Diets lean on meat and rich in healthy fats like olive oil were most effective at promoting weight loss and lowering blood sugar among people with diabetes in a review of evidence from the last 10 years.
Benefits were also seen with diets low in carbohydrates, high in protein or low in simple sugars.
"If you look at different types of diets, these four can improve various aspects of diabetes control," lead author Dr. Olubukola Ajala, a diabetes specialist at Western Sussex Hospitals in the UK, told Reuters Health.
More than 24 million Americans have type 2 diabetes. People with the disease cannot store glucose in their cells effectively, and their blood sugar levels can go dangerously high. Lifestyle changes like weight loss and cutting calorie intake can improve blood sugar control and reduce the risk of complications from the disease, but it has not been clear which diet plans work best.
Ajala and her colleagues reviewed the results of 20 studies comparing the effect of seven popular diets on adults with type 2 diabetes. Mediterranean diets, low-carb diets, high-protein diets and low glycemic index diets - which rank foods by how quickly their carbs turn into glucose - all lowered participants' blood sugar.
After following the diet for at least six months, the people on a Mediterranean eating plan also lost an average of 4 pounds. No other diet had a significant impact on weight, according to the findings published in the American Journal of Clinical Nutrition.
"We were quite surprised by the Mediterranean diet in particular," Ajala said. "I would have thought that low-carb would have been the best for losing weight, but Mediterranean seems to be better."
A Mediterranean-style diet emphasizes fruits, vegetables and legumes, whole grains, fish, and using olive oil and herbs in place of butter and salt. Saturated fats from red meat and dairy products are typically less than eight percent of total calories consumed.
Other studies have linked Mediterranean diets with reduced risks of cancer, Alzheimer's disease, Parkinson's disease and death from heart attack.
Though the review found no evidence that vegetarian, vegan or high-fiber diets aided in weight loss, they might still have promise for improving blood sugar control, the report notes.
In addition, low-carb, low-glycemic and Mediterranean diets all led to increases in markers of heart health - "good" cholesterol rose by 4 percent to 10 percent, and triglycerides fell by up to 9 percent.
The authors caution that the study could not tease apart the beneficial effects of weight loss - versus the types of foods consumed - in the results seen with some of the diets.
"One has to take this with a pinch of salt," Ajala said.
"Weight loss is important, but so is nutrient quality," Katherine Zeratsky, a registered dietician at the Mayo Clinic in Rochester, Minnesota, told Reuters Health.
Portion control and sustainability are the most important elements of a diet plan for type 2 diabetes, according to Zeratsky, who was not involved in the study. She thinks Mediterranean diets may be more successful because they are easier to maintain than restrictive low-carb or high-protein diets.
"I think we're all still recovering from that low-fat diet phase of the 1990s," Zeratsky said. Olive oil makes food more palatable and satisfying, and may curb the urge to snack later in the day, she added.
But a Mediterranean diet is not the only way to achieve weight loss and improve heart health, Zeratsky said.
It's more important to take a balanced approach, including fruits and vegetables, eating moderate portions and talking to a doctor before embarking on a plan.
"It's not just about dumping olive oil on a salad," Zeratsky said.