4.8: Summary

Carbohydrates are created by photosynthesis: The energy of the sun is captured by the green chlorophyll in plants to create hydrated carbons— carbohydrates—from carbon dioxide and water. In fact, plant life and animal life provide a natural balance—plants produce food and oxygen, and animals take in that food and oxygen and turn it back into carbon dioxide and water.

The simplest carbohydrates are single sugars (monosaccharides). The most common single sugars in food are glucose, fructose, and galactose. When two sugars are linked together, they are called double sugars (disaccharides). The most common ones are sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (glucose + glucose).

When many sugars are linked together, they are called polysaccharides or complex carbohydrates. We can digest only some of them, depending on whether we have the proper digestive enzymes. (Enzymes are biological catalysts that enable life’s chemical reactions to take place.) The digestible complex carbohydrates include starch and glycogen. Starch is the storage form of glucose in plants; glycogen is the storage form of glucose in animals.

There are various indigestible complex carbohydrates, which are collectively called fiber. In fact, the definition of fiber is simply the edible but indigestible parts of plants, since fibers aren’t necessarily complex carbohydrates. But most fibers, are chains of sugar or sugar-like substances. Fibers include a wide variety of substances, broadly classified according to whether they dissolve in water —soluble fibers and insoluble fibers.

Insoluble fibers include cellulose, lignin, and some hemicellulose. These form the structural components of plants and add bulk and softness to stools. Soluble fibers include pectin, gums, and some hemicellulose. Many of these impart gel-like qualities to food, and can bind to bile products in the intestine. All plants contain an intimate mixture of various fibers which vary in their physical properties, health effects, and how well they are fermented by bacteria in the colon. Carbohydrate-rich foods—plant foods—are the staple food for most of the world’s population. The typical American diet—a high-fat, low-fiber diet—can be improved by including more of these plant foods.

Carbohydrates have a caloric value of 4 calories per gram, and must be broken down to single sugars (monosaccharides) before they are absorbed from the digestive tract. Practically speaking, fiber, indigestible as it is, has no caloric value (though short-chain fatty acids produced from some fibers by bacterial in the colon do). Alcohol, a fermentation product of carbohydrate, has 7 calories per gram.