Using an observational case-control cohort design, the investigator aims to define the mechanisms underlying microbiome regulation of metabolism in adolescence before and after medically indicated therapies for obesity. The investigator's overarching hypothesis is that gut bacteria control host weight, insulin sensitivity, and response to weight-loss treatments by modifying host metabolism.

Intervention Cohorts: A prospective, longitudinal observational case control study will be performed. All subjects will receive Healthy Lifestyle Program (HLP) standard of care, which includes intensive lifestyle modification and free participation in Bull City Fit. Four groups of participants receiving obesity treatment will be enrolled: (1) HLP-only, (2) HLP + Low carbohydrate diet, (3) HLP + weight loss medication(s) and (4) HLP + Bariatric surgery. Consistent with the observational design of this study, study participants will be assigned to each group as per usual HLP clinical care, based on established standards and guidelines, expert medical provider recommendations, and family preference.

Comparison Cohort: two comparison groups be enrolled:(1c) healthy siblings of enrolled intervention subjects and (2c) age/sex matched controls recruited from the Duke Children's Primary Care Clinic.

Up to 350 participants will be enrolled, with the goal of 295 evaluable participant data. The study period is six months for all groups with the exception of the bariatric surgery group which will last from baseline until after surgery, up to 12 months. The comparison groups will have one visit at baseline only.

Stool Sample Collection: Participants in both intervention and comparison groups will provide a fecal sample at enrollment or within 3 days of enrollment. Participants in intervention groups (Groups 1-4) will provide additional fecal samples at 1.5, 3, 4.5, and 6 months. Given that the ability to collect fecal samples is unpredictable, the investigator will develop a standard operating procedure along the following guidelines:

1. At the first visit, if the subject cannot provide a sample at the time of enrollment, the investigator will give the participant/parent a cooler/ice pack and sample collection kit. The investigator will ask the subject to provide the "baseline" sample within 3 days of the first visit. The sample will be refrigerated for up to 24 hours prior to pick up by the clinical research staff or delivery by the parent/guardian, with compensation for miles. With pick up or delivery a new cooler and freezer pack as well as sample collection kit will be provided for the following sample collection.
2. Within a 2-week window, centered on collection points 1.5 and 4.5 months, the subjects will provide additional samples. They may either have the samples picked up within 24 hours of collection (with refrigeration from collection to pick up) or get reimbursed for mileage.
3. For the 3-month and 6-month collections, which occur in conjunction with a regularly scheduled Healthy Lifestyles visit, the patients will collect a sample within 24 hours prior to the visit and bring with them to clinic. At the 3-month visit they will be sent home with new collection kits, freezer packs, and coolers.
4. If a patient cannot provide a sample within 24 hours of a clinic visit or a sample is missed, we will still collect a sample as soon as the patient is able to provide it and arrange for pickup or delivery as outlined below, noting the sample as "out of window."

Plasma sample collection: Participants in intervention groups will provide a 20mL blood samples at baseline, 3 and 6 months at the time of a regular HLP visit. The standard HLP clinical protocol is to draw fasting labs at baseline and 6 months, so no additional blood draws beyond what is required for clinical care will be required at those visits. However, HLP patients do not typically have a blood draw at 3 months, so participants will be asked for a blood sample at 3 months they would not otherwise require. Participants in comparison groups (Groups 1c, 2c) will be asked for a blood sample at enrollment only; they would not otherwise require this sample.

Analysis of samples: Fecal samples will be analyzed for the composition of the microbiota using DNA sequencing methods. Specific bacteria associated with obesity will be isolated from fecal samples and studied in the laboratory for their metabolism. A subset of fecal samples will be transferred into the intestinal tracts of germ free mice to study the effects on their metabolism. Participant plasma will be analyzed for panels of metabolites that are associated with diabetes mellitus and obesity. Blood mononuclear cell DNA will be used for focused genetic testing to identify genetic changes associated with obesity.

Inclusion Criteria:

• Child aged 10-18 years
• Sexual Maturity Rating 2 or above (pubertal)
• Age- and gender-specific BMI ≥ 95th percentile
• Expected enrollment in Healthy Lifestyles Clinical Program > 6 months

Exclusion Criteria:

• Antibiotic use in the 1 month prior to expected enrollment
• Anticipated antibiotic use during the course of the study.
• Immunodeficiency or prior transplantation
• Type 1 diabetes mellitus
• Inborn error of metabolism
• No fluency in English or Spanish
• Identifiable cause of obesity including hypothyroidism, Cushing syndrome, known genetic obesity syndrome including melanocortin receptor 4 mutation, drug-induced obesity including steroids or anti-psychotics
• Significant medical or mental health condition that precludes treatment
• Receiving insulin
• Plans to move away from area in next 6 months
• Weight loss of >5% total body weight in prior 3 months
• Pregnancy (self-report, confirm with noted last menstrual period in chart)
• Home outside of a 50 mile radius from the HL Clinic.

Duke Healthy Lifestyles (HL) Clinic Patients with BMI>95%

1. c) Normal-weight siblings of patients presenting to the Duke Healthy Lifestyles Clinic
2. c) Normal weight age/sex-matched controls presenting for well-child care to Duke Children's Primary Care clinic