Epidemiological studies consistently show an association between sucrose or high fructose corn syrup intake and metabolic diseases. It is proposed that this relationship is mainly due to the fructose component of sugars. Prospective cohort studies show an association between consumption of added sugars containing fructose and body weight gain, and intervention trials suggest that the relationship is due to a fructose-induced increase in energy intake. The mechanisms through which fructose alters energy intake remain debated. In addition, many short term studies also indicate that dietary fructose exert specific metabolic effects in humans independently of changes in body weight. It increases fasting and postprandial blood triglyceride concentration, impairs hepatic insulin sensitivity, and increases uric acid and intrahepatic fat concentrations. It is proposed that excess fructose associated with excess energy causes a hepatic metabolic overload which may contribute to the development of non-alcoholic fatty liver disease, insulin resistance and cardiovascular diseases.