Seasonal Variation in Vitamin D Levels and Association With Albuminuria in Patients With Diabetic Nephropathy: A Retrospective Cross-Sectional Study

Background: Diabetic nephropathy (DN) is a major cause of chronic kidney disease and remains a substantial contributor to morbidity worldwide. This study aimed to assess seasonal variation in serum 25-hydroxyvitamin D (25(OH)D) concentrations among patients attending the clinic and to examine associations between these concentrations and albuminuria, as well as other metabolic and biochemical parameters, in patients with DN. Methods: This retrospective cross-sectional study included 298 patients with DN admitted to a tertiary university hospital between 2017 and 2023. Patients were divided into four groups based on the season in which their blood samples were collected. Serum 25(OH)D levels, albuminuria, lipid profiles, and glycemic markers were measured. Differences in clinical parameters across seasons among clinic attendees and their associations with serum 25(OH)D levels were assessed using group comparisons, correlation analysis, and multiple linear regression. Results: The mean serum 25(OH)D concentration was 16.1 ± 8.2 ng/mL and varied significantly across seasons (p < 0.001). Vitamin D concentrations were lowest in winter and spring and highest in summer and autumn. Albuminuria levels differed significantly across the four seasonal groups (p = 0.016); however, none of the pairwise comparisons remained significant after Bonferroni correction. In multivariable regression analysis, albuminuria was independently and inversely associated with serum Vitamin D levels (p < 0.001). Conclusions: Serum 25(OH)D concentrations varied by the season of sample collection, and lower concentrations were cross-sectionally associated with higher urine albumin-to-creatinine ratio (UACR). Prospective longitudinal and interventional studies are needed to establish temporality and determine whether Vitamin D supplementation affects renal outcomes.