The purpose of the study is to evaluate whether the addition of Diamel, a nutritional supplement, to hypocaloric diet and exercise could improve the histological results (steatosis, necro-inflammatory activity and fibrosis), insulin resistance, aminotransferase levels and anthropometric measures in comparison with a placebo-controlled group with hypocaloric diet and exercise during 52 weeks of treatment in patients with nonalcoholic steatohepatitis.