Even without weight loss, adhering to a diet rich in fresh produce, chicken, fish and olive oil is 40% more effective in heading off the development of Type 2 diabetes than following a low-fat diet, a new study has found.
The research suggests that for the nation's 78 million obese adults, a diet that minimizes red meat and sweets but incorporates plant-based fats may be a sustainable way to improve health — even if permanent weight reduction proves elusive.
The findings add to mounting research that suggests a traditional Mediterranean diet may be easier to adhere to and more likely to improve health than more restrictive regimens.
Compared with those on a low-fat diet, trial participants whose Mediterranean-style diet was supplemented with a daily dose of tree nuts — almonds, walnuts and hazelnuts — were 18% less likely to develop Type 2 diabetes. The researchers called that a positive trend but acknowledged that the difference fell short of demonstrating beyond doubt the superiority of such a diet over a standard low-fat diet.
Published Monday in the Annals of Internal Medicine, the latest entry in the diet fray followed for more than four years a group of 3,541 older Spaniards who were at high risk of developing cardiovascular disease. They were a subgroup of a larger clinical trial that demonstrated the effectiveness of the Mediterranean diet in reducing the risk of heart attacks and strokes.
That trial of 7,447 subjects — documented in February in the New England Journal of Medicine — found that those placed on a Mediterranean diet supplemented with either nuts or extra-virgin olive oil were 30% less likely than those prescribed a low-fat diet to suffer a heart attack, stroke or death due to cardiovascular disease.
Nearly half of those recruited for the parent trial already had Type 2 diabetes.
The subjects used in the current subgroup analysis started the trial with at least three risk factors for developing premature cardiovascular disease: They were active smokers; were overweight or obese; had a family history of premature heart disease; or had hypertension or worrisome cholesterol readings. None had been diagnosed with Type 2 diabetes at the start of the trial.
Some 273 participants went on to develop Type 2 diabetes. Among those in the Mediterranean diet-supplemented-with-extra-virgin-olive-oil arm, 6.9% developed diabetes; among those in the Mediterranean-diet-plus-nuts group, 7.4% did so; and among the low-fat dieters, 8.8% developed Type 2 diabetes.
Dr. Meir J. Stampfer, professor of medicine and epidemiology at Harvard University, called the research published Monday a significant step in further demonstrating the clinical benefits of the diet that until recently predominated in southern Europe. In showing the Mediterranean diet to be sustainable and beneficial, Stampfer said, the study should help put to rest many health-conscious Americans' aversion to nuts and oils, which are as calorie-dense as they are rich in unsaturated fats.
But Dr. David Heber of UCLA's Center for Human Nutrition cautioned that Americans should not give up efforts to cut fat from their diets in a bid to improve health. He pointed to copious evidence supporting a widely available regimen known as the Diabetes Prevention Program: When people at risk for diabetes lose 5% to 10% of their body weight using a program that reduces calorie and fat consumption and boosts their exercise, they drive down the likelihood of developing diabetes over the next five years by close to 60%.
"Saying that it's beneficial to consume more olive oil, which has over 100 calories per tablespoon, without weight loss encourages magical thinking about diabetes," Heber added. For obese patients, he said, driving down one's risk of developing Type 2 diabetes requires weight management.
Dr. James B. Meigs, an internal medicine specialist at Harvard, noted that the latest research suggests a Mediterranean diet drives down diabetes risk as much as preventive use of the drug metformin. But that's still only half as powerful an effect as that seen in subjects participating in the Diabetes Prevention Program, which recommends at least 30 minutes a day of exercise and a low-fat, reduced-calorie diet.
Meigs said that while physicians still should advise obese patients to lose weight and exercise more, he sees "little harm … of also encouraging" Mediterranean-style diets.
In the parent trial, subjects were urged to minimize sodas and fats that came in spreadable form, as well as limit consumption of commercially baked sweets and pastries to three times a week. They were told either to eat about a quarter-cup a day of either almonds, walnuts or hazelnuts, or to consume at least 4 tablespoons a day of extra-virgin olive oil.
Mediterranean dieters were told they could drink wine moderately — about seven glasses per week.
Aside from those guidelines, subjects in the Mediterranean diet arms of the trial had an "energy unrestricted" diet: They did not have a calorie limit, and fats made up between 35% and 40% of their daily calorie intake.
Low-fat dieters were told to avoid nuts and vegetable oils of all kinds, to limit store-bought sweets to less than one per week and to remove visible fat from meats. In addition to fruits and vegetables, they were encouraged to eat three servings of low-fat dairy products and three or fewer servings of bread, potatoes, pasta or rice each day.
The researchers also reported that, compared with subjects in the low-fat arm of their trial, those randomized into the two Mediterranean diet arms showed much stronger adherence to the guidelines.
"These differences were probably critical" to the divergent patterns of diabetes between those in the Mediterranean diet groups and those in the low-fat diet group, they wrote.