An analysis of previously carried out study reviews on the association between whole grain foods and type 2 diabetes risk appears to have backed the theory that such foods may reduce the likelihood of developing the disease, especially in people who consume a diet high in unrefined grains.
The authors have, however, warned 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, a nutritionist and epidemiologist at the Center for Medical Biomics, University Medical Center Groningen in the Netherlands.
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 given rise to the theory that there is a connection between the two.
The researchers reviewed 12 studies that examined relationships between whole grain intake and type 2 diabetes, which consisted of a single randomized controlled clinical trial and 11 prospective studies.
In the prospective studies, groups of people without diabetes were followed over long periods to see whether those who consumed more whole grain foods were less likely to get the disease than other participants were.
The review appearing in the latest issue of The Cochrane Library, a publication of The Cochrane Collaboration, suggests that these studies consistently showed a reduction of risk for the disease in those with a high intake of whole grain foods or cereal fibre.
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.
The authors write that scientists consider evidence from prospective studies to be weaker than that from randomized controlled trials. They also say that other factors, such as an overall healthy lifestyle, could also influence the development of type 2 diabetes, and that it is not possible to completely correct for known and possibly unknown factors in this study design.
Priebe said that she was surprised to find that there existed only one randomized trial on this topic.
"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," she said.
Dr. Osama Hamdy, medical director of the Clinical Obesity Program at the Joslin Diabetes Center in Boston, agreed that the concept of a whole-grains-rich diet as a possible diabetes preventative was interesting.
He, however, insisted that the kinds of data used within the review were troubling. According to him, studies about diabetes prevention should be randomized controlled trials over long durations.
He said that 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. 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," Hamdy said.
"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. The findings of this review are in line with those recommendations," Priebe said.