www.ncbi.nlm.nih.gov/pubmed/26365989

Many women ask if they can reduce their risk of breast cancer by changing their diet or adding specific supplements. To date, the results of epidemiological studies have been inconsistent. The Mediterranean diet is a popular diet that is rich in plant foods, fish, and olive oil that has been found be beneficial for cardiovascular health. Because of the lower incidence of breast cancer in Mediterranean countries, the effect of the Mediterranean diet on breast cancer risk was evaluated in the PREDIMED (Prevencion con Dieta Mediterranea) trial, a multicenter randomized trial from Spain. The primary objective of the study was primary prevention of cardiovascular disease. Prevention of breast cancer was a secondary outcome. Women between 55 and 80 years of age were randomized to 1 of 3 diets: the Mediterranean diet supplemented with extra virgin olive oil (EVOO), a Mediterranean diet with mixed nuts, and a control diet that recommended a low-fat diet. The patients met with dieticians regularly and were also asked to fill out questionnaires. The outcome was first invasive breast cancer.

A total of 4,282 women were randomized to 1 of the 3 groups over a 6-year period. The median follow-up was 4.8 years. The groups were evenly matched with respect to patient characteristics. During the follow-up period, there were 35 cases of invasive breast cancer. Those who were randomized to the Mediterranean diet with EVOO had the lowest incidence of breast cancer of the 3 groups. When compared to the control diet group, the Mediterranean diet had a 62% relative risk reduction of invasive breast cancer. There was also a reduced risk of breast cancer in the Mediterranean diet with nuts group compared to the control diet group, but it was not as significant. When the 2 Mediterranean diets were combined, there was a 51% risk reduction for invasive breast cancer compared to the control diet.

The PREDIMED trial is first study to address the effect of the Mediterranean diet on breast cancer. The results are promising given the other health benefits derived from Mediterranean diet. However, there are some limitations to the study. The most important concern is that the primary goal of the study was not to study breast cancer prevention. Therefore, participants’ breast imaging was not reviewed during the trial and data on other factors known to increase the risk breast cancer were not collected, so we do not know for sure if the groups were evenly matched. Also, the number of breast cancer cases was small, so any difference between groups may not be statistically significant. Overall, the results of the PREDIMED trial are exciting, but a larger randomized study focused on breast cancer risk reduction is needed to confirm these findings.