Effects of Vitamin C Supplementation on Glycemic Control and Cardiovascular Risk Factors in People With Type 2 Diabetes: A GRADE-Assessed Systematic Review and Meta-analysis of Randomized Controlled Trials

BACKGROUND Evidence suggests that vitamin C supplementation could be a potential therapy in type 2 diabetes. However, its effectiveness and evidence quality require further evaluation. PURPOSE To investigate the efficacy of oral vitamin C supplementation in improving glycemic control, cardiovascular risk factors, and oxidative stress in people with type 2 diabetes. DATA SOURCES Databases (PubMed, Embase, Scopus, Cochrane Library) and clinical trial registries were searched for randomized controlled trials up to 8 September 2020. STUDY SELECTION Trials in adults with type 2 diabetes were included. Trials were excluded if supplements were not exclusive to vitamin C and if <2 weeks in duration. DATA EXTRACTION Primary outcomes were HbA1c, glucose, cholesterol, triglycerides, and blood pressure (BP). Data were extracted for changes in outcomes between vitamin C and control groups. Evidence certainty was assessed using Grading of Recommendations, Assessment, Development, and Evaluation methods. DATA SYNTHESIS Twenty-eight studies (N = 1,574 participants) were included in the review. Outcomes that changed to a statistically and clinically significant extent with vitamin C were systolic BP (mean difference −6.27 [95% CI −9.60, −2.96] mmHg; P = 0.0002), with moderate evidence certainty, and HbA1c (−0.54% [−0.90, −0.17]; P = 0.004) and diastolic BP (−3.77 [−6.13, −1.42] mmHg; P = 0.002) with very low evidence certainty. LIMITATIONS Studies were predominantly short term (<6 months) with a small number of participants (n < 100). CONCLUSIONS While evidence from short-term studies suggests that vitamin C supplementation may improve glycemic control and BP in people with type 2 diabetes, vitamin C supplementation cannot currently be recommended as a therapy until larger, long-term, and high-quality trials confirm these findings.