Fructose is converted to glucose after phosphorylation. Although two enzymes are available for the phosphorylation of fructose, one of these, fructokinase, is present only in the liver. Hexokinase can catalyze the phosphorylation of fructose. However, fructokinase is a much more active enzyme. Its activity is so high that, in fact, most of the dietary fructose, whether as the free sugar or a component of sucrose, is metabolized in the liver. This is in contrast to glucose, which is metabolized by all the cells in the body. As a result, fructose- or sucrose-rich diets fed to rats or mice will result in a fatty liver. This occurs because the dietary overload of fructose or sucrose exceeds the capacity of the liver to oxidize it, so the liver uses sugar metabolites as substrates for fatty acid and triacylglyceride synthesis. Until the hepatic lipid export system increases sufficiently to transport this lipid to the storage depots, the lipid accumulates, hence, the occurrence of fatty liver. Adaptation to a high-fructose intake can and does occur in normal individuals.