Loss of Nocturnal Dipping in Type 2 Diabetes Mellitus According to Diabetic Kidney Disease: The Role of Vitamin D Levels

Background/Objectives: Abnormal nocturnal dipping (ND) in diabetes mellitus is linked to adverse outcomes. Its association with vitamin D (VD), cardiovascular autonomic neuropathy (CAN), and diabetic kidney disease (DKD) remains unclear. We aim to evaluate the association of VD status with ND loss as well as CAN in type 2 diabetes mellitus (T2DM) across different DKD stages. Methods: 83 T2DM patients were included in this cross-sectional study: 36 with early-stage DKD (urine albumin-to-creatinine ratio [UACR] 30–299 mg/g; Group 1) as well as 47 with advanced-stage DKD (UACR ≥ 300 mg/g; Group 2). Results: Patients with advanced DKD exhibited higher CAN prevalence (72.70% vs. 52.80%; p < 0.05), lower 25(OH)D (26.90 ± 9.90 vs. 29.60 ± 8.00 ng/mL; p = 0.05), and lower systolic ND (0.82 ± 8.40 vs. 4.50 ± 7%; p < 0.05). Among patients with advanced DKD, lower 25(OH)D levels, classified according to the Institute of Medicine (IOM) criteria (normal ≥ 20 ng/mL), were associated with reduced systolic ND (1.90 ± 7.80 vs. −5.60 ± 9.30%; p < 0.05), while the Endocrine Society criteria showed a progressive reduction in systolic ND across VD classification (insufficiency 20–29.9, deficiency <20, and sufficiency ≥30 ng/mL), (−5.60 ± 9.30 vs. −0.20 ± 7.30 vs. 3.30 ± 8%, respectively; p < 0.05). Patients with definite/severe CAN exhibited lower systolic ND (−4.38 ± 7.41 vs. 3.34 ± 8.00%; p < 0.01) than those with absent/early CAN. In addition, lower VD levels were associated with greater CAN severity according to both the Endocrine Society criteria (p < 0.01, r = 0.40), and the IOM criteria (p < 0.05, r = 0.30). In the regression model, VD remained an independent predictor of absence of diastolic ND (Rsqr = 0.393; p < 0.05). Conclusions: These findings suggest a potential link between vitamin D status, autonomic dysfunction, and ND in patients with T2DM and advanced DKD.