Metformin is investigational for use in patients with atrial fibrillation. Metformin is approved by the Food and Drug Administration (FDA) for use in patients with type 2 Diabetes Mellitus. Metformin is used with a proper diet and exercise program to control high blood sugar, and has been shown to help people lose weight. This study aims to look at the effects of metformin on weight loss and heart rhythm in patients with atrial fibrillation.