Serum 25-Hydroxyvitamin D Levels are Associated with Impaired Glomerular Filtration Rate in Type 2 Diabetes Patients

Background: Diabetic kidney disease (DKD) is a major microvascular complication of type 2 diabetes mellitus (T2DM) and a leading cause of end-stage renal disease (ESRD). Vitamin D deficiency is common in patients with T2DM and chronic kidney disease (CKD), however its association with DKD severity remains unclear. Objective: This study aimed to evaluate the relationship between serum 25-hydroxyvitamin D levels and the severity of DKD in patients with T2DM. Methods: This cross-sectional observational study included 135 participants: 115 patients with T2DM and 20 healthy controls. Diabetic patients were classified according to CKD stages and albuminuria grades based on kidney disease improving global outcome (KDIGO) 2023 guidelines. Clinical assessment and laboratory investigations were performed, including renal function tests, glycemic indices, lipid profile, calcium parameters, albumin-to-creatinine ratio (ACR), estimated glomerular filtration rate (eGFR), and serum 25(OH)D measured by ELISA. ？ Results: Serum 25(OH)D levels were significantly lower in T2DM patients than in controls (median 29.0 vs. 78.0 nmol/L; p < 0.001). Vitamin D levels declined progressively with advancing CKD stages and increasing albuminuria severity (p < 0.001). Serum vitamin D showed significant negative correlations with HbA1c, serum creatinine, blood urea, ACR, and lipid parameters, and positive correlations with eGFR, serum albumin, hemoglobin, and calcium. ROC analysis demonstrated excellent diagnostic performance of vitamin D for renal impairment severity (AUC = 0.998). Conclusion: Vitamin D deficiency was strongly associated with renal dysfunction and albuminuria severity in T2DM, suggesting its potential value as an adjunctive marker of DKD progression.