Xxx

Cellulose

Figure 4-9

Starch and Cellulose Molecules Compared (Small Segments)

The bonds that link the glucose molecules together in cellulose are different from the bonds in starch (and glycogen). Human enzymes cannot digest cellulose. See Appendix C for chemical structures and descriptions of linkages.

When the hormonal message "Release energy" arrives at the storage sites in a liver or muscle cell, enzymes respond by attacking all the many branches of each glycogen simultaneously, making a surge of fuel available.*

Starches

Just as the human body stores glucose as glycogen, plant cells store glucose as starches—long, branched or unbranched chains of hundreds or thousands of glucose molecules linked together (see the middle and right side of Figure 4-8 on p. 95). These giant molecules are packed side by side in grains such as wheat or rice, in tubers such as potatoes, and in legumes such as peas and beans. A cubic inch of food may contain as many as a million starch molecules. When you eat the plant, your body hydrolyzes the starch to glucose and uses the glucose for its own energy purposes.

All starchy foods derive from plants. Grains are the richest food source of starch, providing much of the food energy for people all over the world—rice in Asia; wheat in Canada, the United States, and Europe; corn in much of Central and South America; and millet, rye, barley, and oats elsewhere.1 Legumes and tubers, such as potatoes, yams, and cassava, are also important sources of starch.

Fibers

Fibers are the structural parts of plants and thus are found in all plant-derived foods—vegetables, fruits, grains, and legumes. Most fibers are polysaccharides, but starch is not one of them; in fact, fibers are often described as nonstarch polysac-charides. Nonstarch polysaccharides include cellulose, hemicelluloses, pectins, gums, and mucilages. Fibers also include some nonpolysaccharides such as lignins, cutins, and tannins.**

Even though most are polysaccharides, fibers differ from starches in that the bonds between their monosaccharides cannot be broken down by human digestive enzymes. The bacteria of the GI tract can break some fibers down, however, and this is important to digestion and to health.

Each of the fibers has a different structure. Most contain monosaccharides, but differ in the types they contain and in the bonds that link the monosaccharides to each other. These differences produce diverse health effects.

  • Cellulose • Cellulose is the primary constituent of plant cell walls and therefore occurs in all vegetables, fruits, and legumes. Like starch, cellulose is composed of glucose molecules connected in long chains. Unlike starch, however, the chains do not branch, and the bonds linking the glucose molecules together resist digestion by human enzymes (see Figure 4-9).
  • Normally, only the liver can return glucose directly from glycogen to the blood; muscle cells use glycogen internally to produce glucose. Muscle cells can restore the blood glucose level indirectly, however, as Chapter 7 explains.
  • The terms crude fiber, neutral-detergent fiber, and dietary fiber reflect different methodologies used to estimate the fiber contents of foods; they do not identify different types of fiber. The structure of cellulose is shown in Appendix C; the other polysaccharide fibers are similar, but differ slightly in their bonding. Besides glucose, their component sugars may include a variety of other monosaccharides. As for the lignins, they are polymers of several dozen molecules of phenol (an organic alcohol), with strong internal bonds that make them impervious to digestive enzymes.
  • Hemicelluloses • The hemicelluloses are the main constituent of cereal fibers. They are composed of various monosaccharide backbones with branching side chains of monosaccharides.* The many backbones and side chains make the hemicelluloses a diverse group; some are soluble, while others are insoluble.
  • Pectins • All pectins consist of a backbone derived from carbohydrate with side chains of various monosaccharides. Commonly found in vegetables and fruits (especially citrus fruits and apples), pectins may be isolated and used by the food industry to thicken jelly, keep salad dressing from separating, and otherwise control texture and consistency. Pectins can perform these functions because they readily form gels in water.
  • Gums and Mucilages • When cut, a plant secretes gums from the site of the injury. Like the other fibers, gums are composed of various monosaccharides and their derivatives. Gums such as gum arabic are used as additives by the food industry. Mucilages are similar to gums in structure; they include guar and carrageenan, which are added to foods as stabilizers.
  • Lignin • This nonpolysaccharide fiber has a three-dimensional structure that gives it strength. Because of its toughness, few of the foods that people eat contain much lignin. It occurs in the woody parts of vegetables such as carrots and the small seeds of fruits such as strawberries.
  • Other Classifications of Fibers • Scientists classify fibers in several ways. The previous paragraphs classified them according to their chemical properties. Fibers can also be classified according to their solubility. The effects of fibers on the body do not divide neatly along the lines of solubility, but some generalizations of significance to health can be made (see Table 4-1).

