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17.1: Carbohydrates in the Diet

  • Page ID
    152245
  • Learning Objectives

    • Describe the metabolism of carbohydrates.
    • Know the source and function of common carbohydrates in the diet.

    Carbohydrates are sugars and sugar derivatives whose formulas can be written in the general form: Cx(H2O)y. (The subscripts x and y are whole numbers.) Some typical carbohydrates are sucrose (ordinary cane sugar), C12H22O11; glucose (dextrose), C6H12O6; fructose (fruit sugar), C6H12O6; and ribose, C5H10O5. Since the atom ratio H/O is 2/1 in each formula, these compounds were originally thought to be hydrates of carbon, hence their general name.

    In scientific literature, the term "carbohydrate" has many synonyms, like "sugar" (in the broad sense), "saccharide", "ose",[2] "glucide",[7] "hydrate of carbon" or "polyhydroxy compounds with aldehyde or ketone". Some of these terms, specially "carbohydrate" and "sugar", are also used with other meanings.

    In food science and in many informal contexts, the term "carbohydrate" often means any food that is particularly rich in the complex carbohydrate starch (such as cereals, bread and pasta) or simple carbohydrates, such as sugar (found in candy, jams, and desserts).

    Carbohydrates may be classified according to their degree of polymerization, and may be divided initially into three principal groups, namely sugars, oligosaccharides and polysaccharides[14]as shown in Table \(\PageIndex{1}\).

    Table \(\PageIndex{1}\) The Major Dietary Carbohydrates. Source: Wikipedia

    Class (DP*) Subgroup Components
    Sugars (1–2) Monosaccharides Glucose, galactose, fructose, xylose
    Disaccharides Sucrose, lactose, maltose, trehalose
    Polyols Sorbitol, mannitol
    Oligosaccharides (3–9) Malto-oligosaccharides Maltodextrins
    Other oligosaccharides Raffinose, stachyose, fructo-oligosaccharides
    Polysaccharides (>9) Starch Amylose, amylopectin, modified starches
    Non-starch polysaccharides Glycogen, Cellulose, Hemicellulose, Pectins, Hydrocolloids

    DP * = Degree of polymerization

    Digestion of Carbohydrates

    The human body breaks down complex carbohydrates into glucose. Glucose in the blood (often referred to as “blood sugar”) is the primary energy source for the body. Sugars provide calories, or “energy,” for the body. Each gram of sugar provides 4 calories. Glucose can be used immediately or stored in the liver and muscles for later use.

    Carbohydrate digestion begins in the mouth (Figure \(\PageIndex{1}\)) where salivary α-amylase attacks the α-glycosidic linkages in starch, the main carbohydrate ingested by humans. Cleavage of the glycosidic linkages produces a mixture of dextrins, maltose, and glucose. The α-amylase mixed into the food remains active as the food passes through the esophagus, but it is rapidly inactivated in the acidic environment of the stomach.

    37c9abefb172b2bf45f1be8f837a6f6f.jpg
    Figure \(\PageIndex{1}\) The Principal events and sites of carbohydrate digestion.

    The primary site of carbohydrate digestion is the small intestine. The secretion of α-amylase in the small intestine converts any remaining starch molecules, as well as the dextrins, to maltose. Maltose is then cleaved into two glucose molecules by maltase. Disaccharides such as sucrose and lactose are not digested until they reach the small intestine, where they are acted on by sucrase and lactase, respectively. The major products of the complete hydrolysis of disaccharides and polysaccharides are three monosaccharide units: glucose, fructose, and galactose. These are absorbed through the wall of the small intestine into the bloodstream.

    Fuel for the Brain

    The brain is a marvelous organ. And it's a hungry one, too. The major fuel for the brain is the carbohydrate glucose. The average adult brain represents about \(2\%\) of our body's weight, but uses \(25\%\) of the glucose in the body. Moreover, specific areas of the brain use glucose at different rates. If you are concentrating hard (taking a test, for example), certain parts of the brain need a lot of extra glucose while other parts of the brain only use their normal amount. Something to think about.

