This clinical trial is being done to determine if metformin, a drug which is normally used in diabetes, can reduce atrial fibrillation in patients who are having an ablation for atrial fibrillation (AF). Atrial fibrillation is an abnormal heart rhythm which research has shown is related in part to obesity and diabetes.

It is anticipated that the participants treated in the metformin arm will have greater freedom from recurrent atrial arrhythmias after ablation.

Eligible participants enrolled in the trial will be assigned to one of the treatment arms (no treatment or metformin) and have follow-up visits up to approximately 1 year after the ablation. Additionally, all patients will also receive education on lifestyle changes and exercise which are standard of care.

Inclusion Criteria:

• Body Mass Index (BMI) >25 kilograms / square meter (kg/m2) with plan for rhythm control of atrial fibrillation by catheter ablation
• All subjects must be able to understand and willing to sign a written informed consent document.

Exclusion Criteria:

• Individuals who are already taking metformin or other antidiabetic medications, including insulin
• Known diabetes
• Known allergy or Food and Drug Administration (FDA)-labeled contraindication to taking metformin (estimated glomerular filtration rate (eGFR)<30 millilitres per minute (mL/min)/1.73 square meters (m2), hypersensitivity to metformin, acute or chronic metabolic acidosis)
• Patients taking carbonic anhydrase inhibitors
• eGFR below 30 mL/min per 1.73 m2 or other clinical diagnosis of advanced renal disease
• Acute or chronic metabolic acidosis (serum bicarbonate <22 milliequivalents per liter (mEq/L))
• History of significant alcohol use (>2 drinks/day on average)
• History of hepatic dysfunction (serum bilirubin 1.5 times greater than ULN)
• History of New York Heart Association (NYHA) Class III or IV heart failure
• Pregnancy or nursing