Does the Mediterranean diet reduce the risk of breast cancer?

The Mediterranean diet, characterized by an emphasis on plant foods, fish, and olive oil, is known to have cardiovascular benefits. This study by Toledo and colleagues is a secondary analysis of a large randomized trial that assessed the impact of the Mediterranean diet versus a recommended low-fat diet in patients at elevated risk for cardiovascular disease (CVD). The larger trial was stopped after 4.8 years of follow-up, when findings of early cardiovascular benefit became evident.

The 4,282 women in the trial, all of whom were white, were randomly allocated to one of the following groups:

• Mediterranean diet supplemented by EVOO. Participants were given 1 L of EVOO a week for the study duration. At baseline and quarterly thereafter, dieticians ran individual and group sessions. In individual sessions, participants completed a 14-item dietary screening questionnaire to assess adherence to the diet.
• Mediterranean diet supplemented by mixed nuts. Participants were given 30 g of mixed nuts per day (15 g walnuts, 7.5 g hazelnuts, and 7.5 g almonds). They also took part in individual and group sessions at baseline and quarterly thereafter, and completed the same screening questionnaire as the first group.
• A control group. Participants were advised to reduce dietary fat. They also underwent dietary training at the baseline visit and completed the 14-item screener. Thereafter, during the first 3 years of the trial, they received annual mailing of a leaflet explaining the low-fat diet. In 2006, however, the protocol was amended to include personalized advice and quarterly group sessions, with use of a separate 9-item dietary screener. The control group also received gifts of nonfood items as incentives.

Physical activity was not promoted in any group.

Findings of the trialAfter a median follow-up of 4.8 years, 35 confirmed cases of invasive breast cancer occurred among participants in the trial, who ranged in age from 60 to 80 years. The observed rate (per 1,000 person-years) of breast cancer was 1.1 for women following the Mediterranean diet supplemented with EVOO, 1.8 for women on the diet supplemented by mixed nuts, and 2.9 for the control group.

Women allocated to the Mediterranean diet with EVOO had a 62% reduced risk of invasive breast cancer (95% confidence interval [CI], 0.16–0.87). Women allocated to the same diet with mixed nuts had a 38% reduced risk of breast cancer, but this finding was not statistically significant.

Note that women in the control group failed to reduce their total fat intake substantially, even though they were advised to do so, although saturated fat intake remained below 10%.

Strengths and limitations of the trialAlthough the incidence of breast cancer is lower in Mediterranean countries, this is the first randomized trial to assess the impact of the Mediterranean diet on risk of this disease.

Because breast cancer was not the primary outcome, investigators were unable to verify if or when participants underwent mammography screening. However, randomization resulted in large groups of participants between whom mammographic status likely was comparable.

The lack of ethnic diversity represents another limitation.

Toledo and colleagues describe differences between olive oils in general and EVOO, including biologic mechanisms that might result in breast cancer prophylaxis.

WHAT THIS EVIDENCE MEANS FOR PRACTICEGiven the known cardiovascular benefits, it is reasonable to suggest to our menopausal patients that a Mediterranean diet with EVOO may reduce their risk of breast cancer.
—Andrew M. Kaunitz, MD

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