Optimal Timing of Exercise Initiation Within a Lifestyle Weight Loss Program

Inclusion Criteria: * Age 18-55 * Body Mass Index 27-42 kilograms/meters squared * Sedentary: defined as regular exercise of less than 100 minutes per week at moderate intensity or greater during the previous 6 months * No self-report of acute or chronic disease (heart disease, diabetes, gastrointestinal disorders and orthopedic problems in particular) * No plans to relocate or for extended travel (more than 1 week) within the next 18 months * No current tobacco use * Capable and willing to give informed consent, understand exclusion criteria, and accept the randomized group assignment * Live or work within 20 minutes of the University of Colorado Anschutz Health and Wellness Center to ensure reasonable ability to comply with requirement to attend group weight loss sessions and supervised exercise sessions. Exceptions can be made at the discretion of the Study Principal Investigator (PI) on a case by case basis for highly motivated subjects. * For Females * Not currently pregnant or lactating * Not pregnant within the past 6 months * Not planning to become pregnant in the next 18 months * Sexually active women of childbearing potential may be enrolled if they have had a tubal ligation or use a reliable means of contraception. Exclusion Criteria: * Presence or history of health problems which would affect appetite, food intake, energy metabolism, or ability to optimally participate in the exercise component including: diabetes, cardiovascular disease, peripheral vascular disease, cerebrovascular disease, uncontrolled hypertension, uncontrolled hyper or hypothyroidism, cancer (within the last 5 years, except skin cancer), human immunodeficiency virus (HIV) infection, significant gastrointestinal, pulmonary, renal, musculoskeletal, neurologic, or hematologic disease * Symptoms suggestive of cardiovascular disease: chest pain, shortness of breath at rest or with mild exertion, syncope * Abnormal resting electrocardiogram (ECG): serious arrhythmias, including multifocal Premature Ventricular Contractions (PVCs), frequent PVC's (defined as 10 or more per minute), ventricular tachycardia (defined as runs of 3 or more successive PVC's), or sustained atrial tachyarrhythmia; 2nd or 3rd degree A-V block, corrected QT interval \> 480 millisecond or other significant conduction defects * Triglycerides \> 400 milligram/deciliter (md/dL) or Low-Density Lipoprotein (LDL) cholesterol \> 190 mg/dL * Use of prescription or over-the-counter medications known to affect appetite, weight, heart rate response to exercise, or energy metabolism * Use of systemic steroids (other than Oral Contraceptive Pills) * Use of obesity pharmacotherapeutic agents within the last 6 months * History of surgical procedure for weight loss * History of major surgery within the past 3 months * Current alcohol or substance abuse * History of clinically diagnosed eating disorders including anorexia nervosa or bulimia * Current severe depression or history of severe depression within the previous year * History of other significant psychiatric illness (e.g. psychosis, schizophrenia, mania, bipolar disorder) which in the opinion of the Study MD would interfere with ability to adhere to dietary or exercise interventions * Currently participating in or planning to participate in any formal weight loss or physical activity programs or clinical trials * Weight loss or gain of \> 5% in past 6 months for any reason except post-partum weight loss * Participation in a weight loss program using the Colorado Weigh curriculum within the past 3 years (exceptions can be made at the discretion of the study PI on a case-by-case basis).