One of three types of nutrients used as energy by the body (the other two are protein and fat). Sugars are the simplest structures, while fiber and starches are more complex. The digestive tract breaks down most sugars and starches into glucose, a simple sugar that the body can use to feed its cells. Dietary intake of fiber-rich carbohydrates can lower blood cholesterol when substituted for saturated fat. The energy produced by carbohydrates is four calories per gram.