The strongest evidence that monounsaturated fatty acids such as oleic acid may influence breast cancer risk comes from studies of southern European populations, which have considerably lower levels of the disease compared with northern counterparts.

But animal research on the effects of olive oil to date has thrown up inconsistent results, possibly because olive oil has been administered as a mixture of several fatty acids and other natural protections and not on its own.

In a new series of laboratory experiments on breast cancer cell lines, a US and Spanish team has found that oleic acid dramatically cuts the levels of an oncogene called Her-2/neu (also known as erb B-2). High levels of Her-2/neu occur in over a fifth of breast cancer patients and are associated with highly aggressive tumours that have a poor prognosis.

Writing in today's issue of the Annals of Oncology, they say that oleic acid not only suppressed over-expression of the gene but other tests on the cell lines showed that it also boosted the effectiveness of trastuzumab (Herceptin), the monoclonal antibody treatment that targets the Her-2/neu gene and has helped to prolong the lives of many breast cancer patients.

Lead researcher Dr Javier Menendez, assistant professor at the Northwestern University Feinberg School of Medicine in Chicago and a research scientist with the Evanston Northwestern Healthcare Research Institute, said: "To our knowledge this is the first report that a dietary monounsaturated fatty acid previously suggested to be protective against breast cancer significantly down-regulates the expression of Her-2/neu, cutting it by up to 46 per cent."

"Her-2/neu is one of the most important oncogenes in breast cancer.”

The findings should not only help in understanding the molecular mechanisms by which individual dietary fatty acids regulate the behaviour of breast cancer cells but also suggest that dietary interventions based on oleic acid may delay or prevent Herceptin resistance in Her-2/neu-postive breast cancer patients, added Dr Menendez.

Dr Menendez and co-researchers Dr Ruth Lupu, director of the Evanston Northwestern Health Research Institute's Breast Cancer Translational Program and Dr Ramon Colomer, head of the Medical Oncology Division at Institut Catala d'Oncologia in Girona, Spain, are now looking to identify the ultimate molecular mechanism through which oleic acid supplementation inhibits the expression of Her-2/neu, as its blocking action appears to work in a different way from that of Herceptin.

They are also seeking funds for a study to see whether a high virgin olive oil diet will modulate the expression of the Her-2/neu oncogene in human breast tumours in animals and make the tumours less aggressive.

In addition, they want to investigate whether oleic acid-rich diets have any effect on the anti-tumour activity of Herceptin.

Dr Menendez emphasised that it is important to be cautious about the implications of the study, as laboratory results did not always translate into clinical practice.

However the findings did suggest that a higher level of oleic acid in breast tissue could provide an effective means of influencing the outcome of breast cancer in patients carrying high levels of the rogue gene.

"They may also help in designing future epidemiological studies and, eventually, dietary counselling to delay or prevent drug resistance developing in patients taking Herceptin," he said.