Breast cancer, the most frequently diagnosed malignant tumor and the leading cause of cancer death among women, has increasing incidence rates. In 2012, 1.7 million women received a diagnosis of breast cancer. Since the 2008 estimates, breast cancer incidence has increased by more than 20% worldwide, while mortality has increased by 14%.1 In European countries, breast cancer is the most common incident cancer and the first or second (after lung cancer) malignant neoplasm implicated in mortality among women.2
Diet has been extensively studied as a modifiable component of lifestyle that could influence breast cancer development. Epidemiological evidence on the effect of specific dietary factors is still inconsistent, and the only convincing evidence relates to an increased risk in women with high alcohol consumption.3
The inconsistent association between foods or nutrient consumption and breast cancer risk may be partly due to the fact that individuals do not consume foods or nutrients in isolation but mixtures of foods with different nutrient constituents that may interact synergistically to influence biological pathways leading to or protecting from cancer. Thus, assessing diet as a whole, based on overall dietary patterns, provides more useful information on the role of diet in breast cancer risk. The Mediterranean dietary pattern has attracted considerable attention because, historically, breast cancer rates have been lower in Mediterranean countries than in Northern or Central European countries or the United States.4,5 The Mediterranean diet (MeDiet) is characterized by an abundance of plant foods, fish, and especially olive oil.5 In the Lyon Diet Heart Study, participants allocated to a cardioprotective Mediterranean-type diet showed a 61% lower risk of cancer (all subtypes) than those participants allocated to a control diet close to the step 1 American Heart Association prudent diet.6 Recent prospective cohort studies have evaluated the association between adherence to a MeDiet pattern and specifically breast cancer risk.7,8 However, the epidemiological evidence is still limited and conflicting.9,10 Moreover, no randomized trial has ever assessed the effect of the MeDiet on the primary prevention of breast cancer.
To further examine the effects of the MeDiet on breast cancer risk, we have analyzed the effect of the MeDiet supplemented with extra-virgin olive oil (EVOO) or nuts in the randomized intervention of the PREDIMED trial on the incidence of breast cancer.
After a median follow-up of 4.8 years, we identified 35 confirmed incident cases of breast cancer. Observed rates (per 1000 person-years) were 1.1 for the Mediterranean diet with extra-virgin olive oil group, 1.8 for the Mediterranean diet with nuts group, and 2.9 for the control group. The multivariable-adjusted hazard ratios vs the control group were 0.32 (95% CI, 0.13-0.79) for the Mediterranean diet with extra-virgin olive oil group and 0.59 (95% CI, 0.26-1.35) for the Mediterranean diet with nuts group. In analyses with yearly cumulative updated dietary exposures, the hazard ratio for each additional 5% of calories from extra-virgin olive oil was 0.72 (95% CI, 0.57-0.90).
Conclusions and Relevance
This is the first randomized trial finding an effect of a long-term dietary intervention on breast cancer incidence. Our results suggest a beneficial effect of a Mediterranean diet supplemented with extra-virgin olive oil in the primary prevention of breast cancer. These results come from a secondary analysis of a previous trial and are based on few incident cases and, therefore, need to be confirmed in longer-term and larger studies.