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.
