Mediterranean Diet may Reduce Breast Cancer Risk - The PREDIMED Trial

 

A paper by Toledo et al published in JAMAInternal Medicine suggests that women could reduce their breast cancer risk by following a version of the Mediterranean diet that goes heavy on extra virgin olive oil.

The study was conducted within the frame of PREDIMED trial a 1:1:1 randomized, single-blind, controlled field trial from 2003 to 2009, on 4282 women aged 60 to 80 years who were at high cardiovascular disease risk.

Participants were randomly allocated to a Mediterranean diet (MeDiet) that was supplemented with extra-virgin olive oil (EVOO), a Mediterranean diet supplemented with mixed nuts, or a control diet (advice to reduce dietary fat).

After a median follow-up of 4.8 years, they identified 35 confirmed cases of breast cancer.  The 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 versus 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. When both MeDiet groups were merged together, they observed a 51% relative risk reduction (95% CI, 0.25-0.94). When they excluded women who were diagnosed with breast cancer during the first year after enrollment, the results remained unchanged.  The information that was collected in this study was on invasive breast cancers only therefore noninvasive tumors such as DCIS, in situ cancers, were not included or analyzed in this trial.

The data from the PREDIMED trial, suggest a significant inverse association between consumption of a MeDiet supplemented with EVOO and breast cancer incidence. A high consumption of EVOO (≥15% of total energy intake) seems to be instrumental in obtaining this significant protection. A non-significant risk reduction was observed with the MeDiet supplemented with nuts.

In a different trial, the Lyon Diet Heart Study, which was also a randomized trial, a protective effect of a Mediterranean-type diet against overall cancer incidence was observed, supporting the hypothesis of an anticancer effect of the MeDiet.

The low rate of breast cancer among women in the PREDIMED trial should not be surprising. If the MeDiet is actually protective against breast cancer, a low incidence is to be expected in a study with these characteristics, especially when overall adherence to such diet was good already at baseline. 

Several biological mechanisms could explain the anti-carcinogenic properties of EVOO. All types of olive oil provide a high supply of monounsaturated fatty acids, mainly oleic acid, as well as squalene, but EVOO also contains additional biologically active compounds as polyphenols.

These compounds are known to have a likely role in breast cancer prevention due to inhibition of tumor growth and proliferation, migration, and invasiveness of breast cancer cells in both in vitro or in vivo breast cancer models.  Olive oil has been associated with increased apoptosis of cultured breast cancer cells. It has also been reported to reduce intracellular reactive oxygen processes and to prevent oxidative DNA damage in both human breast epithelial cells and human breast cancer cells.

The authors conclude that the findings of their trial suggest a beneficial effect of the Mediterranean diet supplemented with extra-virgin olive oil as a primary prevention of breast cancer.  The authors also state that their findings need to be confirmed with longer-term and larger studies, as breast cancer in addition to being the most prevalent cancer in women worldwide, with 1.7 million new cases diagnosed in 2012 while its incidence is has increased by 20% due to growth and aging of the population.

JAMA Intern Med. published online September 14, 2015.