Although carbohydrates are the most abundant compounds on earth, they normally constitute only about 1% of the mammalian organism. At rest, the central nervous system consumes more carbohydrate than any other organ system. Mammalian enzymes involved in carbohydrate metabolism recognize only the D-isomers. Fructose is a 6-carbon ketose found in sucrose. Maltose is an intermediate in the intestinal digestion of glycogen and starch. Monosaccharides such as galactose, glucose, ribose, and fructose usually exist in ring rather than linear forms. Reduction of NAD+ and NADP+ by dehydrogenases changes the character of the nicotinamide ring, with resultant changes in absorption spectra.