Adaptive Mechanisms Responsible for Weight Change in Youth With Obesity

In this study, doctors want to find out more about why people who lose weight often regain the weight that they have lost once they resume a regular diet and whether hormones might play a role in weight regain. The study is divided into two parts, called the meal replacement period and the follow-up period. The meal replacement period will consist of drinking a shake for breakfast and lunch and eating a frozen meal for dinner that is calorie controlled. Individuals will also be asked to eat two servings of fruit and three servings of vegetables each day. The study will provide the shakes and the frozen entrees, participants are asked to supply the fruits and vegetables.

Participation in this study will last for up to 35 weeks. There will be 10 in-person visits and 13 visits by phone or over Zoom over the 35 weeks.

Individuals who are found to be eligible to participate will have two study interventions: An 8-week meal replacement therapy period in which they are asked to reduce their BMI by \>5%. Participants will be asked to strictly follow the individually-prescribed eating regimen which will include meal replacement shakes and/or frozen meals to be used for breakfast and lunch. For dinner, the study will provide pre-packaged frozen entrée meals to be consumed with two servings of fruit and three servings of vegetables per day. Meal replacement compliance will be assessed by requiring participants to maintain a dietary log throughout the study. The percentage of days for which the protocol was followed (no additional calories consumed) will serve as the primary metric of compliance.

Participants will also receive a standardized lifestyle/behavioral modification counseling every 2 weeks throughout the entire study, which will be delivered at each in-person study visit and via virtual platform of phone when there is no in-person study visit scheduled. The lifestyle curriculum will incorporate evidence-based behavior change principles including dietary modification, energy expenditure modification, behavior modification and family involvement and support. To ensure fidelity and adherence to the delivery protocol, a manual will be developed for each session that trained staff will follow when administering the curriculum.

Eligibility Criteria:
Inclusion Criteria:

* 11 to less than 16 years old
* BMI \> 30 kg/m\^2 or 95th BMI percentile
* Tanner stage 2, 3, or 4

Exclusion Criteria:

* Tanner stage 1 and 5
* Prior bariatric surgery
* Current or recent (\< 3 months prior to enrollment) use of anti-obesity medication(s) defined as orlistat, metformin, phentermine, topiramate, combination phentermine/topiramate, liraglutide, and/or combination naltrexone/bupropion (monotherapy use of naltrexone or bupropion is not an exclusion)
* Monogenic and hypothalamic obesity
* Polycystic ovary syndrome (diagnosed by a physician)
* Pregnancy or planned pregnancy
* Current use of supplemental hormones
* Individuals with a diagnosed eating disorder of anorexia nervosa, bulimia or binge eating disorder
* Type 1 or 2 diabetes
* Treatment with growth hormones
* Thyroid disease/problem
* Has had cancer in the last 10 years