5.3: Complex Carbohydrate

Table 5-1: Approximate Composition of Grains, Beans, and Tubers

Starch

A complex carbohydrate is made of three or more sugars linked together—a complex of many sugars. Starch, found in plants, is the most common one. It’s made of hundreds to thousands of glucoses linked together—a very complex carbohydrate!

The two forms of starch are amylose and amylopectin. In amylose, the glucoses are linked in a straight chain; the chain in amylopectin is highly branched (Figure 5.1). Amylose is a better thickener than amylopectin; starches higher in amylose (e.g., cornstarch) are good in thickening gravy. Amylopectin is “stickier” than amylose. “Japanese rice” (rich in amylopectin) is stickier (and works better in sushi) than “Chinese rice.”

Plants store energy as starch, so starch is found mostly in seeds (e.g., rice, beans) and tubers (e.g., potato), major sources of calories for people worldwide. Compositions of some staple foods are given in Table 5-1.

Grains (seeds) are about 75% starch (dry weight). Seeds are quite dry, so they store well. Refined grain (e.g., white rice) stores even better because most of the germ and outer coat, which are more susceptible to spoilage and more likely to be eaten by vermin, is removed. Rice is the staple food for about half the world’s population. Wheat and corn (maize) are two other predominant staple grains.

Beans (also seeds) are about 60% starch and about 25% protein (more than twice the protein of grains). Beans (also called legumes) are an important source of protein for vegetarians. Lima beans, chick peas (garbanzo beans), lentils, and green peas are beans/legumes. Soybeans are also, but are higher in protein and fat and lower in carbohydrate.

Tubers such as potatoes, yams, and cassava also are rich in starch. The 20% starch content of cooked potatoes doesn’t sound high, but it only seems low because potatoes are 75% water. If dried to about the same water content as grains and beans, it would be about 80% starch. Cassava is a staple food in some countries where there’s little else to eat. It’s very low in protein, and protein deficiency is common in these countries.

Glycogen

Glycogen is the animal version of starch (animal starch); it’s made entirely of glucose. Its structure resembles amylopectin in starch but with even more branches (Figure 5.1). It’s a negligible source of energy in food (trace amounts in meat), but an important source of energy within the body. Its concentration is low in tissues, but the body has a lot of tissue, so it has a sizable store of glycogen and potential energy. We store about 300 grams (about 11 oz) of glycogen. Carbohydrate has 4 cal/g, so 300 g of glycogen provides 1200 calories.

We store glycogen in liver and muscle. In liver, glycogen is used to keep a steady level of glucose in the blood. In muscle, it’s used to fuel muscle action. Knowing how glycogen is used in the body makes it clear why its highly branched structure is so important: Glucose is released at the end of the chains, and the branching gives many more ends from which glucose can be released, allowing a very fast release of glucose. If the chain were a long single strand, as in amylose, there’d be only two ends from which to release glucose.

Fiber

Fiber is a general term for the indigestible parts of plants. All plants have fiber (it isn’t found in animal products). It’s indigestible, so it isn’t an energy-providing nutrient. Most fibers are made of many sugar or sugar-like molecules linked together. They’re indigestible simply because our digestive tract lacks the enzymes to break the bonds that link the sugars. Fiber isn’t necessarily fibrous, e.g., cellulose is fibrous whereas pectin isn’t.

Cellulose gives plants structural support. Like amylose in starch, cellulose is a straight chain of many glucoses linked together (Figure 5.1). But in cellulose, the connecting link is different. Our digestive enzymes can’t break this link, though they can break the one that links glucoses in amylose. Enzymes are very specific.

Some animals can break down cellulose. Termites make a feast of wood (mostly cellulose). A ruminant (e.g., cow) can use the cellulose in grass and hay because bacteria living in its rumen break the links in cellulose, enabling a cow to absorb the resulting sugars.* In other words, grass has calories for cows but not for us.

Like starch, cellulose absorbs water. Hemicellulose, another fiber, is especially good at this. Thus, eating foods rich in these fibers adds bulk and softness to the stools, helpful in preventing constipation. Prunes, peanuts, and bran are good sources of cellulose and hemicellulose.

Pectin is a fiber made of galactose and other less-familiar sugars. Apples, oranges, and carrots are good sources. Unlike cellulose (which doesn’t dissolve in water), pectin dissolves quite well. It gels, so is used to thicken jams and jelly. It can also bind to bile products in the intestine and carry them out in the stool (cellulose doesn’t do this). Lignin, another fiber, is also effective this way. As will be discussed in Chapters 6 and 8, bile-binding substances such as pectin and lignin in food can help lower blood-cholesterol. Apples are a good source of pectin, so perhaps it’s the pectin in an apple a day [that] keeps the doctor away.

^{*Ruminants are animals with several chambers in their stomach. One chamber is the rumen. Food in the rumen is regurgitated back into the mouth where it (the cud) is chewed a second time. This is why ruminating means thinking it over—to go over it again.}