Literature reviews
Dietary Fiber Intake and Type 2 Diabetes Mellitus: An Umbrella Review of Meta-analyses

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Abstract

Objective

The purpose of this study was to review previously published meta-analyses on the effectiveness of dietary fiber on type 2 diabetes.

Methods

An umbrella review of all published meta-analyses was performed. A PubMed search from January 1, 1980, to April 30, 2017, was conducted using the following search strategy: (fiber OR glucan OR psyllium) AND (meta-analysis OR systematic review). Only English-language publications that provided quantitative statistical analysis on type 2 diabetes, fasting blood glucose concentrations, or glycosylated hemoglobin were retrieved.

Results

Sixteen meta-analyses were retrieved for inclusion in this umbrella review. In the meta-analyses comparing highest versus lowest dietary fiber intake, there was a statistically significant reduction in the relative risk (RR) of type 2 diabetes (RR = 0.81-0.85), with the greatest benefit coming from cereal fibers (RR = 0.67-0.87). However, statistically significant heterogeneity was observed in all of these meta-analyses. In the meta-analyses of supplementation studies using β-glucan or psyllium fibers on type 2 diabetic participants, statistically significant reductions were identified in both fasting blood glucose concentrations and glycosylated hemoglobin percentages.

Conclusion

This review suggests that those consuming the highest amounts of dietary fiber, especially cereal fiber, may benefit from a reduction in the incidence of developing type 2 diabetes. There also appears to be a small reduction in fasting blood glucose concentration, as well as a small reduction in glycosylated hemoglobin percentage for individuals with type 2 diabetes who add β-glucan or psyllium to their daily dietary intake.

Introduction

The Centers for Disease Control and Prevention estimates there are 30 million people with type 2 diabetes in the United States, and most will, at some point in their life, develop some form of vascular complication.1 These vascular complications can present in the form of microvascular disease (retinopathy, nephropathy, and neuropathy) and/or macrovascular disease (coronary heart disease, stroke, or peripheral artery disease).2, 3 These serious complications of diabetes confer substantial morbidity and impair patient quality of life. It has been estimated that diabetes costs the United States more than $174 billion per year when taking into account medical costs and loss of productivity.4

Both microvascular and macrovascular complications are thought to be due to prolonged hyperglycemia, which promotes an increase in oxidative stress, inflammation, and vascular damage.2, 3 Evidence indicates that medical interventions designed to lower blood glucose concentrations can reduce the risk of developing microvascular and macrovascular complications.5 One of these interventions includes a low glycemic index diet. Because increased fiber content decreases the glycemic index of foods, the American Diabetes Association encourages diabetics to consume a variety of fiber-containing foods.6 Unfortunately the blood glucose–lowering effects of fiber intake have not been consistently reported in the literature, with some clinical intervention studies reporting improvements in fasting blood glucose control and glycosylated hemoglobin (HbA1c) percentages,7, 8, 9, 10 but others reporting no improvement.11, 12, 13, 14, 15

Many of the clinical trials conducted to study the effects of dietary fiber intake on type 2 diabetes may have had sample sizes that did not provide sufficient statistical power to detect small, potentially meaningful changes in effect. Given the inconsistency of the existing literature and the insufficient statistical power as a result of small sample sizes, a pooling of information from individual trials could provide a more precise and accurate estimate of dietary fibers role in reducing the incidence of type 2 diabetes. To achieve this result, many investigators have turned to performing a powerful statistical method known as meta-analysis. Meta-analyses are fundamental in providing the highest level of evidence to best inform health care decision making. Therefore, the purpose and objective of this paper is to summarize the evidence from previously published meta-analyses regarding the effectiveness of dietary fiber as a therapeutic agent for type 2 diabetes.

Section snippets

Methods

An umbrella review was selected for this study. An umbrella review provides a summary of existing published meta-analyses and systematic reviews and determines whether authors addressing similar review questions independently observe similar results and arrive at similar conclusions.16

Because meta-analyses began appearing in medical literature in the early 1980s, a systematic literature search of PubMed and CINAHL from January 1, 1980, to April 30, 2017, was conducted using the following search

Results

The initial search strategy identified 528 articles and, after careful review, 17 meta-analyses were retrieved for inclusion into this umbrella review.17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33 One meta-analysis was excluded because it analyzed studies that administered a soluble fiber (resistant maltodextrin) for acutely reducing glycemic response with participants on a high-carbohydrate diet.20 A flow chart of the meta-analyses selection process is provided in Figure 1.

Discussion

In this umbrella review, 3 meta-analyses presented with statistically significant findings ranging between a 15% to 19% reduction in the incidence of developing type 2 diabetes when comparing participants with the highest intakes of total dietary fiber to those with the lowest intakes (see Table 1).17, 19, 21 Moreover, when total dietary fiber was separated into cereal, fruit, and vegetable fiber groups, it appeared that only cereal fiber significantly reduced the incidence of developing type 2

Conclusion

This umbrella review suggests that there may be some evidence for dietary fiber intake, especially from cereal fibers, to be beneficial in the prevention of type 2 diabetes; however, these results should be considered with caution because of the statistically significant heterogeneity. There also appears to be a small reduction in fasting blood glucose concentration, as well as a small reduction in HbA1c percentage, for individuals with type 2 diabetes who add β-glucan or psyllium to their

Funding Sources and Conflicts of Interest

No funding sources or conflicts of interest were reported for this study.

Contributorship Information

Concept development (provided idea for the research): M.P.M.

Design (planned the methods to generate the results): M.P.M.

Supervision (provided oversight, responsible for organization and implementation, writing of the manuscript): M.P.M.

Data collection/processing (responsible for experiments, patient management, organization, or reporting data): M.P.M.

Analysis/interpretation (responsible for statistical analysis, evaluation, and presentation of the results): M.P.M.

Literature search (performed

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