Carbohydrates are widely distributed in plants and animals, where they fulfil both structural and metabolic roles. In plants, glucose is synthesise from carbon dioxide and water by photosynthesis and stored as starch or is converted to the cellulose of the plant framework. Animals can synthesise some carbohydrate from fat and protein, but the bulk of animal carbohydrate is derived ultimately from plants.
Knowledge of the structure and properties of the carbohydrates of physiologic significance is essential to understanding their fundamental role in the economy of the mammalian organism. The sugar glucose is the most important carbohydrate. It is as glucose that the bulk of dietary carbohydrate is absorbed into the bloodstream or into which it is converted in the liver, and it is from glucose that all other carbohydrates in the body can be formed. Glucose is a major fuel of the tissues of mammals (except ruminants) and a universal fuel of the fetus. It is converted to other carbohydrates having highly specific functions, eg., glycogen for storage; ribose in nucleic acids; galactose in lactose of milk, in certain complex lipids, and in combination with protein in glycoprotein and proteoglycans.
Most carbohydrates in the diet form glucose, galactose, or fructose upon digestion. These are transported to the liver via the hepatic portal vein. Galactose and fructose are readily converted to glucose in the liver.
Glucose is specifically required by many tissues but does not have to be provided as such in the diet, since other dietary carbohydrates are readily converted to glucose, either during digestion (eg., starch) or subsequently in the liver (eg., fructose, galactose. Glucose is also formed from the glycerol moiety of fats and from glucogenic amino acids by gluconeogenesis. However, a minimum daily intake of carbohydrate (50 g) is recommended in humans to prevent ketosis and loss of muscle protein.

Diseases associated with carbohydrates include diabetes mellitus, galactosemia, glycogen storage diseases, and milk intolerance.