Complex carbohydrates are carbohydrates that contain at least three single sugar molecules. Plants use long chains of sugar molecules called starches to store energy.
When you eat those plants, your body breaks down the carbohydrates so you can use them for your body's energy needs.
Another 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. Vegetables like 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 -- they have the most 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.