Whole Grain, Bran, and Germ Intake and Risk of Type 2 Diabetes: A Prospective Cohort Study and Systematic Review

Abstract

Background

Control of body weight by balancing energy intake and energy expenditure is of major importance for the prevention of type 2 diabetes, but the role of specific dietary factors in the etiology of type 2 diabetes is less well established. We evaluated intakes of whole grain, bran, and germ in relation to risk of type 2 diabetes in prospective cohort studies.

Methods and Findings

We followed 161,737 US women of the Nurses' Health Studies (NHSs) I and II, without history of diabetes, cardiovascular disease, or cancer at baseline. The age at baseline was 37–65 y for NHSI and 26–46 y for NHSII. Dietary intakes and potential confounders were assessed with regularly administered questionnaires. We documented 6,486 cases of type 2 diabetes during 12–18 y of follow-up. Other prospective cohort studies on whole grain intake and risk of type 2 diabetes were identified in searches of MEDLINE and EMBASE up to January 2007, and data were independently extracted by two reviewers. The median whole grain intake in the lowest and highest quintile of intake was, respectively, 3.7 and 31.2 g/d for NHSI and 6.2 and 39.9 g/d for NHSII. After adjustment for potential confounders, the relative risks (RRs) for the highest as compared with the lowest quintile of whole grain intake was 0.63 (95% confidence interval [CI] 0.57–0.69) for NHSI and 0.68 (95% CI 0.57–0.81) for NHSII (both: p-value, test for trend <0.001). After further adjustment for body mass index (BMI), these RRs were 0.75 (95% CI 0.68–0.83; p-value, test for trend <0.001) and 0.86 (95% CI 0.72–1.02; p-value, test for trend 0.03) respectively. Associations for bran intake were similar to those for total whole grain intake, whereas no significant association was observed for germ intake after adjustment for bran. Based on pooled data for six cohort studies including 286,125 participants and 10,944 cases of type 2 diabetes, a two-serving-per-day increment in whole grain consumption was associated with a 21% (95% CI 13%–28%) decrease in risk of type 2 diabetes after adjustment for potential confounders and BMI.

Conclusions

Whole grain intake is inversely associated with risk of type 2 diabetes, and this association is stronger for bran than for germ. Findings from prospective cohort studies consistently support increasing whole grain consumption for the prevention of type 2 diabetes.

Abstract

Jeroen de Munter and colleagues found that, in women in the US Nurses' Health Studies, whole grain intake was inversely associated with risk of type 2 diabetes. The association was stronger for bran than for germ.

Editors' Summary

Background.

Type 2 diabetes mellitus (also sometimes called adult-onset or noninsulin-dependent diabetes) is increasing worldwide and is the most common form of diabetes. It puts people at risk of poor health and death by increasing their risk of heart disease and stroke, and a range of other conditions including blindness, kidney disease, and ulcers. It has long been recognized that there is a link between diet and developing type 2 diabetes, because people who are overweight (because the amount of energy in their diet is greater than the energy they use up) run a greater risk of getting type 2 diabetes. However, it has not been clear which particular nutrients or foods might increase the risk or might give protection.

Cereals—such as rice, wheat, corn (maize), etc.—make up a major part of most people's diets. During the refining of cereal grains, much of the outer part of the grain (kernel) are usually removed. Foods are described as “whole grain” if all components of the kernel (the bran, germ, and endosperm) are still present in their natural proportions. There is good evidence that consumption of whole grains may reduce the risk of several diseases, including various types of cancer, heart attacks, and strokes. Some evidence also suggests that eating a diet rich in whole grains might help protect against diabetes, but this has not been firmly established.

Why Was This Study Done?

The authors of this study wanted to find out how much whole grain was eaten by a large number of people over several years and to record how many of these people developed type 2 diabetes. If these two things were closely associated it would provide more evidence to support the idea that whole grain consumption helps protect against type 2 diabetes.

What Did the Researchers Do and Find?

The researchers drew on information recorded in a very large and continuing study in the US, the Nurses' Health Study, which began in 1976, when over 100,000 female registered US nurses completed and returned a mailed questionnaire to assess their health and lifestyle. More nurses were added in 1989. It is an example of what is known as a “cohort study.” Every two years, questionnaires have been mailed to the nurses. Questions asked include the nurses' age, weight, their diet, whether they smoke, their use of oral contraception; and their personal history of diabetes, cardiovascular disease, and cancer. The researchers calculated each nurse's whole grain intake in grams per day. They found that by 2004 about 6,500 of them had developed type 2 diabetes. From an analysis of the data it was clear that the greater the consumption of whole grains the lower the risk of getting type 2 diabetes.

An additional part of the study was that the researchers searched the medical literature for other cohort studies that examined whole grain intake in relation to risk of type 2 diabetes. (This type of research is called a “systematic review,” and it requires that researchers define clearly in advance the kind of studies they are looking for and how they will analyze the data.) They found five such studies. They added together the results of all the studies, including their own. This gave a total of nearly 11,000 cases of type 2 diabetes, out of around 286,000 people. From their analysis they calculated that a two-serving-per-day increment in whole grain consumption was associated with a 21% decrease in risk of type 2 diabetes.

What Do These Findings Mean?

Scientists say that association can never prove causation. (That would require a different sort of study called a trial, where two similar groups of people would be given either a diet high in whole grains or one that was low.) Nevertheless, the research does strongly suggest that a healthy diet that reduces the risk of developing type 2 diabetes should include the consumption of several servings of whole grains daily. The authors do point out that people who choose to eat a lot of whole grains also tend to have a healthy lifestyle in other respects, and that it was hard to calculate intake accurately. However, they do not consider that these limitations to their study would have affected the overall result too seriously.

Additional Information.

Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0040261.

• Good introductory information about diabetes (type 1 and type 2) may be found on the Web sites of the National Diabetes Clearing House (US) and Diabetes UK

• More detailed information is available on Medline Plus, a Web site that brings together authoritative information from several US government agencies and health-related organizations

• Wikipedia has an entry on whole grain (Wikipedia is a free online encyclopedia that anyone can edit)

• The Nurses' Health Study has a Web site