Checking Out Carbohydrates

Carbohydrates come in three varieties: simple carbohydrates, complex carbohydrates, and dietary fiber. All are composed of units of sugar. What makes one carbohydrate different from another is the number of sugar units it contains and how the units are linked together.

• Simple carbohydrates: These carbohydrates have only one or two units of sugar.
• A carbohydrate with one unit of sugar is called a simple sugar or a monosaccharide (mono = one; saccharide = sugar). Fructose (fruit sugar) is a monosaccharide, and so are glucose (blood sugar), the sugar produced when you digest carbohydrates, and galactose, the sugar derived from digesting lactose (milk sugar).
• A carbohydrate with two units of sugar is called a double sugar or a disaccharide (di = two). Sucrose (table sugar), which is made of one unit of fructose and one unit of glucose, is a disaccharide. _ Complex carbohydrates: Also known as polysaccharides (poly = many), these carbs have more than two units of sugar linked together. Carbs with three to ten units of sugar are sometimes called oligosaccharides (oligo = few).
• Raffinose is a trisaccharide (tri = three) that’s found in potatoes, beans, and beets. It has one unit each of galactose, glucose, and fructose.
• Stachyose is a tetrasaccharide (tetra = four) found in the same vegetables mentioned in the previous item. It has one fructose unit, one glucose unit, and two galactose units.
• Starch, a complex carbohydrate in potatoes, pasta, and rice, is a definite polysaccharide, made of many units of glucose. Because complex carbohydrates are, well, complex, with anywhere from three to a zillion units of sugars, your body takes longer to digest them than it takes to digest simple carbohydrates. As a result, digesting complex carbohydrates releases glucose into your bloodstream more slowly and evenly than digesting simple carbs. (For more about digesting carbs, see the section “Carbohydrates and energy: A biochemical love story,” later in this chapter.)
• Dietary fiber: This term is used to distinguish the fiber in food from the natural and synthetic fibers (silk, cotton, wool, nylon) used in fabrics. Dietary fiber is a third kind of carbohydrate.
• Like the complex carbohydrates, dietary fiber (cellulose, hemicellulose, pectin, beta-glucans, gum) is a polysaccharide. Lignin, a different kind of chemical, is also called a dietary fiber.
• Some kinds of dietary fiber also contain units of soluble or insoluble uronic acids, compounds derived from the sugars fructose, glucose, and galactose. For example, pectin — a soluble fiber in apples — contains soluble galacturonic acid.
• Dietary fiber is not like other carbohydrates. The bonds that hold its sugar units together cannot be broken by human digestive enzymes. Although the bacteria living naturally in your intestines convert very small amounts of dietary fiber to fatty acids, dietary fiber is not considered a source of energy.