c57bl/6 mice develop signs and symptoms comparable, in part, to the human metabolic syndrome. the objective of the present study was to evaluate the effects of exercise training on carbohydrate metabolism, lipid profile, visceral adiposity, pancreatic islet alterations, and nonalcoholic fatty liver disease in c57bl/6 mice. animals were fed one of two diets during an 8-week period: standard (sc, n = 12) or very high-fat (hf, n = 24) chow. an exercise training protocol (treadmill) was then established and mice were divided into sc and hf sedentary (sc-sed, hf-sed), exercised groups (sc-ex, hf-ex), or switched from hf to sc (hf/sc-sed and hf/sc-ex). hf/hf-sed mice had the greatest body mass (65% more than sc/sc-sed; p < 0.0001), and exercise reduced it by 23% (p < 0.0001). hepatic enzymes alp (+80%), alt (+100%) and ast (+70%) were higher in hf/hf mice than in matched sc/sc. plasma insulin was higher in both the hf/hf-sed and hf/sc-sed groups than in the matched exercised groups (+85%; p < 0.001). pancreatic islets, adipocytes and liver structure were greatly affected by hf, ultimately resulting in islet β-cell hypertrophy and severe liver steatosis. the hf group had larger islets than the sc/sc group (+220%; p < 0.0001), and exercise significantly reduced liver steatosis and islet size in hf. exercise attenuated all the changes due to hf, and the effects were more pronounced in exercised mice switched from an hf to an sc diet. exercise improved the lipid profile by reducing body weight gain, visceral adiposity, insulin resistance, islet alterations, and fatty liver, contributing to obesity and steatohepatitis control.