The Effect of Vitamin C Supplements on Urinary Stone Risk: A Systematic Review and Meta-Analysis

Introduction and Objective: With the increased intake of vitamin C supplements during the COVID pandemic, physicians have reported an increase in stone incidence and complexity. The purpose of this systematic review and meta-analysis was to study the effect of vitamin C supplements on urine parameters associated with stone formation. Methods: A comprehensive review was performed to retrieve relevant articles published from 1965 until February 2023. Studies were included if they were interventional studies investigating the effect of vitamin C supplements on 24-hour urinary parameters (oxalate, citrate, and calcium) in both stone formers (SFs) and nonstone formers (NSFs), compared with either baseline values or a placebo group. Meta-analyses were performed to calculate the mean difference (MD) measuring the effect of vitamin C supplements on these parameters. Results: The initial database search yielded 808 titles and after rigorous screening, nine studies were included. Pooled analysis revealed a significant increase in urinary oxalate after vitamin C supplementation compared with baseline (MD = 9.72 mg/24 h; p < 0.001). When compared with placebo, vitamin C also significantly increases urinary oxalate excretion (MD = 6.45 mg/24 h; p < 0.001). Subgroup analyses of both comparisons showed that vitamin C increases urinary oxalate in both SFs and NSFs ( p < 0.05 for all). Analyses of studies reporting on 24-hour urinary citrate or calcium revealed no significant effect of vitamin C on either parameter. Conclusions: Vitamin C supplements significantly increase urinary oxalate excretion in both SFs and NSFs. This may contribute to stone formation and may worsen stone presentation among those who consume these supplements.