Many have touted whole grain foods as a way to prevent type 2 diabetes, and a new review finds a reduction in risk for people who consume a diet high in unrefined grains. However, the authors caution that more research is necessary before scientists can confirm a causal relationship.
“At the moment, because there is only weak evidence, no definite conclusion can be drawn concerning the protective effect of whole grain foods for the development of type 2 diabetes,” said lead review author Marion Priebe.
Refined cereal food products remove the nutrient- and fiber-rich bran and germ of the grain, leaving only the starchy inner parts. A decrease in consumption of whole grain cereals over the last decade, occurring at the same time as an increase in type 2 diabetes, has lead to the theory that there is a connection between the two.
Priebe, a nutritionist and epidemiologist at the Center for Medical Biomics, University Medical Center Groningen in the Netherlands, and colleagues reviewed 12 studies that examined relationships between whole grain intake and type 2 diabetes. These studies consisted of a single randomized controlled clinical trial and 11 prospective studies.
The review appears in the latest issue of The Cochrane Library, a publication of The Cochrane Collaboration, an international organization that evaluates medical research. Systematic reviews draw evidence-based conclusions about medical practice after considering both the content and quality of existing medical trials on a topic.
In the prospective studies, researchers followed groups of people without diabetes over long periods to see whether those who consumed more whole grain foods were less likely to get the disease than other participants were. These studies consistently showed a reduction of risk for the disease in those with a high intake of whole grain foods or cereal fiber.
Two of the studies that looked at the effect of whole grain consumption on weight, an important diabetes risk factor, found only a slight improvement.
Scientists consider evidence from prospective studies to be weaker than that from randomized controlled trials. Other factors, like an overall healthy lifestyle, can also influence the development of type 2 diabetes and it is not possible to completely correct for known and possibly unknown factors in this study design.
In randomized controlled trials, which are more difficult to perform, researchers can exclude — or control for — other influences on the development of the disease.
Priebe said she was surprised that only one randomized trial on this topic exists: “As type 2 diabetes mellitus is reaching epidemic proportions and diet is considered as a modifiable risk factor, it is important to have a sound knowledge of which kinds of food can contribute to the prevention of this disease and to identify gaps in this knowledge.”
Osama Hamdy, M.D., medical director of the Clinical Obesity Program at the Joslin Diabetes Center in Boston, said the kinds of data used within the review are troubling. He said that studies about diabetes prevention should be randomized controlled trials over long durations. Although the concept of a whole–grains-rich diet as a possible diabetes preventative is interesting, he said, none of the review studies would enable any kind of cause-and-effect conclusion.
“This is an additional piece of information that tells us diets rich in whole grains will probably do some good in the prevention of type 2 diabetes,” Hamdy said. “It is not a shortcut to tell you exactly what you need. It is just more support of a concept that has been around for a long time.”
“Whole grain foods are rich in dietary fiber and nutrients and they are recommended to be consumed together with plenty of fruit and vegetables for a healthy diet,” Priebe said. The findings of this review are in line with those recommendations.”
Reference: Priebe MG, et al. Whole grain foods for the prevention of type 2 diabetes. The Cochrane Database of Systematic Reviews 2008, Issue 1.
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