In a trial that followed participants for more than eight years, those following a so-called Mediterranean diet went significantly longer before needing diabetes medication and more of them had their diabetes go into remission, compared to those on a low-fat diet.
"There's been lots of epidemiology suggesting that a Mediterranean diet was beneficial with metabolic syndrome and diabetes," Dr Leanne Olansky told Reuters Health.
"But this was a randomized controlled trial, so we know it really was the diet causing the results," she said. "This is the kind of evidence that we use to determine if drugs are effective."
"Everybody thinks of fat as being bad, but this shows that it depends on what kind of fat," said Olansky, an endocrinologist at the Cleveland Clinic who was not involved in the new study.
People diagnosed with diabetes should aim to have a healthy diet, and a Mediterranean diet is a good, healthy option, lead study author Katherine Esposito told Reuters Health in an email.
Cutting calories is important, and cutting fat is an easy way to cut calories, but according to this study, maintaining the right levels of healthy fats is important, she said.
"One of the main aspects of the Mediterranean diet is the percentage of daily fat, which is higher than 30 percent of daily calories, however, the main fat is monounsaturated, usually from olive oil in the Mediterranean basin," said Esposito, of the Diabetes Unit at University Hospital at the Second University of Naples in Italy.
She and her colleagues continued to follow participants in a previous study who had been divided into two groups - one assigned to follow a Mediterranean diet and the other a low-fat diet - when they were first diagnosed with type 2 diabetes.
Both diets were designed to help prevent the disease from getting worse and to keep blood sugar under control without medication for as long as possible.
On both diets, women aimed to consume 1,500 calories per day and men aimed for 1,800 per day. Mediterranean dieters ate lots of vegetables and whole grains and replaced most red meat with poultry and fish. Monthly sessions with nutritionists helped them keep less than half of their calories coming from carbohydrates and at least 30 percent of calories from fat, mainly olive oil.
The low-fat diet restricted fatty or sugary snacks, limiting fats to less than 30 percent of daily calorie intake.
At the end of a four-year study period, some of the participants in each group still hadn't gone on medication.
At the six-year mark, all the people in the low-fat diet group had gone on diabetes medication, but it wasn't until the eight-year mark that all people in the Mediterranean diet group needed medication.
Diabetes "remission," in which blood sugar levels appear healthy with no signs of diabetes, was rare overall but slightly more common in the Mediterranean group, according to the results published in Diabetes Care.
Avoiding saturated fat, which often comes from red meat, could be important for diabetics, Olansky said.
"Although we don't know exactly what it is about Mediterranean diets that helps control blood sugar, it likely has to do with high levels of fiber, less red meat, and more olive oil and fish, a good source of protein with unsaturated fat," she said.
"The Mediterranean diet represents an easy way to combine healthy foods with taste and flavor," Esposito said. "Most of our patients continue to follow Mediterranean diet, even after the study ended."
People on the Mediterranean diet tended to lose more weight than those on the low-fat diet, which may be because the Mediterranean diet is easier to stick to, Olansky said.
"Patients often ask us what they can do besides medication," Olansky said.
"Often they want to try a lifestyle intervention before medicine, and this is a great example of something you could offer a patient."