simple carbohydrates include monosaccharides and disaccharides. The chemical names of the six sugars to be discussed all end in “-ose,” which means sugar.
MONOSACCHARIDES
Monosaccharides include these simple sugars:
1. Glucose 2. Fructose 3. Galactose
The prefix mono- means “one”; these sugars consist of a single ring of atoms.
Monosaccharides are the building blocks of other carbohydrates, such as disaccharides and starch.
Glucose is the most abundant sugar found in nature. In photosynthesis, plants make glu-cose, which provides energy for growth and other activities. Also called dextrose, glucose is our primary energy source as well. As already mentioned, most of the carbohydrates you eat are converted to glucose in the body. The concentration of glucose in the blood, referred to as the blood glucose level, is vital to the proper functioning of the human body. Glucose is found in fruits such as grapes, in honey, and in small amounts in many plant foods.
Fructose, the sweetest natural sugar, is also found in honey as well as in fruits. Although it is a natural sugar, honey (made by bees) is primarily fructose and glucose, the two compo-nents of white sugar. Fructose is about 1.3 times as sweet as white sugar. Fructose and glu-cose are the most common monosaccharides in nature.
The last single sugar, galactose, is almost always linked to glucose to make milk sugar, a disaccharide.
DISACCHARIDES
Most naturally occurring carbohydrates contain two or more monosaccharide units linked together. Disaccharides, the double sugars, include sucrose, maltose, and lactose. They each contain glucose (Figure 3-1). Sucrose is the chemical name for what is commonly called white sugar, table sugar, granulated sugar, or simply sugar. Sugar cane and sugar beets both contain much sucrose but have to be refined to extract the sucrose from them. As Figure 3-1 indicates, sucrose is simply two common single sugars—glucose and fructose—linked together.
Although the primary source of sucrose in the American diet is refined sugar, sucrose occurs naturally in small amounts in many fruits and vegetables. Table sugar is more than 99 percent pure sugar and provides virtually no nutrients for its 16 kcalories per teaspoon.
Maltose, which consists of two bonded glucose units, does not occur in nature to any appreciable extent. It is fairly abundant in germinating (sprouting) seeds and is produced in the manufacture of beer.
The last disaccharide, lactose, is commonly called milk sugar because it occurs in milk.
Although milk is not a food you think of as sweet, there is some sugar there. However, if you look at Figure 3-2, you will see that lactose is one of the lowest-ranking sugars in terms of sweetness. Lactose, or milk sugar, is present in milk and products made from milk. Unlike most carbohydrates, which are in plant products, lactose is one of the few carbohydrates asso-ciated exclusively with animal products.
84 Chapter 3 Carbohydrates MONOSACCHARIDES Simple sugars, including glu-cose, fructose, and galac-tose, which consist of a
sin-gle ring of atoms and are the building blocks for found in the blood; glu-cose is vital to the proper
functioning of the body.
linked to glucose to form lactose, or milk sugar.
DISACCHARIDES Double sugars such as
sucrose.
SUCROSE (SUGAR) A disaccharide commonly
called table sugar, granu-lated sugar, or simply sugar.
MALTOSE milk and milk products that
is made of glucose and galactose.
f
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Simple Carbohydrates (Sugars) 85