March 25, 2021
Around 1.7 million Australians have diabetes, with type 2 diabetes accounting for 85-90 per cent of cases. It is the fastest growing chronic condition in Australia, bringing with it complications such as increased risk factors for cardiovascular disease.
Research has found that vitamin C supplementation may have benefits for people with type 2 diabetes. Dr Shaun Mason, a Lecturer in Nutrition Science at IPAN, is part of a team investigating this growing evidence base more comprehensively and specifically, to evaluate the certainty of this evidence for future clinical decision making.
The team’s systematic review of randomised controlled trials (RCTs) investigated the effects of vitamin C supplementation on blood glucose control, blood pressure, triglycerides, cholesterol, and oxidative stress markers. They further assessed the quality and certainty of the evidence for outcomes using Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) methods. GRADE is a framework that provides a summary of evidence using a systematic approach for making clinical recommendations.
Dr Mason’s team examined 28 RCTs (with a total of 1,574 participants). Vitamin C supplementation was found to significantly improve glucose control (measured using HbA1c, which measures average blood glucose over the prior 3 months).
“Vitamin C also significantly decreased blood glucose, blood pressure, triglycerides, total cholesterol, and a plasma marker of oxidative stress,” Dr Mason said.
“While many of these improvements were small, some outcomes improved to a level regarded as clinically important (of sufficient magnitude to exert meaningful health benefits).”
The clinically important outcomes included HbA1c (which decreased on average by 0.54%), systolic blood pressure (which decreased on average by 6.27 mmHg), and diastolic blood pressure (which decreased on average by 3.77 mmHg).
Additional analyses found that improvements in most outcomes with vitamin C tended to be largest when study participants had a worse baseline HbA1c, when vitamin C supplements were taken for a longer period of time, and when study sample sizes were larger.
Despite clinically important improvements found for HbA1c and blood pressure, the certainty of evidence found using GRADE methods was regarded as moderate for systolic blood pressure but very low for most other outcomes. According to the GRADE framework, a high level of evidence suggests that authors are confident that the true effect of the treatment is similar to the estimated study effect found, while a moderate level of evidence suggests that the authors believe that the true effect of the treatment is likely close to the estimated study effect. Low and very low levels of evidence are suggestive that the true effect of the treatment could be quite different from the estimated effect.
How does vitamin C exert its effects?
Dr Mason said it was unclear how vitamin C might exert beneficial effects on blood glucose control, however some evidence suggests that by acting as an antioxidant vitamin C can improve the underlying insulin resistance in people with type 2 diabetes.
“This means that it might help to improve how effectively the hormone insulin works in the body,” he explained.
“In terms of blood pressure, vitamin C may, as an antioxidant, promote a decrease in levels of oxidant compounds that can damage the blood vessels.
“Vitamin C may also improve production of nitric oxide, which helps blood vessels to dilate. We need to further investigate are required to better understand vitamin C’s mechanisms of action in people with type 2 diabetes.”
What were the study limitations?
Dr Mason said studies included in the review were predominantly short-term (<6 months) and with a small number of participants (< 100). Furthermore, most studies included used vitamin C as an add-on therapy to existing diabetes medications.
“As a result, we can’t be certain about the effectiveness of vitamin C supplementation as a sole therapeutic approach,” he said.
Also, studies with important outcomes such as diabetic complications, cardiovascular disease incidence, or mortality were not included. Dr Mason said more investigation was needed to address the lack of available evidence on these outcomes in RCTs with vitamin C supplementation in people with type 2 diabetes.
What is the significance of these findings?
Dr Mason said the findings, based largely on short-term studies with low evidence certainty, suggest that vitamin C supplementation may be potentially effective for improving blood glucose control and blood pressure in people with type 2 diabetes.
Effects may be greatest in people with a higher HbA1c and in those who regularly supplement with vitamin C. Furthermore, vitamin C supplements are relatively inexpensive and widely available.
“Vitamin C might be a potentially cost-effective treatment for people with type 2 diabetes – but given limitations of studies in our review, we can’t recommend vitamin C supplements for type 2 diabetes management until larger, long-term, and high-quality trials confirm clinically beneficial findings,” he said.
Dr Shaun Mason is a member of IPAN. He has been employed as a full-time lecturer in Nutrition Science at Deakin University’s School of Exercise and Nutrition Sciences since January 2018. He completed his PhD at Deakin University investigating antioxidant therapy for type 2 diabetes management in 2015. He is currently undertaking research in nutrition and metabolism, with a specific focus on using antioxidant-based strategies to help manage metabolic diseases.
More information about the Institute for Physical Activity and Nutrition (IPAN) at Deakin can be found here