Original Research
Health Benefits of Dietary Whole Grains: An Umbrella Review of Meta-analyses

https://doi.org/10.1016/j.jcm.2016.08.008Get rights and content

Abstract

Objective

The purpose of this study is to review the effectiveness of the role of whole grain as a therapeutic agent in type 2 diabetes, cardiovascular disease, cancer, and obesity.

Methods

An umbrella review of all published meta-analyses was performed. A PubMed search from January 1, 1980, to May 31, 2016, was conducted using the following search strategy: (whole grain OR whole grains) AND (meta-analysis OR systematic review). Only English language publications that provided quantitative statistical analysis on type 2 diabetes, cardiovascular disease, cancer, and weight loss were retrieved.

Results

Twenty-one meta-analyses were retrieved for inclusion in this umbrella review, and all the meta-analyses reported statistically significant positive benefits for reducing the incidence of type 2 diabetes (relative risk [RR] = 0.68-0.80), cardiovascular disease (RR = 0.63-0.79), and colorectal, pancreatic, and gastric cancers (RR = 0.57-0.94) and a modest effect on body weight, waist circumference, and body fat mass. Significant reductions in cardiovascular and cancer mortality were also observed (RR = 0.82 and 0.89, respectively). Some problems of heterogeneity, publication bias, and quality assessment were found among the studies.

Conclusion

This review suggests that there is some evidence for dietary whole grain intake to be beneficial in the prevention of type 2 diabetes, cardiovascular disease, and colorectal, pancreatic, and gastric cancers. The potential benefits of these findings suggest that the consumption of 2 to 3 servings per day (~45 g) of whole grains may be a justifiable public health goal.

Introduction

Dietary whole grain consumption has been postulated to reduce the risk of type 2 diabetes, cardiovascular disease, colorectal cancer, and obesity.1 A whole grain kernel contains the endosperm, germ, and bran. The bran’s outer coating is rich in fiber and the inner germ contains vitamins, minerals, lignans, and phytochemicals (phenolic acids, polyphenols, and phytosterol compounds). Examples of whole grains include whole wheat, dark bread, brown rice, oats, barley, and rye. In the grain-refining process the most potent protective components of whole grains found in the bran and germ are removed, leaving behind the only the starch-rich endosperm.

There was a significant inverse association between dietary whole grain intake and all-cause mortality when comparing participants with dietary whole grain intakes in the top quintile to those whose intakes were in the bottom quintile.2, 3 Therefore, it is believed that a deficiency in dietary whole grain intake might contribute to the epidemics of type 2 diabetes, cardiovascular disease, cancer, and obesity.3

There is much discrepancy when it comes to randomized controlled studies on the effects of whole grain on clinical endpoints. Several studies on dietary whole grain intake in relation to type 2 diabetes risk have reported inverse associations with higher intake, but some studies found no significant association.4 This discrepancy may be due to differences in study design, selected populations, and the type and amount of whole grain foods consumed. Given the inconsistency of the existing literature and the insufficient statistical power because of small sample sizes, a pooling of information from individual trials could provide a more precise and accurate estimate of whole grain’s role in ameliorating chronic diseases such as type 2 diabetes, cardiovascular disease, cancer, and obesity. To achieve this result, many investigators have turned to performing a powerful statistical method known as meta-analysis. Meta-analyses are fundamental to provide 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 the role of whole grain as a therapeutic dietary agent.

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.5

A systematic literature search of PubMed, Cochrane Library, and CINAHL from January 1, 1980, to May 31, 2016, was conducted using the following search strategy: (whole grain OR whole grains) AND (meta-analysis OR

Results

The initial search strategy identified 126 articles and after careful review 22 meta-analyses were retrieved for inclusion into this umbrella review.4, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 One meta-analysis was excluded because it was not published in English; this meta-analysis investigated the use of whole grains on intestinal motility for bowel function disorders.26

A flow chart of the meta-analyses selection process is provided in Figure 1, and Table

Discussion

Almost all meta-analyses in this umbrella review reported positive benefits for dietary whole grain intake in the prevention and management of the most serious health care problems such as type 2 diabetes, cardiovascular disease, stroke, and colorectal, gastric, and pancreatic cancers. However, we must appreciate these positive results with some caution because of statistically significant heterogeneity in the type 2 diabetes meta-analyses and statistically significant publication bias in the

Conclusion

This review suggests that there is some evidence for dietary whole grain intake to be beneficial in the prevention of type 2 diabetes, cardiovascular disease, and colorectal, pancreatic, and gastric cancers. The potential benefits of these findings suggest that the consumption of 2 to 3 servings per day (~45 g) of whole grains may be a justifiable public health goal.

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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