If you Google “Mediterranean diet may,” you will be faced with pages and pages of studies and articles purporting its possible health benefits, from protecting against asthma to delaying cognitive decline, reducing the risk of diabetes and Parkinson’s to helping us live longer. Now, we have something else to add to this ever-growing list: a decreased risk of breast cancer.
This may not come as a total shock. Not purely because the benefits of this diet – rich in fruit, veg, fish and olive oil – seem to be never ending, but also because lower rates of breast cancer in Mediterranean countries, compared with the U.S. or other areas of Europe, are not a new phenomenon. Furthermore, the possibility that diet could affect the development of breast cancer has already been extensively investigated, albeit results have largely been inconsistent.
But such inconsistencies are not an uncommon problem when it comes to diet studies, particularly those conducted in an observational manner. That’s because people adopting certain types of diets may also be more likely to engage in other lifestyle choices that could impact health. For example, those that eat lots of fast food and sugar-laden drinks may tend to be more sedentary than those who eat more healthily.
And although we moan and groan about studies such as this latest piece of research, the study does have some merits. Rather than comparing groups of individuals with different self-reported diets, participants were randomly assigned one of three diets: a Mediterranean diet supplemented with extra virgin olive oil, the same diet supplemented with mixed nuts, or a control diet where they were advised to slash fat intake. And the supplements were not unrealistic: one liter per week of oil for the participants and their families, and 30 grams per day of a mix of walnuts, hazelnuts and almonds.
The study, published in JAMA Internal Medicine, also involved a large number of participants, but it should be noted that they were not recruited with the intention of examining breast cancer incidence. This was a secondary outcome measured in a study that was actually designed to look at the effects of the Mediterranean diet on the prevention of cardiovascular disease. Consequently, all of the participants – 4,282 women between the ages of 60 and 80 – had a high risk of cardiovascular disease.
The trial was stopped back in 2010, after nearly five years of study, because of early evidence for the heart benefits of both Mediterranean diets. During this time, 35 of the participants developed breast cancer. The Mediterranean diet plus olive oil had the lowest rate, while the control group had the highest.
From their observations, they calculated that those following the oil-supplemented diet had a 68% lower risk of developing breast cancer than those advised to follow a reduced fat diet. Those in the nut-supplemented group also experienced lower rates of breast cancer, but they weren’t low enough to be significant.
Although the study can’t prove cause and effect, the researchers note that extra virgin olive oil contains numerous biologically active compounds, such as polyphenols, that have been shown to suppress cell proliferation by altering expression of genes that can drive cancer development. One in particular, oleocanthal, has also previously been linked with reduced tumor growth and invasiveness in breast cancer.
But we still shouldn’t be too hasty in drawing conclusions since, as pointed out in an accompanying editorial, the number of cases of breast cancer was very small, and the study was limited to a population of Caucasian women who were all at risk of heart disease.