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

Diabetes Care
Q1
Jan 2021
Citations:96
Influential Citations:6
Systematic Reviews / Meta-Analyses
87
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Enhanced Details

Methods
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.
Results
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 moderate; authors conclude short-term vitamin C may improve glycemic control and BP but cannot yet be recommended as therapy pending longer-term, high-quality trials, with effects potentially influenced by baseline HbA1c, duration, and dose.
Limitations
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 indirectness reduce certainty of effects.

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 supplementa...