PREDIMED: a Mediterranean diet with extra-virgin olive oil and fewer breast cancers
In a Spanish cardiovascular prevention trial, women assigned to a Mediterranean diet supplemented with extra-virgin olive oil had about two-thirds fewer invasive breast cancers than a low-fat control group, although the number of cases was small.
PREDIMED randomised older Spanish adults at high cardiovascular risk to a Mediterranean diet supplemented with extra-virgin olive oil, a Mediterranean diet supplemented with nuts, or advice to follow a low-fat diet. This secondary analysis examined invasive breast cancer in the 4,282 women over a median 4.8 years.
Thirty-five incident breast cancers occurred. The olive-oil group had a hazard ratio of 0.32 versus control; the nut group's reduction was not statistically significant. The original 2018 retraction and republication of PREDIMED (for randomisation irregularities at some sites) led to a re-analysis with similar estimates.
It is the only randomised trial evidence that a whole-diet intervention reduces cancer incidence, but it rests on very few events.
- Invasive breast cancer HR 0.32 (95% CI 0.13-0.79) for Mediterranean diet plus extra-virgin olive oil vs control
- Mediterranean diet plus nuts: HR 0.59 (95% CI 0.26-1.35), not significant
- Only 35 breast cancers occurred among 4,282 women over about 5 years
- Effect estimates were similar after the 2018 re-analysis excluding sites with randomisation problems
A Mediterranean dietary pattern rich in olive oil may lower breast cancer risk, and it is safe and good for the heart anyway. The evidence is suggestive, not definitive, because of the small number of cancers; it should not be presented as proven cancer prevention.
- Breast cancer was not a pre-specified primary endpoint and cases were few
- Randomisation irregularities at some sites forced a retraction and republication of the parent trial
- Older Spanish women at high cardiovascular risk; not generalisable to younger or other populations
- No mammographic screening protocol, so ascertainment could differ between arms
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