Even without increased exercise or calorie restrictions, older white men and women at high risk for cardiovascular disease who followed a Mediterranean diet enriched with extra-virgin olive oil were 40% less likely to develop type 2 diabetes than those who were simply advised to reduce their fat intake, according to a report published online Jan. 6 in Annals of Internal Medicine.
Those on the olive oil–enriched Mediterranean diet also were less likely to develop diabetes than were those who followed a Mediterranean diet supplemented with nuts.
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A Mediterranean diet with plenty of olive oil ‘is palatable and has a high potential for long-term sustainability, with obvious public health implications for primary prevention of diabetes,’ the study authors wrote.
None of the study’s three diets restricted caloric intake, nor did patients increase their physical activity. Thus, there was no appreciable weight loss or reduction in waist circumference in any of the three groups: The protective effect was attributed solely to the change in the overall dietary pattern, said Jordi Salas-Salvadó, M.D., Ph.D., of the Universitat Rovira i Virgili in Reus, Spain, and his associates.
"Of note, this dietary pattern is palatable and has a high potential for long-term sustainability, with obvious public health implications for primary prevention of diabetes," they added (Ann. Intern. Med. 2014 Jan. 6 [doi 10.7326/M13-1725]).
Dr. Salas-Salvadó and his colleagues compared three diets in a subset of 3,541 participants in the PREDIMED (Prevención con Dieta Mediterránea) clinical trial: men aged 55-80 years and women aged 60-80 years who had three or more cardiovascular risk factors such as smoking, hypertension, hypercholesterolemia, low LDL cholesterol, overweight or obesity, and family history of premature cardiovascular disease.
Those patients were randomly assigned to follow a Mediterranean diet supplemented with extra-virgin olive oil (EVOO) (1,154 patients), a Mediterranean diet supplemented with mixed walnuts, almonds, and hazelnuts (1,240 patients), or a diet advising reduced intake of all types of fat (1,147 control patients). The two intervention groups were given either free EVOO or free mixed nuts, and the control group was given nonfood gifts such as kitchenware.
The two intervention groups also received personal advice from dietitians regarding the use of EVOO or mixed nuts; increased intake of vegetables, fruits, legumes, and fish; switching from red or processed meat to white meat; avoiding butter, fast food, sweets, or sugar-sweetened drinks; increasing the use of sofrito sauce (tomatoes, garlic, onion, and spices simmered in olive oil) to add flavor to food; and reducing the consumption of alcohol except for red wine.
The two intervention groups met with dietitians every 3 months to receive information on Mediterranean foods, seasonal shopping lists, meal plans, and recipes, as well as personalized advice to enhance adherence. The control group also met with dietitians for sessions "with the same frequency and intensity," but with advice pertaining to a low-fat diet rather than a Mediterranean diet, Dr. Salas-Salvadó and his associates wrote.
At a median follow-up of 4.1 years (range, 2.5-5.7 years), rates of both nonadherence to the diet and withdrawal from the study were significantly higher in the control group than in either intervention group.
A total of 273 participants developed new-onset type 2 diabetes: 6.9% of patients following the Mediterranean diet plus EVOO, 7.4% of patients following the Mediterranean diet plus nuts, and 8.8% of the control group, the researchers reported.
After adjusting for smoking status, fasting glucose level, total energy intake level, and physical activity level, the hazard ratio for developing diabetes was 0.60 for the Mediterranean diet plus EVOO and 0.82 for the Mediterranean diet plus nuts, compared with the control group. That reflects a significant 40% reduction in risk for the first intervention group but a nonsignificant 18% reduction in risk for the second, the investigators said.
Those results were consistent across subgroups of patients defined by age, sex, comorbidities, family history of CVD, and degree of adiposity. The findings from sensitivity analyses also aligned with those of the main analysis: The relative risk for developing diabetes was 0.70 with the Mediterranean diet supplemented with EVOO and 0.82 for the Mediterranean diet supplemented with nuts.
The study was stronger than previous observational studies of this issue, the authors noted, because of its randomized design and statistical control of many potential confounding variables. It was limited, however, in that it assessed only a subgroup of participants in the PREDIMED clinical trial and because that trial did not have the development of diabetes as a primary endpoint. Moreover, the study involved only older white patients at high risk for CVD, "which limits the generalizability of our results to other age groups or ethnicities," Dr. Salas- Salvadó and his associates said.