After completing the tests, the child will begin treatment with metformin or placebo, plus a daily vitamin tablet. Participants will be followed once a month with a brief history and physical examination, including a blood test. After 6 months, all of the tests described above will be repeated. All children who complete the second round of tests-both those who took metformin and those who took placebo-will be offered metformin for an additional 6 months and will be seen once a month for follow-up evaluations. Parents will not be told which children received metformin and which received placebo until all children in the study complete the first 6 months of the trial.