Vitamin C and diabetes: Supplementation improves glucose level and blood pressure, but larger trials needed – meta-analysis

Levels of glycated haemoglobin (HbA1c), fasting glucose, postprandial glucose, triglycerides, systolic and diastolic blood pressure were shown to had significantly decreased in subjects on vitamin C supplementation.  

The meta-analysis was conducted by researchers from Deakin University in Australia and findings published in Diabetes Care. ​

The 28 RCTs, found from databases such as Cochrane Library, Scopus, Embase, and Pubmed, took place in India, Malaysia, Thailand, Australia, New Zealand, US, UK, Egypt, and Iran etc.

In the case of glycated haemoglobin, the meta-analysis showed that vitamin C supplementation could reduce HbA1c by a mean decrease of 0.54%. This is a significant result where the p-value is 0.004.

Systolic and diastolic blood pressure also recorded a mean decrease of 6.27mmHg and 3.77 mmHg respectively. These are also statistically significant results where the p-values are 0.0002 and 0.002 respectively.

Modifying factors ​

At the same time, the researchers found that there are a number of factors that could modify the efficacy of vitamin C supplementation.

This include the length of intervention and the dosage amount.

Notably, the efficacy was greater when the study duration was at least 12 weeks.

For instance, to achieve a significant reduction in triglyceride levels, a daily dose of less than 1,000mg of vitamin C for at least 12 weeks was found to be more effective.

“Findings of increased efficacy with lower vitamin C doses implies an upper dose limit of effectiveness of vitamin C.​

“Vitamin C might have greater effects on glycaemic control when taken long term,” ​he researchers said.

In addition, a larger sample size and an increase in baseline HbA1c are the other factors that could affect the efficacy of vitamin C supplementation.

“Subgroup and meta-regression analyses of potentially modifying factors revealed that the most prominent effects on heterogeneity and efficacy were due to study sample size, study duration, and baseline HbA1c.”​

For example, with each increase of one participant per study, vitamin C supplementation significantly reduced HbA1c level by 0.009%, fasting glucose by 0.007 mmol/L, triglycerides by 0.003 mmol/L, and total cholesterol by 0.004 mmol/L.

Similarly, with every one percent increase in baseline HbA1c, vitamin C supplementation significantly decreased HbA1c by 0.47%, fasting glucose by 0.57 mmol/L, triglycerides by 0.18 mmol/L, and total cholesterol by 0.32 mmol/L.

In contrast, baseline age, BMI, length of diabetes history, and vitamin C concentration do not have significant modifying effects on the efficacy of vitamin C supplementation on blood glucose and blood pressure levels.

Limitations ​

Due to study biases and other factors, the researchers have cautioned that the certainty of the evidence is low and higher quality trials are required.

In addition, the meta-analysis mostly included short term RCTs that are less than six months long, with a small number of participants at less than 100 subjects.

“This means that while on average vitamin C might have beneficial effects, it might not always be beneficial in an individual setting,” ​the researchers said.

“This review highlights that vitamin C supplementation may be beneficial for improving glycaemic control and reducing risk factors associated with cardiovascular disease in type II diabetes.​

“However, further investigations using larger sample sizes and longer supplementation periods are required to confirm these potential cardiometabolic benefits,” ​the researchers concluded.

Source: Diabetes Care ​

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​

DOI: 10.2337/dc20-1893​

Authors: Shaun A Mason, et al​