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Carbohydrates provide a great part of the energy in most human diets. Foods rich in carbohydrates are usually the most abundant and cheapest, when compared with foods high in their protein and fat content. Carbohydrates are burned during metablism to produce energy, liberating carbon dioxide and water. Humans also get energy less efficiently from fats and proteins in the diet, and also from alcohol. The two kinds of carbohydrates are starches, which are found mainly in the grains, the legumes, and the tubers, and sugars, which are found in plants and fruits. Carbohydrates are used by the cells in the form of glucose (q.v.), the body's main fuel. After absorption from the small intestine, glucose is processed in the liver, which stores some as glycogen, starch-like substance, and passes the rest into the bloodstream. In combination with fatty acids, glucose forms triglycerides, which are fat compounds that can be easily broken down into combustible ketones.|
Glucose and triglycerides are carried by the bloodstream to the muscles and organs to be oxidized, and excess quantities are stored as fat in the adipose and other tissues, to be retrieved and burned at times of low carbohydrate intake. The carbohydrates containing the most nutrients are the complex carbohydrates, such as unrefined grains, tubers, vegetables, and fruit, which also provide protein, vitamins, minerals, and fats. A less beneficial source is foods made from refined sugar, such as candy and soft drinks, which are high in calories but low in nutrients and fill the body with what nutritionists call empty calories.
Carbohydrates are found in the Grain Food Group. Foods like whole grain breads and cereal, and rice and noodles, are rich sources of carbohydrate.