Carbohydrates provide the quickest source of energy for the body and depending on the type of carbohydrate consumed, may also be a short-lived source of energy. When carbohydrates are digested, the body breaks down the carbohydrates into glucose, which is released into the bloodstream. Body tissues, such as the brain, use glucose as an energy source. If more carbohydrates are consumed than is required, the glucose will be turned into glycogen and stored in the muscles and liver for future use. When the liver and muscle tissues exceed their capacity for storing glycogen, the excess is converted into fat. Proponents of a low-carb lifestyle maintain that reducing the intake of carbohydrates causes the body to use stored fat for energy rather than creating additional fat.
Carbohydrates considered "good" (due to a higher nutritional value) are contained in foods that have not been processed and contain a fair amount of fiber. These foods include oatmeal, whole-grain bread, legumes, vegetables, fruit, and sugar-free whole grain cereals. Carbohydrates considered "bad" (because they are less nutritious) are contained in foods that have been processed (or refined). These foods include white bread, white pasta, ice cream, candy, and pop.