Mediterranean-style diet better than low-fat diet in early type 2 diabetes

In patients with newly diagnosed type 2 diabetes, a low-carbohydrate Mediterranean-style diet improved glycaemic control and reduced the need for drug treatment compared to a high-carbohydrate, low fat diet.

US guidance on initial therapy of type-2 diabetes recommends starting therapy with pharmacotherapy in addition to dietary measures: either low fat, high carbohydrate, or low carbohydrate calorie-restricted diets are recommended, however there have been no direct comparisons of the two types of diet. This study investigated the medium-term efficacy and safety of a low-carbohydrate Mediterranean-style diet containing a high-proportion of unsaturated fat, compared with a standard low-fat, high carbohydrate diet.

Participants were patients newly diagnosed with type 2 diabetes at the diabetic clinic of one Italian university hospital. Eligibility criteria included BMI >25kg/m2, haemoglobin A1c (HbA1c) <11%, minimal physical activity level (<1 hour/week), and stable weight for at least six months. They were randomised to either a low-fat diet (based on American Heart Association guidelines) in which no more than 30% of energy was from fat, or a Mediterranean-style diet with no more than 50% energy from carbohydrates and at least 30% from fat (mainly as olive oil). Both were calorie-controlled (women 1,500 daily, men 1,800 daily). All patients received guidance on increasing their energy levels. Primary outcome was time to initiation of hypoglycaemic drug therapy according to a specified protocol, and planned follow-up was for four years: those who assessed study outcomes were blind to group allocation. Power calculations based on a pilot study indicated a study size of 87 patients per group.

Of 283 patients assessed for eligibility, 215 were randomised to the study (108 to the Mediterranean diet and 107 to the low fat diet); only 20 patients (10 in each group) were lost to follow-up. Average age at baseline was about 52, average BMI just under 30kg/m2, and average HbA1c 7.75% (Mediterranean diet) and 7.71% (low-fat diet). On intention to treat analysis at four years, those in the Mediterranean diet group were significantly less likely to require drug treatment: 44% vs. 70% (absolute difference, –26.0 percentage points; 95% CI, –31.1 to –20.1 percentage points; hazard ratio [HR], 0.63; 95% CI, 0.51 to 0.86).

More patients in the Mediterranean diet group lost weight (absolute difference -2kg; 95% CI -3.0 to -0.9kg): after adjustment for weight loss, the difference in likelihood of requiring drug treatment remained significant (HR 0.70; 95% CI, 0.59 to 0.90; P < 0.001). Reported energy intake and physical activity levels were similar in the two groups. Adverse events were similar in both groups, and one patient died in each group (of causes unrelated to the study).

The authors conclude that in patients with newly diagnosed type 2 diabetes, a Mediterranean-style diet delayed the need for hypoglycaemic drug therapy compared with a high-carbohydrate low fat diet. It was also associated with greater weight loss and some improvements in cardiac risk factors. They note that the study was inevitably only single blind, and that participants received intensive encouragement to continue with the recommended treatment. Nevertheless, they suggest that their study results reinforce the potential benefits of lifestyle intervention in type 2 diabetes.

Ann Intern Med 2009; 151: 306-14 (link to abstract)