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Effects of vitamin C supplementation on glycaemic control: a systematic review and meta-analysis of randomised controlled trials

Abstract

Randomised controlled trials (RCTs) have observed contrasting results on the effects of vitamin C on circulating biomarkers of glycaemic and insulin regulation. We conducted a systematic review and meta-analysis of RCTs testing the effect of vitamin C administration on glucose, HbA1c and insulin concentrations. Four databases (PubMed, Embase, Scopus and Cochrane Library) were used to retrieve RCTs published from inception until April 2016 and testing the effects of vitamin C in adult participants. The screening of 2008 articles yielded 22 eligible studies (937 participants). Overall, vitamin C did not modify glucose, HbA1c and insulin concentrations. However, subgroup analyses showed that vitamin C significantly reduced glucose concentrations (−0.44 mmol/l, 95% CI: −0.81, −0.07, P=0.01) in patients with type 2 diabetes and in interventions with a duration greater than 30 days (−0.53%, 95% CI: −0.79, −0.10, P=0.02). Vitamin C administration had greater effects on fasting (−13.63 pmol/l, 95% CI: −22.73, −4.54, P<0.01) compared to postprandial insulin concentration. Meta-regression analyses showed that age was a modifier of the effect of vitamin C on insulin concentration. Furthermore, the effect size was associated with baseline BMI and plasma glucose levels, and with the duration of the intervention. In conclusion, greater reduction in glucose concentrations observed in patients with diabetes, older individuals and with more prolonged supplementation. Personalised interventions with vitamin C may represent a feasible future strategy to enhance benefits and efficacy of interventions. Nevertheless, results need to be interpreted cautiously due to limitations in the primary studies analysed.

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Acknowledgements

AWA is funded by the Republic of Iraq. PROSPERO Database registration: CRD42015023344.

Author contributions

The authors’ were responsible for the following tasks: AWA drafted the manuscript; AWA, JL, MS and JCM conceived the idea for the study and developed the search strategy; AWA, NDW and ADW conducted the search and summarised the data. All authors contributed to the data analysis, verification, writing and revising the manuscript.

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Correspondence to A W Ashor.

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Supplementary Information accompanies this paper on European Journal of Clinical Nutrition website

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Ashor, A., Werner, A., Lara, J. et al. Effects of vitamin C supplementation on glycaemic control: a systematic review and meta-analysis of randomised controlled trials. Eur J Clin Nutr 71, 1371–1380 (2017). https://doi.org/10.1038/ejcn.2017.24

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