Carbohydrates are organic compounds containing carbon, hydrogen and oxygen. They are the human body�s key source of energy. Carbohydrates are the most abundant organic compounds in nature. The carbohydrate group consists principally of sugar, starch, dextrin, cellulose, and glycogen, substances that form an important part of the human diet and that of many animals. Within living organisms, carbohydrates serve both structural and energy-storage functions.