Spanish researchers are predicting that a traditional Mediterranean diet including olive oil or nuts can reduce heart problems.

Two groups of study subjects who added olive oil or nuts to their diets did better than people on a low-fat diet, researchers with PREDIMED, a long-term trial looking at the Mediterranean diet, said in a news release Monday.

"After the three-month intervention period, the two groups following the Mediterranean diet were compared with the low-fat diet group and showed lower blood pressure and decreased blood glucose levels, cholesterol, triglycerides and inflammation markers," said Dr. Ramon Estruch, general co-ordinator of PREDIMED in Barcelona.

The subjects also had higher HDL (good) cholesterol levels.

After reporting early results, "it's easy to foresee that the participants who follow the Mediterranean diet supplemented with olive oil or with nuts will show in the long run a 50 per cent reduction in the incidence of cardiovascular complications," he predicted.

A Mediterranean diet is rich in legumes, whole grains, fish, nuts, fruits, vegetables and olives, but contains little meat and dairy products; as a result it is low in saturated fat and high in monounsaturated fat and dietary fibre.

The investigators tested 372 subjects aged 55 to 80 with a high risk of coronary heart disease. One group of 121 was put on a low-fat diet, a second group ate the traditional Mediterranean diet supplemented with a litre a week of free olive oil and a third group ate the traditional diet with 30 grams of free nuts — mostly walnuts — a day.

After three months, oxidized low-density lipoprotein (LDL, or bad cholesterol) fell in the two groups following the traditional diet, but there was no change in the low-fat group.

The study, published in the June 11 edition of the Archives of Internal Medicine, was the first to test what many people had suspected, said Estruch.

Because the Mediterranean diet is rich in antioxidants found in fruits and vegetables, "a lot of people simply figured that it would be beneficial. But nobody has tested the antioxidant effects of this dietary pattern in a randomized trial," he said.

The researchers are looking ahead to a test of 9,000 people, expected to be completed in late 2010.

PREDIMED is supported by the Spanish Health Ministry. It includes 17 groups of Spanish researchers in 200 health centres in the country.

LDL cholesterol promotes the buildup of plaque in artery walls, reducing the flow of blood, which greatly increases the risk of heart attack.

There is also evidence that the Mediterranean diet can significantly protect against Alzheimer's disease.