Obesity has been increasing all over the world. This has lead to a significant increase of a liver disease in children called non-alcoholic fatty liver disease (NAFLD). NAFLD is a liver disease that ranges from excess fat being stored in the liver to an inflamed and fatty liver with fibrosis to cirrhosis. NAFLD is thought to be caused by changes in energy, fat and carbohydrate metabolism induced by diets high in in processed foods. Sugary (especially high fructose corn syrup or HFCS) and fatty foods in processed foods have been shown to produce more insulin resistance, a factor that is thought to cause a fatty liver. Currently the main treatment for NAFLD is weight loss. However, it unknown the best way to achieve this. The investigator has shown previously that adolescents with NAFLD eat a lot of fatty and sugary foods, and that when they decrease the amount of foods they eat that contain HFCS, experience some improvements in insulin resistance and liver dysfunction even when they don't lose weight. The plan is to compare and contrast how two different diets (high vs low HFCS containing diets) may affect how much fat gets deposited in the liver and whether or not a lower diet in HFCS can help decrease liver damage in adolescents with NAFLD.