Effectiveness of the Optifast Program Compared With a Reduced-energy Food Based Diet Plan on Body Weight

Inclusion Criteria: * Adult males and females between 18 and 70 years * Obese (BMI \> 30 kg/m2 and 55 kg/m2) * Non-smokers or smoking cessation \> 6 months * \< 14 alcoholic beverages per week * Willing and able to give informed consent Exclusion Criteria: * Active participation in any weight loss program within previous 3 months * Weight changes of \> 5% body weight within previous 3 months * Participated in an Optifast program within prior 5 years * Prior bariatric surgery or liposuction * Use of any medication prescribed for weight loss in the past 3 months * Current major disease or GI disease that is poorly controlled (Crohn's, ulcerative colitis) * Type 1 DM * Current ESRD * Current COPD * Any major or active hepatic disease requiring inpatient or outpatient treatment * History of acute pancreatitis in the past year * Active cancer treatment in the past 2 years other than non-malignant skin cancers * Uncontrolled hypertension (Blood pressure 160/100 or greater) * Hemoglobin A1c \> 10% * Recent CV event in past 6 months * Pregnancy, childbirth, or nursing within prior 6 months * Eating Attitudes Test (EAT-26) \> 30 * Current major depressive disorder with Center for Epidemiological Studies Depression Scale-Revised (CESD-R) score \> 16 * Schizophrenia, history of bipolar disorder * Recent hospitalization for psychiatric illness in past 6 months * Dependence on alcohol or sedative-hypnotic drugs * Intolerance or allergy to Optifast product * Unable to read/speak English * Orthopedic limitation preventing participation in regular physical activity * Untreated thyroid disease, abnormal TSH, non on stable dose of hormone replacement for hyperthyroidism * Major surgery defined as any surgical procedure that might require prolonged convalescence or limit participation in the program in any way * Inability to complete the 7-day run-in satisfactorily