Some researchers classify fibers according to other physical properties that affect GI function and nutrient absorption. Physical properties of fibers include:

  • Water-holding capacity—the capacity to capture water like a sponge, swelling and increasing the bulk of the intestines' contents.
  • Viscosity—the capacity to form viscous, gel-like solutions.
  • Cation-exchange capacity—the ability to bind minerals.
  • Bile-binding capacity—the ability to bind to bile acids.
  • Fermentability—the extent to which bacteria in the GI tract can break down fibers to fragments that the body can use.**

Clearly, the fibers are a diverse group of compounds. The accompanying table summarizes the carbohydrate family of compounds.

  • In hemicelluloses, the most common backbone monosaccharides are xylose, mannose, and galactose; the common side chains are arabinose, glucuronic acid, and galactose (see Appendix C for structures).
  • Dietary fibers are fermented by colon bacteria to short-chain fatty acids, which are absorbed and metabolized by the GI mucosa and liver.

TTi SUMMARY

_ The Carbohydrate

Simple Carbohydrates (sugars)

• Monosaccharides

Glucose Fructose Galactose

• Disaccharides

Maltose Sucrose Lactose

Complex Carbohydrates

• Polysaccharides

Glycogen3 Starches

Fibers (nonstarch polysaccharides) Soluble Insoluble aGlycogen is a complex carbohydrate (a polysaccharide), but not a dietary source of carbohydrate.

  • In hemicelluloses, the most common backbone monosaccharides are xylose, mannose, and galactose; the common side chains are arabinose, glucuronic acid, and galactose (see Appendix C for structures).
  • Dietary fibers are fermented by colon bacteria to short-chain fatty acids, which are absorbed and metabolized by the GI mucosa and liver.

Type of Fiber

Major Food Sources

Action in the Body

Soluble fibers

Gums, pectins, some hemicelluloses, mucilages

Fruits (apples, citrus), oats, barley, legumes

Delay GI transit. Delay glucose absorption. Lower blood cholesterol.

Insoluble fibers

Cellulose, many hemicelluloses, lignins

Wheat bran, corn bran, whole-grain breads and cereals, vegetables

Accelerate GI transit. Increase fecal weight (promotes bowel movements). Slow starch hydrolysis. Delay glucose absorption.

Note: These generalizations are useful, but exceptions occur. For example, insoluble rice bran also lowers blood cholesterol, and the soluble fiber of the psyllium plant effectively promotes bowel movements.

Fibers: Their Solubilities, Sources, and Actions

Table 4-1

Fibers: Their Solubilities, Sources, and Actions phytic (FYE-tick) acid: a nonnutrient component of plant seeds; also called phytate (FYE-tate). Phytic acid occurs in the husks of grains, legumes, and seeds and is capable of binding minerals such as zinc, iron, calcium, magnesium, and copper in insoluble complexes in the intestine, which the body excretes unused.

Examples Legumes Vegetables
When a person eats carbohydrate-rich fruits, vegetables, and legumes, the body receives a valuable commodity—glucose.

The short chains of glucose units that result from the breakdown of starch are known as dextrins. The word sometimes appears on food labels because dextrins can be used as thickening agents in foods.

amylase (AM-ih-lace): an enzyme that hydrolyzes amylose (a form of starch). Amylase is a carbohydrase, an enzyme that breaks down carbohydrates.

Reminder: A bolus is a portion of food swallowed at one time.

satiety (sah-TIE-eh-tee): the feeling of fullness and satisfaction that food brings (Chapter 8 provides a more detailed definition).

• sate = to fill maltase: an enzyme that hydrolyzes maltose. sucrase: an enzyme that hydrolyzes sucrose. lactase: an enzyme that hydrolyzes lactose.

A compound not classed as a fiber but often found with it in foods is phytic acid. Because of this close association, researchers have been unable to determine whether it is the fiber, the phytic acid, or both, that binds with minerals, preventing their absorption.2 This binding presents a risk of mineral deficiencies, but the risk is minimal when fiber intake is reasonable and mineral intake adequate. The nutrition consequences of such mineral losses are described further in Chapters 12 and 13.

IN SUMMARY

The complex carbohydrates are the polysaccharides (chains of mono-^ saccharides): glycogen, starches, and fibers. Both glycogen and starch are storage forms of glucose—glycogen in the body, and starch in plants—and both yield energy for human use. The fibers also contain glucose (and other monosaccharides), but their bonds cannot be broken by human digestive enzymes, so they yield little, if any, energy.

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