The study included 13,380 Spanish university graduates (age range 20 to 90, average age 36) who were followed for about four years. They completed a dietary survey when the study started and follow-up questionnaires every two years after that.
The dietary survey included questions about foods, cooking methods, and olive oil consumption.
Based on the survey, participants were scored on a scale from 0 to 9 to show how closely they followed a Mediterranean diet. High scores meant they consumed a Mediterranean diet, meaning they favored legumes, grains, fruits, nuts, vegetables, fish, and moderate drinking and downplayed meat and dairy products.
A total of 33 people were diagnosed with type 2 diabetes during the follow-up period. Those who followed a Mediterranean diet were less likely to develop type 2 diabetes during the study.
For every two-point increase in the Mediterranean diet score, the odds of being diagnosed with type 2 diabetes dropped by 35%.
The study doesn't prove that the Mediterranean diet prevented type 2 diabetes. But the results held regardless of other factors such as physical activity and family history of diabetes.
The researchers -- who included Miguel A. Martinez-Gonzalez, MD, PhD, MPH, epidemiology professor at Spain's University of Navarra -- report their findings in the advance online edition of BMJ.
Because few participants developed type 2 diabetes -- and because the study only included college graduates in Spain -- the researchers call for further studies to validate their findings.