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
What this study found
Vitamin C supplementation produced statistically significant improvements in HbA1c (−0.54 percentage points) and fasting glucose (−0.74 mmol/L), and decreased systolic BP (−6.27 mmHg) and diastolic BP (−3.77 mmHg). Triglycerides (−0.20 mmol/L) and total cholesterol (−0.27 mmol/L) decreased; LDL cholesterol showed no significant change (−0.23 mmol/L; P=0.08); HDL cholesterol rose slightly (0.06 mmol/L; P=0.06). Plasma malondialdehyde decreased (standardized MD −1.25). Some lipid/oxidative markers showed no significant change. Overall certainty for primary outcomes ranged from very low to…
- Study & population
- Adults with type 2 diabetes; randomized controlled trials (including parallel and crossover designs); 28 trials with 1,574 participants; data extracted by two independent reviewers; risk of bias assessed; evidence certainty rated with GRADE.
- Intervention
- Oral vitamin C supplementation; regimens included 1,000 mg/day (commonly) and 500 mg/day in some trials; durations varied from short-term (<6 months) to longer-term (≥12 weeks); taken daily.
- Key limitation
- Predominantly short-term (<6 months) trials with small sample sizes; substantial heterogeneity and risk-of-bias concerns; dosing and duration of vitamin C varied across studies; many trials included concomitant diabetes therapies; adverse events rarely or inconsistently reported; potential publication bias and…
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Open PDFOriginal abstract
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.