Abstract
Vitamin D deficiency is highly prevalent among individuals with diabetes mellitus and may contribute to the development and progression of microvascular complications, including nephropathy, neuropathy, and retinopathy. This systematic review assessed the relationship between vitamin D status and these complications. A comprehensive search of PubMed, European PMC, Google Scholar, CORE, and DOAJ was conducted for studies published between January 2015 and March 2025, focusing on original research evaluating vitamin D levels and diabetic microvascular outcomes in adults with type 1 or type 2 diabetes. Forty-nine studies met the inclusion criteria, and due to substantial methodological heterogeneity, a narrative synthesis was performed. The majority of included studies demonstrated a significant association between low serum vitamin D levels and increased severity of diabetic complications. For nephropathy, vitamin D supplementation was commonly linked to reduced albuminuria, improved glomerular filtration rate, and modulation of the renin–angiotensin–aldosterone system. In diabetic neuropathy, vitamin D repletion was associated with improvements in pain scores, nerve conduction parameters, and inflammatory cytokine profiles. Although evidence for diabetic retinopathy was comparatively limited, several clinical and observational studies reported that sufficient vitamin D levels may enhance responsiveness to anti-VEGF therapy, improve visual outcomes, and reduce retinal edema. Overall, the findings suggest that vitamin D may play a protective role in mitigating the burden of diabetic microvascular complications. However, variations in study design, dosing strategies, and outcome measures highlight the need for well-designed, large-scale randomized controlled trials to clarify causality and determine optimal supplementation protocols for diabetic populations.

Key words: Vitamin D, Diabetes Mellitus, Retinopathy, Neuropathy, Nephropathy, Microvascular Complications