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Carbohydrates are the sugars, starches and fibers found in all plant foods, including fruits, vegetables, legumes, beans and whole grains. They are also found in some dairy foods, such as milk. The body converts carbohydrates into a simple sugar called glucose for immediate energy. Carbohydrates not needed for immediate energy are converted into a substance called glycogen and stored in the body for reserve energy.

Carbohydrates provide the main source of energy to the body and are broken down during digestion into glucose. After glucose is absorbed into the bloodstream, a hormone called insulin moves it into the cells, where it is converted to energy. Fibers are not broken down and digested by the body, but they do provide numerous health benefits such as improved gastrointestinal function.

Carbohydrates also are the source of glycogen. This substance is stored primarily in the liver and muscles and can be broken down into glucose when the body does not receive enough carbohydrates from outside food sources. For example, glycogen stores may be called upon to provide energy during bursts of extra activity. The body can store about 400 grams of glycogen at any one time. When carbohydrate intake produces more glucose than the body can use and store, the excess glucose is turned into body fat.

The body gains energy by breaking down three major classes of foods. These groups, known as macronutrients, include fats, proteins and carbohydrates. All carbohydrates are made up of sugar molecules composed of various combinations of chains of carbon, hydrogen and oxygen molecules.

When a person consumes carbohydrates, the process of breaking down these substances begins in the mouth, with the help of amylase, an enzyme in saliva. The process continues in the stomach and then into the small intestine, where foods are broken down into simple sugars. nutrients are then absorbed and then moved by blood to the liver. In the liver, the substance undergoes the final conversion to glucose, which is then carried into the cells with the help of insulin (hormone secreted by the pancreas) to fuel body activities. Such activities include autonomic functions such as breathing, digestion and heartbeat, and forms of exertion such as running and lifting. In addition, glucose is the main fuel source for the brain.

To meet the body’s daily nutritional needs while minimizing risk for chronic diseases such as heart disease, the Institute of Medicine (IOM) dietary reference intakes report on macronutrients recommends that adults consume 45 to 65 percent of their daily calories from carbohydrates.