A Mediterranean diet high in vegetables, fruits, and cereal grains and low in meat can significantly reduce the risk of diabetes in initially healthy people, according to a large Spanish cohort study published online in BMJ.
In a median of 4 years of follow-up with more than 13,000 Spanish university graduates without diabetes at baseline, researchers from the University of Navarra, Pamplona, found that those with food intake at baseline that conformed strongly with a Mediterranean diet had an adjusted 0.17 incidence rate ratio of type 2 diabetes, compared with those whose diet scored poorly on the 9-point dietary index. Those with a moderate Mediterranean diet score had an adjusted 0.4 incidence rate ratio of diabetes, compared with those who scored poorly.
Rich in vegetables, fruits, cereal grains, legumes, fish, and olive oil, and low in meat intake, a Mediterranean diet has, in previous studies, been found to be protective against coronary death. It also has been associated with a reduction in diabetes incidence among patients who have survived myocardial infarctions, the researchers noted.
“Two trials have shown that virgin olive oil protects against insulin resistance and the metabolic syndrome,” wrote the researchers, led by Dr. Miguel Ángel Martínez-González, chairman of the department of preventive medicine and public health at the university.
“Apart from olive oil, adherence to an overall Mediterranean-type food pattern is related to lower plasma concentrations of inflammatory markers and markers of endothelial dysfunction. These biomarkers are predictive of the future occurrence of type 2 diabetes,” the researchers wrote.
Researchers recruited university graduates and registered nurses to enroll in a long-term prospective cohort study beginning in December 1999. Of the enrollees, 13,380 had completed at least a 4-year follow-up questionnaire and did not report cardiovascular disease or diabetes at baseline (BMJ 2008 [Epub doi:10.1136/bmj.39561.501007.BE]).
The researchers assessed enrollees' dietary habits at baseline using a food frequency questionnaire with 136 items. Adherence to the diet was based on a score that added a point for every item of protective food that was consumed over the median and a point for every item of nonprotective food that was consumed under the median, researchers said. High scores were those between 7 and 9 and moderate scores were between 3 and 6.
At baseline, those scoring highest also had the highest mean body mass index, the greatest percentage with a family history of diabetes, and the largest percentage of members with hypertension, the researchers said.
In 4 years of follow-up, there were 33 confirmed cases of new onset diabetes in 58,918 person-years, according to the researchers. Unadjusted cumulative incidence rates were 0.4% in the low-scoring group, 0.23% in the moderate-score group, and 0.13% in the high-scoring group, the researchers said.
The small number of diabetes cases may have limited the statistical power of the study, the researchers acknowledge, although they add that the cohort is a young one (mean age 37.8 years) in which diabetes onset is less likely.
They added that the higher educational levels and high absolute consumption of favorable foods in the cohort may make its findings difficult to generalize either to people with less education or to non-Mediterranean countries in which consumption rates of the favorable foods are much lower. Additional trials and cohorts may be necessary to confirm their findings, the researchers wrote.
A Mediterranean diet is high in essential fatty acids and includes fish, olive oil, fruits, vegetables, legumes, and grains. ©Dušan