Metformin and Vascular Function in Prediabetes

The goal of this clinical trial is to investigate whether metformin improves vascular function in individuals with prediabetes. The main questions it aims to answer are: 1. Does metformin improve large conduit artery endothelial function in individuals with prediabetes? 2. Does metformin improve microvascular endothelial function in individuals with prediabetes? Researchers will compare metformin to a placebo to see if metformin improves vascular function in prediabetes. The trial duration is 12 weeks. Participants will take metformin or a placebo once a day for 2 weeks. After 2 weeks, participants will take metformin or a placebo twice a day for the remaining 10 weeks. There will be a screening visit, two baseline visits, a 4-week safety visit, and two 12-week end-of-study visits. Adherence will be calculated from pill count and adverse events will be quantified via a questionnaire.