Complex carbohydrates are chains of three or more single sugar molecules linked together. Long chains of sugar molecules called starches serve as a storage form of energy in plants.
When you eat those plants, your body breaks down the carbohydrates so you can use them for your body's energy needs.
Another type of complex carbohydrate, called cellulose, forms the structures that give plants their shape, and it's the main component of dietary fiber.
Foods with large amounts of complex carbohydrates include legumes, starchy vegetables like potatoes and corn, rice and grain products. Other vegetables such as green beans, broccoli and spinach contain less starch, but they have more fiber.
Complex carbohydrates should supply about half the calories in your diet. The best sources are legumes, vegetables, breads, pasta and cereals. Choose 100-percent whole wheat or 100-percent whole grain products because they contain more fiber. The extra fiber slows down the absorption of the carbohydrates, so you'll feel full longer and be less likely to over-eat later.
Gropper SS, Smith JL, Groff JL. "Advanced Nutrition and Human Metabolism." Fourth Edition. Belmont, CA. Wadsworth Pub Co. 2005.