Scientists have discovered why eating a Mediterranean diet rich in fruits, vegetables and particularly olive oil can help to protect women from developing breast cancer.

The key is oleic acid, the main component of olive oil.

Dr Javier Menendez, of Northwestern University Feinberg School of Medicine in Chicago, said oleic acid blocks the action of a cancer-causing oncogene called HER-2/neu which is found in about 30 percent of breast cancer patients.

"We have something now that is able to explain why the Mediterranean diet is so healthy," Menendez stated.

Doctors and researchers had been aware that eating a Mediterranean diet reduced the risk of breast cancer and other illnesses such as heart disease. But until now they did not know how.

Menendez and his colleagues in the United States and Spain studied the impact of oleic acid in laboratory studies of breast cancer cells.

"We are able to demonstrate that the main component of olive oil, oleic acid, is able to down-regulate the most important oncogene in breast cancer," Menendez explained.

"The most important source of oleic acid is olive oil."

They found that oleic acid not only suppressed the action of the oncogene, it also improved the effectiveness of the breast cancer drug Herceptin, a targeted therapy made by Swiss drug maker Roche Holding AG that works against the HER-2/neu gene.

Breast cancer patients with HER-2/neu positive tumors suffer from an aggressive form of the disease and have a poor prognosis.

"There is no evidence at all that olive oil is toxic," said Menendez, who reported his findings in the journal Annals of Oncology, explained.

"It is totally safe to consume olive oil," he added.

More than one million cases of breast cancer are diagnosed worldwide each year. In 1998, the disease caused 1.6 percent of all female deaths, according to the International Agency for Research on Cancer in Lyon, France.

Although oleic acid works against the oncogene in a different manner than Herceptin and enhanced the drug's effectiveness.

But Menendez stressed that although the laboratory results are promising, more research is needed. They are hoping to uncover the mechanism by which the acid targets the oncogene and are planning studies of animals with breast cancer to see if a diet high in olive oil can alter the activity of the oncogene and the impact of Herceptin.

"We have a molecular link than can explain why the Mediterranean diet is demonstrating all these benefits," Menendez added.