The Vascular Effects of Exenatide Versus Metformin in Patients With Pre-Diabetes

Inclusion Criteria: * Greater than or equal to 18 years old * Impaired glucose tolerance: 2-hr oral glucose tolerance test (OGTT) plasma glucose \>140 mg/dL OR impaired fasting glucose: fasting glucose \> or = 100 mg/dL OR elevated glycosylated hemoglobin: Hemoglobin A1c \> or = 5.7% * Abdominal obesity: waist circumference \>102 cm (men) and \>88 cm (women) * Stable cardiovascular medication regimen (or other medications known to affect endothelial function) at least 1 month prior to enrollment and throughout the study Exclusion Criteria: * Type 2 diabetes * Current use of glycemic control medications within one month of randomization * Fasting glucose \>126 mg/dL * Current use of weight loss medication * Previous weight loss surgery * History of severe gastrointestinal disease * Standard clinical contraindications to exenatide or metformin therapy * Unstable angina * Heart failure * Stroke or coronary artery bypass graft within 3 months of screening * Women who are currently pregnant or planning to become pregnant * Breastfeeding women * Clinically significant liver disease * Creatinine \> 1.5 mg/dL * Hepatic function greater than 3 times upper limit of normal * Patients who are mentally incompetent and cannot sign a Patient Informed Consent