Any compound containing carbon, hydrogen, sometimes oxygen. There is considerable variety in these compounds and only some of them are available to humans as a fuel source. Cellulose, for instance, is a carbohydrate, but humans and all non-cud chewing animals (except termites and some microorganisms) can make no use of it. Only a few of the simple sugars (mono-saccharides) and even fewer of the di-saccharides (e.g., lactose) in food are available to humans. Others, if they contribute to human nutrition, do so after processing by intestinal bacteria (some estimates are that, in humans, less than 10% of caloric benefit comes from fermentation in the large intestine, as contrasted to other primates in which the proportion is rather larger). Most carbohydrates that can be absorbed and used for fuel by humans (e.g., starch and glycogen—both chains of glucose molecules).are eventually broken down to glucose during digestion. They eventually are part of the primary metabolic control mechanism.

Fructose, on the other hand, is a carbohydrate which is neither; it is absorbed, but in humans processed only in the liver and in sperm, the only tissues which have the required enzymes. Ingested carbohydrate calories in non glucose forms are, in a special sense, invisible to the body. There has been recent speculation that the increase in such carbohydrates since sucrose (table sugar—half glucose and half fructose) became available in quantity after about 1700 with the discovery of a practical source (sugar cane), accounts for some of the diseases of civilization, including diabetes. Fructose had been quite rare in human diet until that time. Dietary fructose also characteristically causes alterations in blood lipid profiles, probably by changing liver operations.