    Some foods that are high in carbohydrates include bread, pasta, and potatoes (Figure \(\PageIndex{2}\)) . Because carbohydrates are easily digested, athletes often rely on carbohydrate rich foods to enable a high level of performance.

    Carbohydrate food sources
    Figure \(\PageIndex{2}\) Foods that serve as carbohydrate sources.

    Common Monosaccharides and Disaccharides

    Monosaccharides and disaccharides commonly found in our diets are listed in Table \(\PageIndex{3}\). Although a variety of monosaccharides are found in living organisms, three hexoses are particularly abundant: D-glucose, D-galactose, and D-fructose (Figure \(\PageIndex{3}\)). Glucose and galactose are both aldohexoses, while fructose is a ketohexose.

    Table \(\PageIndex{2}\) . Monosaccharides and disaccharides. Source: US FDA
    Simple sugars (monosaccharides) are small enough to be absorbed directly into the bloodstream. They include: Sugars that contain two molecules of sugar linked together (disaccharides) are broken down in your body into single sugars. They include:
    Fructose

    Sucrose (table sugar ) = glucose + fructose

    Galactose Lactose (milk sugar) = glucose + galactose
    Glucose Maltose (malt sugar) = glucose + glucose

    Figure \(\PageIndex{3}\) Structures of three important hexoses. Each hexose is pictured with a food source in which it is commonly found. Source: Photos © Thinkstock.

    Glucose

    D-Glucose, generally referred to as simply glucose, is the most abundant sugar found in nature; most of the carbohydrates we eat are eventually converted to it in a series of biochemical reactions that produce energy for our cells. It is also known by three other names: dextrose, from the fact that it rotates plane-polarized light in a clockwise (dextrorotatory) direction; corn sugar because in the United States cornstarch is used in the commercial process that produces glucose from the hydrolysis of starch; and blood sugar because it is the carbohydrate found in the circulatory system of animals. Normal blood sugar values range from 70 to 105 mg glucose/dL plasma, and normal urine may contain anywhere from a trace to 20 mg glucose/dL urine.

    Galactose

    D-Galactose does not occur in nature in the uncombined state. It is released when lactose, a disaccharide found in milk, is hydrolyzed. The galactose needed by the human body for the synthesis of lactose is obtained by the metabolic conversion of D-glucose to D-galactose. Galactose is also an important constituent of the glycolipids that occur in the brain and the myelin sheath of nerve cells. For this reason it is also known as brain sugar. The structure of D-galactose is shown in Figure \(\PageIndex{3}\). Notice that the configuration differs from that of glucose only at the fourth carbon atom.

    Fructose

    D-Fructose, also shown in Figure \(\PageIndex{3}\), is the most abundant ketohexose. Note that from the third through the sixth carbon atoms, its structure is the same as that of glucose. It occurs, along with glucose and sucrose, in honey (which is 40% fructose) and sweet fruits. Fructose (from the Latin fructus, meaning “fruit”) is also referred to as levulose because it has a specific rotation that is strongly levorotatory (−92.4°). It is the sweetest sugar, being 1.7 times sweeter than sucrose, although many nonsugars are several hundred or several thousand times as sweet (Table \(\PageIndex{1}\)).

    Sucrose

    Sucrose, probably the largest-selling pure organic compound in the world, is known as beet sugar, cane sugar, table sugar, or simply sugar. Most of the sucrose sold commercially is obtained from sugar cane and sugar beets (whose juices are 14%–20% sucrose) by evaporation of the water and recrystallization. The dark brown liquid that remains after the recrystallization of sugar is sold as molasses.

    Maltose

    Maltose occurs to a limited extent in sprouting grain. It is formed most often by the partial hydrolysis of starch and glycogen. In the manufacture of beer, maltose is liberated by the action of malt (germinating barley) on starch; for this reason, it is often referred to as malt sugar. Maltose is about 30% as sweet as sucrose. The human body is unable to metabolize maltose or any other disaccharide directly from the diet because the molecules are too large to pass through the cell membranes of the intestinal wall. Therefore, an ingested disaccharide must first be broken down by hydrolysis into its two constituent monosaccharide units.

