The sum of the physical and chemical changes involved in the breakdown and synthesis of carbohydrates in the body. Carbohydrates are digested to hexose monosaccharides which are absorbed by the small intestine; the liver converts fructose and galactose to glucose. In the liver and muscles, glucose may be converted to glycogen. In all cells, glucose is oxidized to carbon dioxide and water, with energy released in the forms of ATP and heat. Lactic acid, a product of the anaerobic breakdown of glucose, causes muscle fatigue.