Insulin Clamp Ancillary study for Assessment of Insulin Resistance will be conducted as a sub-study to the main protocol, Metformin Therapy for Overweight Adolescents With Type 1 Diabetes (NCT01881828). The purpose of this ancillary study is to assess if metformin will improve tissue-specific insulin resistance (IR) in type 1 diabetes using a hyperinsulinemic euglycemic clamp; a 3-month randomized trial comparing metformin versus placebo in 12 to <20 year olds with BMI >85th percentile ( total daily insulin dose >0.7 units/kg, and HbA1c 7.0%-9.9%).