This is a 12-month, single center, three-arm parallel design, double-blind, randomized clinical trial, to compare the effects of supplemental dietary fiber and metformin (MET) alone and in combination over 12 months on glucose metabolism (insulin resistance [IR]), inflammation and BMI in adolescents with obesity and IR, and to assess the relationship between therapeutic intervention(s) and changes in gut microbiome composition and function.

Since MET and FIBER have been shown to reduce weight and increase insulin sensitivity through distinct but overlapping mechanisms of action, our central hypothesis is that the combination of FIBER + MET will have a synergistic effect and be more effective than FIBER or MET alone in improving metabolic function (IR) and reducing BMI and inflammation in adolescents with obesity, IR and family history (FM) of T2DM.

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Inclusion Criteria:

1. Age 12-18 years
2. BMI percentile > 95% for age/sex;
3. Total weight fluctuation over past 6 months < 10%;
4. HOMA-IR > 3.16;
5. FH of T2DM (first or second-degree relative).

Exclusion Criteria:

1. Current use of insulin or diagnosis of T2DM;
2. Systolic or diastolic blood pressure (BP) > 99th percentile for age and sex;
3. Acute infectious or inflammatory condition over the preceding 1 month; hospitalization > 48 hrs;
4. History of chronic disease such as inflammatory bowel disease, chronic severe liver or kidney disease or neurologic disorders;
5. Active malignancy;
6. Concomitant use of medication/investigational drug known to affect body weight in the past year;
7. Antibiotic use in past 60 days; probiotic and/or prebiotic supplements use in the past 30 days; use of lipid-lowering and anti-inflammatory medication.

Participants and research team will be blinded to the type of intervention allocated to each arm until the end of the study. Concealment will be protected by secure, password protected website. The group not on MET (fiber alone) will receive a placebo pill. The group not on fiber will also receive a placebo, composed of the same base solution without added fibers.