Association of Serum Vitamin D Levels with Urinary Tract Infections in Adult Patients: A Cross-Sectional Prospective Study

Background Urinary tract infections (UTIs) pose a significant global health burden. While vitamin D plays a known role in immune modulation, its association with UTI risk in adult populations remains underexplored. Objectives To investigate the association between serum vitamin D levels and the risk of upper and lower UTIs in adults, and to evaluate the relative risk contributions of comorbid conditions and modifiable risk factors. Methods We conducted a prospective case-control study involving 404 adult participants (203 controls, 73 with upper UTI, 128 with lower UTI). Serum 25(OH)D levels were categorized into deficient, insufficient, and sufficient. Logistic regression was used to calculate adjusted odds ratios (aOR). Absolute Risk Difference (ARD) and Number Needed to Harm (NNH) were computed. Results Vitamin D deficiency was significantly associated with both upper UTIs (aOR = 1.85; ARD = 12.9%; NNH = 8) and lower UTIs (aOR = 1.65; ARD = 10.3%; NNH = 10) compared to controls. Additional risk factors included diabetes mellitus (aOR = 1.92), recent antibiotic use (aOR = 2.77), and Foley catheter presence (aOR = 2.34). A causal hypothesis model supports biologic plausibility through impaired antimicrobial defense and barrier integrity. Conclusion Vitamin D deficiency is an independent risk factor for adult UTIs. Routine screening and targeted supplementation may provide a cost-effective adjunct to UTI prevention, particularly among high-risk populations. Future longitudinal studies and randomized controlled trials are warranted to confirm causality and guide implementation.