Prospective study shows that a strict adherence to a traditional Mediterranean diet yielded an 83% relative reduction in the risk of developing type 2 diabetes.

by Judith Groch, Contributing Writer, MedPage Today

PAMPLONA, Spain, May 29 — Strict adherence to a traditional Mediterranean diet yielded an 83% relative reduction in the risk of developing type 2 diabetes, a prospective study found.

Action Points
Explain to interested patients that the traditional Mediterranean diet, known to be protective for cardiovascular disease, may also help prevent diabetes.

Explain that the diet includes a high intake of fiber and vegetable fat, low intake of trans-fatty acids, moderate alcohol consumption, with abundant use of olive oil, plus vegetables, fruits, nuts, cereals, and legumes, instead of meat and dairy products.

Even moderate adherence was associated with a 59% relative reduction in diabetes risk, Miguel Martínez-González, M.D., Ph.D., of the University of Navarra here, and colleagues reported online in BMJ.

Many studies have shown that the Mediterranean diet has a protective role in cardiovascular disease, but little is known about its role in preventing diabetes in healthy people, the researchers said.

Typically, the diet is characterized by high intake of fiber and vegetable fat, low intake of trans-fatty acids and saturated fats, and a moderate intake of alcohol.

It also makes abundant use of olive oil for cooking, frying, spreading on bread, or in dressing salads, which leads to a high ratio of monounsaturated fatty acids to saturated fatty acids.

Fruits, nuts, grains, legumes, and fish are also featured, whereas consumption of meat and dairy products is relatively low.

In addition to having a long tradition of use without evidence of harm, a Mediterranean diet is highly palatable, and people are likely to comply with it, the researchers said.

Diets rich in monounsaturated fatty acids improve lipid profiles as well as insulin resistance and glycemic control in people with diabetes, they wrote. This suggested that following a Mediterranean diet might protect against developing diabetes.

To find out, the researchers conducted the SUN (Seguimiento Universidad de Navarra) prospective cohort study, which included 13,380 university graduates and registered nurses from Spanish provinces. None had diabetes at baseline. Participants were followed for a median 4.4 years.

Dietary habits were assessed at baseline with a validated 136-item food frequency questionnaire and were scored on a nine-point index.

New cases of diabetes were confirmed through medical reports and an additional detailed questionnaire posted to those who reported a new diagnosis of diabetes by a doctor during follow-up. The main outcome was confirmed cases of type 2 diabetes.

Participants who adhered closely to a Mediterranean diet had a lower risk of diabetes, the researchers found.

The incidence rate ratio adjusted for sex and age was 0.41 (95% CI 0.19 to 0.87) for those with moderate adherence (score 3-6) compared with those with low adherence (score <3).

For those with the highest adherence, scoring 7 to 9, the rate ratio was 0.17 (95% CI 0.04 to 0.75).

In the fully adjusted analyses, the results were similar. A two-point increase in the score was associated with a 35% relative

reduction in the risk of diabetes (incidence rate ratio 0.65,

95% CI 0.44 to 0.95), with a significant inverse linear trend (P=0.04) in the multivariate analysis.

Interestingly, the researchers said, those with high adherence to the diet (score > 6) also had a higher baseline prevalence of most risk factors for diabetes. They were older, had a higher BMI, a higher total energy intake, were more likely to have high blood pressure or a family history of diabetes, and were more likely to be former smokers.

These individuals would have been expected to be at greater risk for diabetes, but actually their risk was lower. It is possible that the diet may have provided substantial protection, the researchers wrote.

Study limitations included a small number of cases of diabetes despite extended follow-up over four years. Also, diabetes might have been underreported despite participants’ high educational level and easy access to medical care (half were health professionals).

Because the participants were university graduates the findings may not be generalizable to groups with less education, the researchers said. Another possible caveat might have been the quality of the food frequency questionnaires, which are known to have a certain degree of measurement error.

Finally, the investigators said, a potential limitation, inherent in every observational study, is the possibility of confounding by unmeasured or unrecorded factors.

The study suggests that substantial protection against diabetes can be achieved by following the traditional Mediterranean diet, they said.

However, the limited number of cases of diabetes and the possibility of underreporting require that further larger cohorts and trials are needed to confirm these findings, the investigators concluded.

The study was funded by the Spanish Ministry of Health and the Navarra Regional Government (Department of Health).

No competing interests were declared.

Additional articles and resources on this topic:

Primary source: BMJ

Source reference: Martínez-González MA, et al “Adherence to Mediterranean diet and risk of developing diabetes: A prospective cohort study” BMJ 2008: DOI:10.1136/bmj.39561.501007.BE.

Reviewed by Dori F. Zaleznik, MD; Associate Clinical Professor of Medicine, Harvard Medical School, Boston.

Published: May 29, 2008

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