Mediterranean Diet for New Onset Type 2 Diabetes: Effective or Not?

Abstract & Commentary

By James C. Scheer, DO, MS. Dr. Scheer is Associate Medical Director of the NorthEast Internal & Integrative Medicine Program, Carolinas Medical Center-NorthEast in Concord, NC; he reports no financial relationship to this field of study.

Synopsis: In this randomized clinical trial, patients with newly diagnosed type 2 diabetes assigned to a Mediterranean-style diet had better glycemic control and were less likely to need oral antihyperglycemic drug therapy than patients assigned to a low-fat diet.

Source: Esposito K, et al. Effects of a Mediterranean-style diet on the need for antihyperglycemic drug therapy in patients with newly diagnosed type 2 diabetes: A randomized trial. Ann Intern Med 2009:151:306-314.

Researchers at a Naples, Italy, teaching hospital conducted a prospective randomized trial to compare the effects of a Mediterranean-style (Med) diet vs. a low-fat (LF) diet on the need for antihyperglycemic drug therapy in patients with newly diagnosed type 2 diabetes. A total of 215 overweight (BMI > 25 kg/m2) adults with newly diagnosed type 2 diabetes were enrolled who had hemoglobin A1c (HbA1c) levels < 11%, had never been treated with antihyperglycemic drugs, had been sedentary, and had a stable weight (± 2 kg) over the prior 6 months. At enrollment, the average age was 52 years, average BMI was 29.6 kg/m2, average HbA1c was 7.73%, 25% were on medication for hypertension, and 15.5% were being treated for hyperlipidemia.

The patients were randomly assigned to either the Med diet or the LF diet. Participants were taught to prepare meals at home. Staff members involved in the intervention were aware of group assignment, but those who assessed achievement of the primary outcome were blinded to the intervention.

The Med diet was rich in vegetables and whole grains yet low in red meat. Poultry and fish were encouraged. Energy intakes were restricted to 1,500 kcal/d for women and 1,800 kcal/d for men in both groups. For the Med diet, no more than 50% of calories were to be eaten from complex carbohydrates, with no less than 30% from fat. The main source of added fat was 30-50 g/d of olive oil.

The LF diet was based on the American Heart Association guidelines; it was rich in whole grains and restricted additional fats, sweets, and high-fat snacks. No more than 30% of calories were to be eaten from fat and no more than 10% of calories from saturated fat.

Nutritionists gave dietary advice to both groups on a monthly basis for the first year and then bimonthly thereafter. Participants kept dietary diaries after being instructed how to record their intake using food models as examples of portion size and using actual weights or amounts in terms of common measures. Dietary adherence was assessed by session attendance and review of diaries. Aerobic exercise was encouraged toward a gradual goal of 175 minutes of moderate-intensity physical activity (e.g., walking) per week.

The primary outcome measure was time to introduction of antihyperglycemic drug therapy. HbA1c was measured at baseline and every 3 months thereafter. Participants who had a HbA1c > 7% were given an additional 3 months to reinforce dietary guidance and physical activity. If the HbA1c level remained > 7%, a drug regimen was introduced.

The investigators found a significant difference in the need for antihyperglycemic drug therapy at 18 months (12% in Med vs. 24% in LF) and at the end of the trial at 4 years (44% vs. 70%). HbA1c decreased by 1.2% in the Med group vs. 0.6% in the LF group at 1 year and by 0.9% vs. 0.5% at 4 years. Dietary energy intakes decreased in both groups (compared to baseline) but were not significantly different in any study year. The percentage of carbohydrate intake decreased and the percentage of monounsaturated and polyunsaturated fatty acid intake increased and persisted throughout the trial in the Med group compared to the LF group. There was no significant difference in the amount of exercise each group reported throughout the study. And while there was a 2 kg greater weight loss (6.2 vs. 4.2) in the first year comparing the Med group to the LF group, the difference was negligible at years 3 and 4. The authors postulate that the consumption of monounsaturated fatty acids, shown to enhance insulin sensitivity,1 best explains the favorable effect of the Med diet on the need for antihyperglycemic drug therapy.

Commentary

Results of several epidemiological and randomized clinical trials suggest that a Mediterranean-style diet improves cardiovascular risk in healthy people and individuals with the metabolic syndrome.2 A prospective cohort study in Spain found that healthy patients adhering closely to a Mediterranean diet had a 35% relative reduction in the risk of developing diabetes.3

In a 2-year trial comparing three weight-loss diets (Med vs. LF vs. low carbohydrate [LC]) in moderately obese participants, among diabetic participants (14% of those in the study), changes in fasting plasma glucose and insulin levels were most favorable in those assigned to the Mediterranean diet, whereas the reduction in HbA1c was greater (0.9%) in the LC diet group.4

The current study does suggest that a Mediterranean-style diet seems to be preferred to a low-fat diet for glycemic control in patients with newly diagnosed type 2 diabetes.

The authors stated their objective was to compare the effects of a low-carbohydrate, Mediterranean-style diet to a low-fat diet in newly diagnosed type 2 diabetics. The Med group consumed approximately 200 g or 800 kcal (at least 42% of total) in complex carbohydrates daily. A low-carbohydrate diet is usually in the 20-100 g/d range. Yet, upon a closer look at the data in this study, relative to the baseline diet, carbohydrate intake (-9.4%) was in fact the biggest change in the Med diet group, followed by monounsaturated fat intake (+5.9%). The change in the LF diet group was less than a 2% reduction in both of these.

Study design weaknesses include that the trial, although randomized, was unblinded, and dietary intake was self-reported. The researchers missed an opportunity to capture additional clinically important data by not including a true low-carbohydrate arm in the trial. Further, no effort was made to quantify fish or fish oil intake, which could be important because omega-3 fatty acids also improve insulin sensitivity.5

Despite these limitations, this study adds to the credibility of the Mediterranean diet in improving cardiovascular risk factors; and now in newly diagnosed type 2 diabetics, in postponing the need for medication. However, it is unclear if the benefit is from a diet lower in carbohydrates, or higher in monounsaturated fats, or a combination thereof.

References

1. Esposito K, et al. Effect of a Mediterranean-style diet on endothelial dysfunction and markers of vascular inflammation in the metabolic syndrome: A randomized trial. JAMA 2004;292:1440-1446.

2. Sofi F. The Mediterranean diet revisited: Evidence of its effectiveness grows. Curr Opin Cardiol 2009;24:442-446.

3. Martinez-Gonzalez MA, et al. Adherence to Mediterranean diet and risk of developing diabetes: Prospective cohort study. BMJ 2008;336:1348-1351.

4. Shai I, et al; Dietary Intervention Randomized Controlled Trial (DIRECT) Group. Weight loss with a low-carbohydrate, Mediterranean or low-fat diet. N Engl J Med 2008;359:229-241.

5. Li JJ, et al. Anti-obesity effects of conjugated linoleic acid, docosahexaenoic acid, and eicosapentaenoic acid. Mol Nutr Food Res 2008;52:631-645.