Chemically, carbohydrates contain carbon, as well as hydrogen and oxygen in the same proportion as in water. Carbohydrates have been typically classified as simple carbohydrates that include monosaccharides and disaccharides and complex carbohydrates that include oligosaccharides and polysaccharides. Polysaccharides stored in plants are mainly referred to as starch, branched or unbranched chain of hundreds or thousands of glucose molecules linked together. There are two forms of starch digestible by humans: amylose and amylopectin. Glycogen is the only stored form of carbohydrates in humans. The main function of carbohydrates is to supply calories for use by the body. Certain tissues in the body, such as red blood cells, can use only glucose as fuel. A diet that supplies enough digestible carbohydrates to prevent the breakdown of proteins for energy needs is considered protein sparing. To many weight loss enthusiasts, carbohydrates are viewed as the "fattening" nutrient. Alcohol is a broad term for a class of organic compounds that have common properties.