Type I diabetes Myocardial infarction or stroke within the previous 6 months Clinically significant renal or hepatic disease (as judged by a study physician) History of cancer in the past 5 years Congestive heart failure requiring diuretics Previous weight loss surgery Endocrine conditions that may cause weight gain, including unstable thyroid disease or hypercortisolism Initiation of therapy with or dose change to any of the following medications within 6 weeks of starting the study: insulin; metformin; sulfonylurea; thiazolidinedione; HMG-CoA reductase inhibitor ("statin"); or SSRI for depression Long-term use of any of the following medications: oral or high-dose inhaled steroid; atypical antipsychotic; tricyclic antidepressant; antiepileptic; any prescription or over-the-counter drug for weight loss Systolic blood pressure > 160 or diastolic blood pressure > 100 Hemoglobin A1c ≥ 10 Pregnancy or lactation Clinically significant psychiatric disease, including major depression, eating disorders, or substance abuse