    In the body, such hydrolysis reactions are catalyzed by enzymes such as maltase.

    Lactose

    Lactose is known as milk sugar because it occurs in the milk of humans, cows, and other mammals. In fact, the natural synthesis of lactose occurs only in mammary tissue, whereas most other carbohydrates are plant products. Human milk contains about 7.5% lactose, and cow’s milk contains about 4.5%. This sugar is one of the lowest ranking in terms of sweetness, being about one-sixth as sweet as sucrose. Lactose is produced commercially from whey, a by-product in the manufacture of cheese. It is important as an infant food and in the production of penicillin.

    Many adults and some children suffer from a deficiency of lactase. These individuals are said to be lactose intolerant because they cannot digest the lactose found in milk. A more serious problem is the genetic disease galactosemia, which results from the absence of an enzyme needed to convert galactose to glucose. Certain bacteria can metabolize lactose, forming lactic acid as one of the products. This reaction is responsible for the “souring” of milk.

    The different disaccharides and the monosaccharides components are illustrated in Figure \(\PageIndex{4}\) below.

    clipboard_e3da7a374eb99cb032278f736fb022bc7.png
    Figure \(\PageIndex{4}\) The three disaccharides.

    Each of these disaccharides contains glucose and all the reactions are dehydration reactions. Also notice the difference in the bond structures. Maltose and sucrose have alpha-bonds, which are depicted as v-shaped above. You might hear the term glycosidic used in some places to describe bonds between sugars. A glycoside is a sugar, so glycosidic is referring to a sugar bond. Lactose, on the other hand, contains a beta-bond. We need a special enzyme, lactase, to break this bond, and the absence of lactase activity leads to lactose intolerance.


    Naturally Occurring and Added Sugars

    Sugars are found naturally in many nutritious foods and beverages and are also added to foods and beverages for taste, texture, and preservation.

    Naturally occurring sugars are found in a variety of foods, including: Added sugars are often found in foods low in other nutrients, including:
    • Dairy products
    • Fruit (fresh, frozen, dried, and canned in 100% fruit juice)
    • 100% fruit and vegetable juice
    • Vegetables
    • Dairy desserts (such as ice cream, other frozen desserts, and puddings)
    • Grain-based desserts (such as brownies, cakes, cookies, doughnuts, pastries, pies, and sweet rolls)
    • Sugar-sweetened beverages (such as energy drinks, flavored waters, fruit drinks, soft drinks, sports drinks, and sweetened coffee and tea)
    • Sweets (such as candies, jams, sweet toppings, and syrups)

    The relative sweetness of the different sugars previously discussed are given in Table \(\PageIndex{3}\).

    Table \(\PageIndex{3}\): The Relative Sweetness of Some Sugars (Sucrose = 100)
    Compound Relative Sweetness
    lactose 16
    maltose 32
    glucose 74
    sucrose 100
    fructose 173

    High-Fructose Corn Syrup

    Figure \(\PageIndex{5}\)U.S. per capita sugar and sweetener consumption

    Opponents claim that high-fructose corn syrup is contributing to the rise in obesity rates. As a result, some manufactures have started releasing products made with natural sugar. You can read about this trend in the following New York Times article in the link below. Also, manufacturers tried to rebrand high-fructose corn syrup as corn sugar to get around the negative perception of the name. But the FDA rejected the Corn Refiners Association request to change the name officially to corn sugar as described in the second link. The last link is a video made by the American Chemical Society that gives some background on how HFCS is produced and how it compares to sucrose.

