Participation in this double-blind study will last between 4 and 6 months. First, participants will undergo a medical screening and medication adjustment period, expected to last 6 months. The medical screening, lasting about 1 hour, will involve completing a medical history, physical exam, pregnancy test if applicable, and blood test to measure various factors that contribute to diabetes control. Participants will also be asked permission to store 1 tablespoon of their blood for up to 10 years to be used in future studies concerning genetics and heart energy metabolism. During the medication adjustment period, study physicians will adjust the participants' medications, offer advice on diabetes education and nutrition, and record any side effects from the medications. For newly diagnosed T2DM participants, the study physician may recommend medication changes to assure a hemoglobin A1c (HgA1c) level of less than 7.5%. If participants have already achieved this level, they will be asked to continue present medications and to also begin taking the medication metformin for the 30 days before they undergo several imaging studies. Next, participants will complete routine tests that evaluate the pumping function of the heart, including an electrocardiogram (ECG) performed before and during exercise, a body composition study using a dual energy x-ray absorptiometry (DEXA) scan, and a magnetic resonance imaging (MRI) test. Each of these tests will last between 30 and 90 minutes.