The effect of vitamin C supplementation on blood pressure in adults: an umbrella review of meta-analyses of randomized clinical trials

Abstract

Uncertainty persists regarding the estimated effects of vitamin C supplementation on blood pressure, including the strength of the evidence supporting these estimates and the magnitude of the observed effects relative to the minimal important difference. We aimed to provide an overview of the effects of vitamin C supplementation on blood pressure among adults through an umbrella review of randomized controlled trials (RCTs). PubMed, Scopus, and Web of Science, and the Cochrane Library were searched from the earliest date available to 25 December 2024. We identified eligible meta-analyses and included all primary trials, adding any RCTs not included in the largest review. For each trial, we extracted the mean change in blood pressure and its SD for both groups to calculate the mean difference as the effect size. The certainty of evidence was rated using the GRADE (Grading of Recommendations Assessment, Development, and Evaluation) approach. We identified six meta-analyses involving a total of 17 RCTs reporting systolic blood pressure (SBP) and diastolic blood pressure (DBP) as outcomes. The findings suggest that vitamin C supplementation may reduce SBP by approximately 3.7 mmHg. Although DBP did not decrease overall, a clinically important reduction was observed in patients with diabetes (−2.27 mmHg). These results indicate that vitamin C supplementation may have a meaningful beneficial effect on blood pressure. Nevertheless, the low certainty of evidence highlights the necessity for larger, long-term randomized trials to definitively confirm its efficacy.