    Links and Video

    Links

    Sugar is back on labels, this time as a selling point - http://www.nytimes.com/2009/03/21/dining/21sugar.html?_r=1&ref=nutrition

    No new name for high-fructose corn syrup - http://well.blogs.nytimes.com/2012/05/31/no-new-name-for-high-fructose-corn-syrup/?_r=0

    Video

    Sugar vs. High Fructose Corn Syrup - What's the Difference? - https://www.youtube.com/watch?v=fXMvregmU1g

    Complex Carbohydrates: Oligosaccharides and Polysaccharides

    An oligosaccharide, from the Greek olígos, "a few", and sácchar, "sugar") is a saccharide polymer containing a small number (typically three to ten[2][3][4][5]) of monosaccharides (simple sugars). Oligosaccharides are a component of fiber from plant tissue. Fructoologosaccharides (FOS) and inulin (Figure \(\PageIndex{6}\)) are present in Jerusalem artichoke, burdock, chicory, leeks, onions, and asparagus. Inulin is a significant part of the daily diet of most of the world’s population. FOS can also be synthesized by enzymes of the fungus Aspergillus niger acting on sucrose. Galatoologosaccharide (GOS) is naturally found in soybeans and can be synthesized from lactose. FOS, GOS, and inulin are also sold as nutritional supplements.[citation needed]

    Figure \(\PageIndex{6}\) The structure of inulin. Source: Wikipedia

    As the name suggests, polysaccharides are substances built up by the condensation of a very large number of monosaccharide units. Cellulose, for example, is a polymer of β-glucose, containing upwards of 3000 glucose units in a chain. Starch is largely a polymer of α-glucose.

    These two substances are a classic example of how a minor difference in the monomer can lead to major differences in the macroscopic properties of the polymer. Good-quality cotton and paper are almost pure cellulose, and they give us a good idea of its properties. Cellulose forms strong but flexible fibers and does not dissolve in water. By contrast, starch has no mechanical strength at all, and some forms are water soluble. Part of the molecular structure of cellulose and starch are shown in Fig. \(\PageIndex{6}\).

    Figure \(\PageIndex{6}\) Structures of (a) cellulose and (b) amylose (starch). The C—O bonds linking glucose units are shown in color and hydrogen bonds are shown as single black lines. Hydrogen atoms have been omitted to simplify the structure. Note how the β-glucose units in cellulose form a linear chain because the C—O hands on either side of the glucose unit are parallel. In amylose, where α-glucose is the monomer, the chain is forced to curve. Note also the numerous hydrogen bonds between the two cellulose chains shown and the hydrogen bonds connecting successive turns of the spiral chain of amylose. When iodine contacts starch, I2 molecules can fit lengthwise within the spiral of each amylose molecule. The starch-iodine complex which results is responsible for the dark blue color observed when iodine contacts starch.

    Cellulose and starch are different not only in overall structure and macroscopic properties. From a biochemical point of view they behave so differently that it is difficult to believe that they are both polymers of the same monosaccharide. Enzymes which are capable of hydrolyzing starch will not touch cellulose, and vice versa. From a plant’s point of view this is just as well since cellulose makes up structural material while starch serves as a storehouse for energy. If there were not a sharp biochemical distinction between the two, the need for a bit more energy by the plant might result in destruction of cell walls or other necessary structural components.

    Starches in Food

    Starch is the most common carbohydrate in the human diet and is contained in many staple foods. The major sources of starch intake worldwide are the cereals (rice, wheat, and maize) and the root vegetables (potatoes and cassava).[39] Many other starchy foods are grown, some only in specific climates, including acorns, arrowroot, arracacha, bananas, barley, breadfruit, buckwheat, canna, colocasia, katakuri, kudzu, malanga, millet, oats, oca, polynesian arrowroot, sago, sorghum, sweet potatoes, rye, taro, chestnuts, water chestnuts and yams, and many kinds of beans, such as favas, lentils, mung beans, peas, and chickpeas.

    Widely used prepared foods containing starch are bread, pancakes, cereals, noodles, pasta, porridge and tortilla.

    Digestive enzymes have problems digesting crystalline structures. Raw starch is digested poorly in the duodenum and small intestine, while bacterial degradation takes place mainly in the colon. When starch is cooked, the digestibility is increased. Starch gelatinization during cake baking can be impaired by sugar competing for water, preventing gelatinization and improving texture.

