Study Offers Limited Support for Mediterranean Diet’s Effect on Breast Cancer Incidence

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A meta-analysis found that a Mediterranean diet with no limits on fat intake may reduce the incidence of breast cancer, as well as several other outcomes, compared with other diets. The evidence remains limited, however, and there may be no effect on all-cause mortality.

“Typical Western diets, which are high in saturated fats, sugar, and refined grains, are causally associated with development of cardiovascular disease, diabetes, and some types of cancer, including breast and colorectal cancer,” wrote study authors led by Hanna E. Bloomfield, MD, MPH, of the Minneapolis VA Medical Center in Minnesota. There has been much interest in a Mediterranean diet in recent years, given previous reports that it can reduce overall mortality.

The diet is considered mostly plant-based, and is characterized by a high ratio of monounsaturated to saturated fats; total fat accounts for between 30% and 40% of daily energy consumption. The new analysis analyzed data from 90 papers on 56 different studies, including 44 primary prevention studies and 12 secondary prevention studies. The results were published online ahead of print today in Annals of Internal Medicine.

One of the studies (the PREDIMED trial) was a large primary prevention trial that reported cancer outcomes. In that study, two groups consuming a Mediterranean diet had a significantly lower risk of breast cancer compared to a control group, with a hazard ratio of 0.43 (95% CI, 0.21–0.88).

Another 28 of the studies were cohort trials that also reported on cancer outcomes. In a pooled analysis of 13 trials with breast cancer data, there were similar incidences of breast cancer at the highest and lowest levels of adherence to a Mediterranean diet, with a relative risk of 0.96 (95% CI, 0.90–1.03).

A pooled analysis of the cohort studies did find that those in the highest quartile of diet adherence had a 14% reduction in total cancer mortality; notably, the direction of the effect was consistent across all except one of the studies included. There was also a 4% reduction in total cancer incidence.

The meta-analysis also found that a Mediterranean diet resulted in a significantly lower incidence of major cardiovascular events and diabetes, and there was inconsistent or limited evidence regarding other outcomes including cognitive function, kidney disease, rheumatoid arthritis, and quality of life.

The authors noted several limitations, including that the analysis only involved English-language publications, as well as the fact that only a few randomized controlled trials are available. Also, “the lack of a clearly articulated and widely accepted definition of a Mediterranean diet is an ongoing problem that must be resolved for this field to move forward,” they wrote.

Still, they concluded that the diet with no restriction on fat intake may indeed be associated with reduced breast cancer incidence, along with other beneficial health outcomes.