A Mediterranean diet enhanced with extra nuts or more olive oil improved certain heart risk factors among people who had them - but did not prevent others from developing them - according to a new study from Spain.

By the end of the trial, which lasted almost five years, fewer people eating the Mediterranean diet had so-called metabolic syndrome - a collection of traits that raise a person’s risk of heart disease and diabetes - largely because the syndrome disappeared from some who had it at the start.

“A healthy diet, like the Mediterranean diet, with a moderate-high intake of vegetable fat (in form of virgin olive oil or nuts) is a good healthy option for the prevention of several cardiovascular risk factors and chronic disease,” the study’s senior author Dr. Jordi Salas-Salvado told Reuters Health by email.

Metabolic syndrome is a cluster of risk factors, including a large waistline, high blood pressure, high blood sugar, low levels of HDL (good) cholesterol and high levels of triglycerides. People who have three or more of these symptoms are at heightened risk of developing cardiovascular disease and diabetes.

About 25 percent of adults around the world, and one-third of American adults, have metabolic syndrome, according to the U.S. Centers for Disease Control and Prevention.

Salas-Salvado, of the Universitat Rovira i Virgili and Hospital Universitari de Sant Joan de Reus, in Reus, Spain, said the foods that make up a Mediterranean diet are likely to help reverse metabolic syndrome and ameliorate several of the syndrome's components.

In the study, for example, even though no one was dieting to lose weight, consuming extra olive oil was linked to loss of the abdominal fat that had helped to qualify some participants for metabolic syndrome to begin with. It was also linked to lowered blood sugar.

Previous research has shown that eating a healthy diet, increasing physical activity and losing weight can help reverse metabolic syndrome, but the authors of the new study wanted to see if just following a Mediterranean-style diet without cutting calories might help.

They analyzed data from a larger trial, in which adults age 55 and older with a high risk of heart disease were randomly assigned to one of three diets: a Mediterranean diet supplemented with additional extra-virgin olive oil, a Mediterranean diet supplemented with nuts, or a low-fat diet.

A Mediterranean eating style already emphasizes healthy fats like those found in olive oil and nuts, as well as vegetables, legumes and lean protein, especially fish.

For the new analysis, the researchers followed 5,801 participants, almost two-thirds of whom (3,707) had metabolic syndrome at the start of the study. Another 2,094 participants developed metabolic syndrome during the study.

After about three years, the researchers didn’t see any differences among the diet groups as far as who developed metabolic syndrome. But, after almost five years they found that participants in the two Mediterranean-diet groups were more likely to have lost some belly fat and to have lower blood sugar levels.

As a result, just under 30 percent of those on one of the Mediterranean diets who started the trial with metabolic syndrome no longer met the criteria, the study team reports in the Canadian Medical Association Journal.

“The higher reversion rate of metabolic syndrome was mainly observed in those individuals allocated to the Mediterranean diet supplemented with extra-virgin olive oil,” Salas-Salvado said.

Since there were no differences in weight loss or energy expenditure from physical activity among the groups, he added, the benefits can be attributed to differences in the dietary pattern.

“We can speculate that a Mediterranean diet, particularly one supplemented with extra-virgin olive oil (which has anti-inflammatory properties), could exert positive effects on fat redistribution,” Salas-Salvado said.

The study authors also note that past research has credited olive oil with lowering high blood sugar and reducing insulin resistance, a difficulty processing blood sugar that often precedes diabetes.

Alice Lichtenstein, director of the Cardiovascular Nutrition Laboratory at Tufts University in Boston, called the study findings interesting, but “depending on how they are superimposed on the U.S. diet, (they) may or may not be of benefit,” she said in an email.

“Taking the habitual U.S. diet, which in two-thirds of Americans is consumed in excess of energy needs, and adding olive oil or nuts, without taking something out - hopefully, refined carbohydrate - would not be predicted to have the desired effect,” said Lichtenstein, who wasn’t involved in the Spanish research.

However, she added, dropping total calories, particularly in the form of sugar and things like white bread and white pasta, and making up some of the calories as unsaturated fat would likely have a benefit.

Lichtenstein also pointed out that the “low fat” control diet used in the study contained about 37 percent of energy from fat, which is high relative to the typical U.S. diet that has about 32 percent of energy as fat.

“Certainly no one is currently recommending low fat diets for anyone,” Lichtenstein said. “The current recommendation for individuals with insulin resistance is that they consume a diet closer to 35 percent of energy as fat.”