Association Between Serum Vitamin D Levels and Metformin Efficacy in Obese Type 2 Diabetes Patients: A Retrospective Cohort Study Based on HbA1c Reduction. - Wang, Yimeng; Liu, Hongli; Li, Yijia; Wang, Xiaoxue; He, Chen; Wei, Zhi (Therapeutics & Clinical Risk Management, 2026)