    Before the advent of processed foods, people consumed large amounts of uncooked and unprocessed starch-containing plants, which contained high amounts of resistant starch. Microbes within the large intestine fermented the starch, produced short-chain fatty acids, which are used as energy, and support the maintenance and growth of the microbes. More highly processed foods are more easily digested and release more glucose in the small intestine—less starch reaches the large intestine and more energy is absorbed by the body. It is thought that this shift in energy delivery (as a result of eating more processed foods) may be one of the contributing factors to the development of metabolic disorders of modern life, including obesity and diabetes.[40]

    Fibers in Food

    Dietary fiber (British spelling fibre) or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes. Dietary fiber consists of non-starch polysaccharides and other plant components such as cellulose, resistant starch, resistant dextrins, inulin, lignins, chitins, pectins, beta-glucans, and oligosaccharides.[1][2]

    Polysaccharide fiber differs from other polysaccharides in that it contains beta-glycosidic bonds (as opposed to alpha-glycosidic bonds). To illustrate these differences, consider the structural differences between amylose and cellulose (type of fiber) in the figure below. Both are linear chains of glucose, the only difference is that amylose has alpha-glycosidic bonds, while cellulose has beta-glycosidic bonds as shown below. The beta-bonds in fiber cannot be broken down by the digestive enzymes in the small intestine so they continue into the large intestine.

    clipboard_e328b6cf7bc4496aeb3115153526cfd6e.png
    Figure \(\PageIndex{7}\) Structures of amylose and cellulose

    Fiber can be classified by its physical properties. In the past, fibers were commonly referred to as soluble and insoluble. This classification distinguished whether the fiber was soluble in water. However, this classification is being phased out in the nutrition community. Instead, most fibers that would have been classified as insoluble fiber are now referred to as nonfermentable and/or nonviscous and soluble fiber as fermentable, and/or viscous because these better describe the fiber's characteristics. Viscous refers to the capacity of certain fibers to form a thick gel-like consistency.

    Fermentable (soluble fiber) dissolves in water and is readily fermented in the colon into gases and physiologically active by-products, such as short-chain fatty acids produced in the colon by gut bacteria;[it is viscous, may be called prebiotic fiber, and delays gastric emptying which, in humans, can result in an extended feeling of fullness.

    Non-fermentable (insoluble fiber) does not dissolve in water and is inert to digestive enzymes in the upper gastrointestinal tract and provides bulking. Some forms of insoluble fiber, such as resistant starches, can be fermented in the colon. Bulking fibers absorb water as they move through the digestive system, easing defecation.

    Table \(\PageIndex{4}\) lists some of the common types of fiber and provides a brief description about each. Specific sources of the different types of soluble and insoluble fiber as well as the amount of fiber found in common foods are discussed in section 17.4.
    Table \(\PageIndex{4}\) Common Types of Fermentable (Soluble) Fiber and Non-Fermentable (Insoluble) Fiber.

    Fermentable

    (Soluble) Fiber

    Description

    Non-Fermentable

    (Insoluble Fiber)

    Description

    Hemicellulose (​​​​soluble type)

    Surround cellulose in plant cell walls Cellulose Main component of plant cell walls
    Pectin Found in cell walls and intracellular tissues of fruits and berries

    Hemicellulose

    (insoluble type)

    Surround cellulose in plant cell walls
    Beta-glucans Found in cereal brans Lignin Non carbohydrate found within "woody" plat cell walls
    Gums Viscous, usually isolated from seeds

    Summary

    • Carbohydrate digestion begins in the mouth (in the presence of salivary α-amylase) and continues in the small intestine. The major products of the complete hydrolysis of disaccharides and polysaccharides are three monosaccharide units: glucose, fructose, and galactose. These are absorbed through the wall of the small intestine into the bloodstream.
    • Simple sugars like glucose, fructose and the disaccharide sucrose function as natural sweeteners.
    • More complex carbohydrates (oligosaccharides and polysaccharides) are now significant parts of the daily diet promoting gut health that leads to other health benefits.

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