Researchers Carlo La Vecchia, from Mario Negri Institute of Pharmacological Research, Italy, and colleagues studied a total of 22,295 patients from Greece. (Agencies)
Among the patients actively followed up for just over 11 years, 2,330 cases of type 2 diabetes were recorded.
Researchers constructed a 10-point Mediterranean diet score (MDS) and a similar scale to measure the available carbohydrate or glycaemic load (GL) of the diet.
People with an MDS of over 6 were 12 percent less likely to develop diabetes than those with the lowest MDS of 3 or under. Patients with the highest available carbohydrate in their diet were 21 percent more likely to develop diabetes than those with the lowest, researchers said.
A high MDS combined with low available carbohydrate reduced the chances of developing diabetes by 20 percent as compared with a diet low in MDS and high in GL.
"The role of the Mediterranean diet in weight control is still controversial, and in most studies from Mediterranean countries the adherence to the Mediterranean diet was unrelated to overweight," researchers said.
"This suggests that the protection of the Mediterranean diet against diabetes is not through weight control, but through several dietary characteristics of the Mediterranean diet," they said.
Researchers point out that a particular feature of the Mediterranean diet is the use of extra virgin olive oil which leads to a high ratio of monounsaturated to saturated fatty acids.
One review of dietary fat and diabetes suggests that replacing saturated and trans fats with unsaturated fats has beneficial effects on insulin sensitivity and is likely to reduce the risk of type 2 diabetes.
However, in a randomized trial of high cardiovascular-risk individuals who were assigned to the Mediterranean diet supplemented with either free extra virgin olive oil or nuts and were compared with individuals on a low-fat diet (comparison group), there was no difference in diabetes occurrence between the two variants of the Mediterranean diet when compared with the comparison group.
"High GL diet leads to rapid rises in blood glucose and insulin levels. The chronically increased insulin demand may eventually result in pancreatic beta cell failure and, as a consequence, impaired glucose tolerance and increased insulin resistance, which is a predictor of diabetes. A high dietary GL has also been unfavourably related to glycaemic control in individuals with diabetes," researchers said.
"A low GL diet that also adequately adheres to the principles of the traditional Mediterranean diet may reduce the incidence of type 2 diabetes," they concluded.
Researchers Carlo La Vecchia, from Mario Negri Institute of Pharmacological Research, Italy, and colleagues studied a total of 22,295 patients from Greece.