Carbohydrate is the major source of energy for both cellular metabolism and one’s diet. Carbohydrates can be either “simple” or “complex”. Simple carbohydrates– for which food science gives the more colloquial name “sugar”–are small in size, making them also sweet to the taste. The two classes of simple carbohydrates are called monosaccharides and disaccharides. As indicated by their prefixes, a monosaccharide comprises 1 sugar unit, while a disaccharide comprises 2.
In contrast, complex carbohydrates contain more than two sugar units. For this reason, they are called “polysaccharides” and are primarily chains of glucose. Unlike monosaccharides, they are larger in structure and not sweet to the taste. Plants store energy in the form of complex carbohydrates or what we commonly call “starch.” Starch is only found in plant products and some of its more familiar sources include all grains, potato, legumes (beans), and many vegetables.
Two major dietary sources of carbohydrate are the food groups “fruits” and “vegetables”. Although both contain large amounts of carbohydrates, the ripening process for each is different. In fact, they are inversely related. So, for instance, when a fruit first buds on a plant, it is primarily a complex carbohydrate. As it ripens, however, it transforms into a simple carbohydrate. On the other hand, vegetables at the start of the ripening process are simple and then ripen to complex. Thus, when they are ripe, fruits taste sweet, whereas vegetables taste sweet when they are younger and not ripened.
The one type of carbohydrate and polysaccharide that human beings do not digest is called fiber. Because we do not digest it, carbohydrate as fiber does not supply energy. For instance, some fibers are metabolized by the bacteria found in the large intestines. In this way, they are thought to be advantageous to the gut bacteria. The amount of fiber in food tends to increase with age of plant product. “Dietary fiber” is fiber found naturally in food. For a variety of reasons, fiber is also added to foods where it is not normally found. For example, manufacturers can add fiber to non-fat yogurt, which makes the yogurt thicker than non-fat yogurt without fiber.
Fiber can either be insoluble or soluble, depending upon how it interacts with water. Insoluble fiber does not dissolve in water and is therefore not metabolized by the bacteria in the large intestines. In contrast, soluble fiber dissolves in water, which leads to swelling and is metabolized by the bacteria in the large intestines.