The fatty acid is present in high levels in olive oil, traditionally consumed by southern European populations, and considered one of the elements that contributes to their low incidence of heart disease and cancer.
The researchers on the new study, published in the Annals of Oncology last week, warn that oleic acid is just one component of olive oil, and further, that as their tests were done in vitro, they do not even prove the benefits of oleic acid in human clinical practice, much less the benefits of olive oil.
But the new findings could significantly speed up knowledge about olive oil's anti-cancer effect.
"Everybody is talking about the good function of the Mediterranean diet but we have no molecular evidence," said Dr Javier Menendez, assistant professor at the Northwestern University in the US, and lead researcher on the new study.
Around 95 per cent of studies investigating the anti-cancer effect of the Mediterranean diet are epidemiological ones, or those that look at the distribution and determinants of health and disease in populations.
However this research is lacking in several key pieces of information.
"Although oleic acid is consumed in the diet it can also be created by the body. Epidemiological studies do not control very well whether the level of fatty acids in breast tissue comes from the diet or the body," he told NutraIngredients.com.
In the new experiments on breast cancer cell lines, Dr Menendez and colleagues found that oleic acid significantly down-regulates the expression of one of the most important oncogenes in breast cancer.
These findings can now be followed up by further investigation of how this process happens, or the molecular signaling, as well as preliminary human trials.
"Because we don't have to look for toxicity of olive oil in animals, we can proceed directly to trials on people," explained Dr Menendez.
Research colleagues in Spain will investigate whether olive oil has any effect on expression of the Her-2/neu gene in a small group of patients with early breast cancer. Plans for the trial are at early stages but Dr Menendez said they are likely to treat one group with a combination of olive oil and drugs, while another will only receive drugs.
It is still too early to recommend consumption of olive oil to protect against cancer. Dr Menendez notes that it contains more than 50 compounds including antioxidants, with no clear understanding of whether its benefits come from the antioxidants, oleic acid, both together or other compounds.
But the strong interest in the research from press around the world underlines the potential suggested by the findings.
"The results are a very easy thing for people to understand. We could be offering a very easy way to target this horrible disease," said Dr Menendez.