This project is aimed at determining whether an exercise-based weight loss intervention causes a compensation in some component of energy expenditure such that the increase in measured energy expenditure is less than the added exercise. The study will compare two separate exercise interventions to determine if this is influenced by exercise intensity.

The primary aim of this proposed study is to determine how total daily activity thermogenesis (TDAT) and total daily energy expenditure (TDEE) are regulated in obese humans attempting to lose weight and maintain weight loss through exercise. Two exercise interventions will be compared; a structured aerobic exercise program and a walking program. exercise activity thermogenesis (EAT) will be verified, and non-exercise physical activity (NEAT), total daily activity thermogenesis (TDAT), and TDEE will be measured using state-of-the-art technologies. Subjects will be studied during a 6 month intervention period and then during a 6 month follow-up period. It is hypothesized that the walking program will have more favorable effects on TDAT and TDEE during both the intervention and follow-up periods. If the investigators hypotheses are correct, then data from the proposed study could be used to improve the effectiveness of exercise-based weight loss and weight loss maintenance programs.

Inclusion Criteria

• Body mass index (BMI) 30-35.0 kg/m2
• Age 18-45 years
• Weight stable (<2 kg weight fluctuation during previous 6 months)
• No regular exercise (> 1 bout of exercise/wk or participation in any sporting activities > 1 hr/wk)
• No self-report of acute or chronic disease (diabetes, heart diseases, and joint problems in particular)
• No current use of prescribed medications
• No plans to relocate within the next year
• No plans for extended travel (> 1 week) within the next 6 months
• No tobacco use

For Females:

• No evidence of amenorrhea (Regular menstrual cycles of 21-35 days)
• Pre-menopausal status (self-report, to be confirmed during screening)
• Pregnancy or lactating within the past year
• No pregnancy or planned pregnancies; sexually active women of childbearing potential may be enrolled if they have had a tubal ligation or use one of the following means of contraception: condom, diaphragm, oral or implanted contraceptives, or intrauterine device. Women in exclusive relationships with male partners who have had a successful vasectomy will not be required to use any additional means of birth control.

Exclusion Criteria:

• Claustrphobia;
• Any contraindication(s) to > MRI, e.g. aneurysm, cochlear implant, cardiac pacemaker, or implantable cardiac defibrillator, artificial heart valve, metallic stents, any implantable devices or shunt, weight less than 300 lbs;
• Left Handed.