A Mediterranean diet supplemented with extra-virgin olive oil (EVOO) cut the risk for developing type 2 diabetes by about a third among adults at high risk for cardiovascular disease (CVD) compared with a low-fat diet, a new analysis finds.
The results, from the subgroup of subjects in the Spanish cardiovascular prevention study Prevencion con Dieta Mediterránea (PREDIMED) who did not have diabetes at baseline, were published January 6 in the Annals of Internal Medicine by Jordi Salas-Salvado, MD, PhD, from Universitat Rovira i Virgili, Reus, Spain, and colleagues.
"The PREDIMED trial provides strong evidence that long-term adherence to a Mediterranean diet supplemented with EVOO without energy restrictions…results in a substantial reduction in the risk for type 2 diabetes among older persons with high cardiovascular risk," Dr. Salas-Salvado and colleagues write.
The Mediterranean diet is high in fat (30% to 40% of total calories) from vegetable sources such as olive oil and nuts and relatively low in dairy products. The diet also commonly includes sauces with tomato, onions, garlic, and spices and moderate wine consumption.
"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," the investigators write.
Commenting on the study for Medscape Medical News, William S. Yancy Jr, MD, associate professor of medicine at Duke University Medical Center, Durham, North Carolina, said that the findings demonstrate overall benefit for the Mediterranean diet, but not necessarily in comparison with a true low-fat diet.
"These results are fairly convincing that the Mediterranean diet prevents the onset of diabetes compared with a usual diet," he observed, noting that the "low-fat" diet used in the study was actually closer to a "usual diet" because adherence to the low-fat diet in the study's control arm wasn't good.
"The Mediterranean diet may ultimately be found to be healthier than a low-fat diet, but this study did not effectively test that," said Dr. Yancy, whose research focus is obesity and nutrition.
Nonetheless, he believes the study lends support to a Mediterranean diet prescription. "It may not be definitive, but it is a very reasonable recommendation, because we are not aware of any harms from the Mediterranean diet — except maybe cost or lack of familiarity/convenience to Americans — and there is a growing body of evidence showing its benefits," he told Medscape Medical News.
Diabetes Risk Cut By a Third
The larger PREDIMED study enrolled 7447 men and women aged 55 to 80 years who did not have CVD at baseline but were at risk for it. They were randomized to 1 of 3 diets: a Mediterranean diet supplemented with either EVOO (50 mL/d) or mixed nuts (30 g/d) or a control diet with advice to reduce intake of all types of fat. Subjects were not calorie-restricted and were given no instructions regarding physical activity.
The main PREDIMED findings, published in February 2013, showed that both Mediterranean diets cut the CVD event risk by as much as 30% compared with the controls at 4.8 years of follow-up. And preliminary results from 1 of the 11 PREDIMED centers suggested that the Mediterranean diet also reduced the incidence of type 2 diabetes, based on 55 incident cases.
The current report includes the substudy population of 3541 who did not have diabetes at baseline and had available information during follow-up.
Over a median of 4.1 years' follow-up, adherence was better with the Mediterranean diet/supplemental foods compared with the low-fat–advice group, as assessed by questionnaire and biomarker measurement in a random subset.
A total of 273 participants developed diabetes, including 6.9% of the Mediterranean-diet/EVOO group, 7.4% of the Mediterranean-diet/mixed-nuts, group, and 8.8% of controls on the low-fat diet. After adjustment for potential confounders, hazard ratios for diabetes among those on the Mediterranean compared with low-fat diet were a significant 0.60 with EVOO supplementation and a nonsignificant 0.81 with added mixed nuts.
When the 2 Mediterranean diet groups were combined, there was a significant overall approximate 30% risk reduction for type 2 diabetes compared with the controls. These results were consistent when examined by subgroups of sex, age, comorbidities, smoking status, CVD family history, and adiposity.
"Many Healthy Foods"
The beneficial CVD effects of the Mediterranean diet are believed to be due to its inclusion of ingredients containing various minerals, polyphenols, and other phytochemicals that combat oxidative stress, inflammation, and insulin resistance, Dr. Salas-Salvado and colleagues note.
Regarding the diet's effect on diabetes risk in particular, Dr. Yancy pointed out that carbohydrates are the main drivers of blood sugar level, and "the Mediterranean diet is typically lower in carbohydrates than usual diets or low-fat diets. It is also a lower glycemic-load diet."
It's not clear why the Mediterranean diet supplemented with mixed nuts failed to show a significant diabetes reduction benefit. The authors and Dr. Yancy suggest that this could be due to chance.
Or, Dr. Yancy speculated, "Another reason may be that the foods that one eats with olive oil are better for preventing diabetes than those one eats with nuts. Or the higher monounsaturated-fat intake of the EVOO is more important than the polyunsaturated fatty acids from the nuts."
Nonetheless, he told Medscape Medical News, "It helps that both Mediterranean diet arms led to reduced diabetes rates, ie, both arms were in the same direction… Diet changes can be quite powerful for improving health, and a Mediterranean diet emphasizes many healthy foods based on prior clinical trials and epidemiological studies."
The study was funded by the funding agency for biomedical research of the Spanish government, Instituto de Salud Carlos III, through grants provided to research networks specifically developed for the trial. The Fundación Patrimonio Comunal Olivarero and Hojiblanca, California Walnut Commission, Borges, and Morella Nuts donated the olive oil, walnuts, almonds, and hazelnuts, respectively, used in the study. Dr. Salas-Salvado has received research funding from the International Nut Council; is a nonpaid member of the scientific advisory board of the International Nut Council; has received fees from Instituto Danone as a member of the scientific committee and from Danone as a consultant; has received fees from Nuts for Life and Eroski Distributors for lectures; and fees for consulting from Font Vella Lanjaron. His institution has received grants from the Nut and Dried Fruit Foundation and Eroski Distributors. Disclosures for the coauthors are listed here. Dr. Yancy has reported no relevant financial relationships.
Ann Intern Med. Published online January 6, 2014. Available at https://annals.org.
Medscape Medical News © 2014 WebMD, LLC
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Cite this: Mediterranean Diet Cuts Type 2 Diabetes Risk by a Third - Medscape - Jan 06, 2014.