Carbohydrates contain four calories per gram and are the main energy source for the body. When three or more 6-carbon sugar molecules are joined, the resulting molecule is known as a complex carbohydrate. One or two 6-carbon sugar molecules linked together comprises a simple sugar. Complex carbs are further sub classified into fibrous and starchy carbohydrates.
When consumed, simple sugars like sucrose and dextrose, as well as refined complex carbohydrates like white flour, provide a burst of energy which often gives way to feelings of lethargy. Typically, unrefined complex carbohydrates are assimilated by the system more slowly than simple sugars and will provide constant and sustained (though less intense) energy levels.
By Dean Ash