Olive oil, which is a key component of the Mediterranean diet, has long been known as a heart-healthy oil due to its high levels of mono-unsaturated fat which help keep LDL cholesterol levels in check. Recent studies have also suggested a link between olive oil and a reduced risk of various types of cancer. The most convincing evidence is for breast cancer where numerous studies have found reductions in breast cancer risk of between 20-40% for those who regularly consume olive oil. Some of these studies are summarised below.
A Spanish study of more than 1,700 women looked at the effects of olive oil and the intake of various fats on the risk of developing breast cancer. The researchers found no link between breast cancer and either total fat intake or intake of any particular type of fat however those in the highest quartile for olive oil consumption had 34% reduced odds of developing breast cancer compared to those in the lowest quartile.
A case control study of Greek women, published in the Journal of the National Cancer Institute in 1995, found that consuming olive oil more than once a day was associated with a 25% reduction in the odds of developing breast cancer. The link between olive oil and breast cancer was strongest for post-menopausal women.
Finally, in a 2009 study published in the journal Carcinogenesis, Spanish researchers from the Autonomous University of Barcelona fed rats that were induced to develop mammary tumours a diet rich in either extra virgin olive oil, corn oil, or a control diet. The researchers found that the tumours that grew in the olive oil fed rats tended to take a slow growing, benign course, while the rats fed corn oil developed more aggressive tumours.
Olive oil appeared to suppress the activity of the p21Ras oncogene which is associated with increased growth, proliferation, and survival of cancerous tumours. Olive oil also reduced the expression and activity of a protein known as Akt which enhances tumour cell survival by inhibiting apoptosis (programmed cell death). Eduard Esrich, who led the study, suggests that women consume 50 ml of extra-virgin olive oil a day to reduce their likelihood of developing breast cancer.
Further evidence of the potential protective effect of olive oil on breast cancer can be found by comparing breast cancer rates across Europe. The graph below shows age standardised rates for breast cancer in selected European nations, lower income countries have been excluded as cancer often goes unreported in these countries due to poor quality health care systems. Countries in red consume high amounts of olive oil (>5kg per person per year) while countries in blue consume low amounts of olive oil (<5kg per person per year).
As can be seen from the graph, with the exception of Italy, which has a moderate breast cancer rate, all of the countries with per capita olive oil consumption above 5kg have very low breast cancer rates relative to other European countries. In-fact Greece, which has the highest per capita consumption of olive oil worldwide (23.7kg / year), has the second lowest breast cancer rate of any developed country in the world (South Korea has the lowest).
Obviously it is difficult to draw any firm conclusions from this data as their are many other dietary and environmental factors that contribute to variations in breast cancer rates across countries, however it does raise the intriguing possibility that the low breast cancer rates in countries such as Greece, Portugal, Spain, and Cyrpus are due, at least in part, to their high consumption of olive oil.
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