Sugar is an important organic compound widely distributed in nature. Sucrose in daily consumption, starch in grains, cellulose in plants and glucose in human blood are all sugars. Sugar plays an important role in life activities and is the main source of energy needed by all living things to maintain life activities. The most important sugars in plants are starch and cellulose, and the most important polysaccharide in animal cells is glycogen.

A variety of foods are rich in sugar, including fruits, soft drinks, bread, pasta, beans, potatoes, rice bran, rice and wheat. Sugar is a common energy source in living things, but it is not a necessary nutrition for human beings. Sugar is not an essential component of any other molecule, and the human body can also get energy from protein and fat. Generally, the brain and cranial nerves cannot burn fat to obtain energy. However, glucose or ketose can also be used instead. During gluconeogenesis, the human body can synthesize some glucose from specific amino acids, glycerol skeleton in triglycerides or fatty acids. Sugar contains 15.8 kilojoules (3.75 kilocalories) per gram, while protein can provide 16.8 kilojoules (4,000 kilocalories) and fat can provide 37.8 kilojoules per gram.