Participants coming on study will be stabilized to a standardized metformin dose: this may take about 10 weeks, and then combination therapy with metformin + sitagliptin will begin during Phase 1 (Week 0 through Week 20). If a participant has already been on a stabilized metformin dose, they will start immediately on combination therapy with metformin + sitagliptin for 20 weeks (Phase 1).