This was a prospective, interventional study in obese adults ages 21 to 50 years of age to determine whether weight loss could improve obesity-related myocardial metabolism and efficiency. Two different mechanisms of weight loss were studied: diet and exercise and gastric bypass surgery. Positron emission tomography (PET) was used to quantitate myocardial oxygen consumption (MVO2) and myocardial fatty acid (FA) metabolism. Echocardiography with tissue Doppler imaging was used to quantify cardiac structure, systolic and diastolic function (left ventricular (LV) relaxation (E') and septal ratio (E/E')).