Investigation of the relationship between vitamin D levels and seasons on clinical and laboratory features in patients with diabetic nephropathy

Objective: Diabetic nephropathy is one of the leading causes of chronic kidney disease worldwide. Albuminuria is an indicator of declining renal function and plays a significant role in the progression of nephropathy. This study aimed to examine the seasonal variations of vitamin D levels in patients with diabetic nephropathy and their effects on clinical and biochemical parameters, particularly albuminuria, lipid profile, and glycemic markers. Materials and Methods: A total of 298 patients with diabetic nephropathy who were admitted to the Süleyman Demirel University Faculty of Medicine Hospital (SDU Research and Education Hospital) between 01.01.2017 and 31.12.2023 (including both dates) were included in the study. Patients with GFR < 30 ml/min/1.73m², stage IV heart failure, undergoing dialysis treatment, diagnosed with malignancy, with persistent urinary tract infections, uncontrolled hypertension, or those receiving vitamin D therapy were excluded. The patients were divided into four seasonal groups, and differences in mean vitamin D levels among these groups, as well as the relationship between vitamin D levels and other parameters, were analyzed. Results: A total of 298 patients with diabetic nephropathy were included in the study. Among them, 51.7% (n=154) were male, and 48.3% (n=144) were female, with a mean age of 59.8 ± 13.1 years. The mean duration of diabetes was 8.8 ± 4.5 years, the mean body mass index (BMI) was 31.8 kg/m², the mean vitamin D level was 16.1 ng/ml, and the mean microalbuminuria level was 104 mg/day. A total of 140 patients were using ACE inhibitors (ACE-I) or angiotensin receptor blockers (ARB), while 158 patients were using neither of these medications, and no statistically significant difference was found between these two groups. Among the four seasonal groups, the lowest vitamin D levels were observed in winter and spring, while the highest levels were recorded in summer and autumn (p=0.000). Albuminuria levels were also found to be lower in summer and autumn (p=0.016). Additionally, male patients had higher albuminuria levels than females (p=0.007). Patients in the winter and spring groups had been living with diabetes for a longer period and had a higher body mass index compared to other seasons. The season in which HbA1c was found to be the lowest among all patients was autumn, with a mean value of 8.0% (p=0.000). A negative correlation was observed between vitamin D levels and albuminuria (rho=-0.430, p=0.000). Similarly, HbA1c and fasting blood glucose (FBG) levels were negatively correlated with vitamin D levels (p=0.014 and p=0.016, respectively). Serum albumin levels, on the other hand, showed a positive correlation with vitamin D levels (p=0.001). Additionally, patients with higher vitamin D levels had lower triglyceride (TG) and parathyroid hormone (PTH) levels (p=0.027 and p=0.014). Conclusion: Vitamin D levels were found to be higher in summer and autumn, and albuminuria levels were lower during these seasons. Seasonal variations appear to have an impact on glycemic markers and renal functions in patients with diabetic nephropathy. Further studies are needed to clarify these effects more comprehensively. Keywords: Diabetic nephropathy, Vitamin D, Albuminuria, Seasons