by ecancer reporter Clare Sansom

Breast cancer is the second most common cancer type worldwide, with about 1.7 million cases diagnosed each year.

Its incidence is increasing in many countries, and this has been linked to several elements of the ‘Western’ lifestyle although the only clear epidemiological evidence is with alcohol consumption.

Historically, rates of breast cancer have been observed to be significantly lower in Mediterranean countries than in northern Europe or in North America.

This has led to suggestions that a traditional ‘Mediterranean’ diet rich in vegetables, fruit, nuts, fish and olive oil may protect against this disease.

This type of diet has already been shown to protect against cardiovascular disease, and in one previous study participants allocated to a Mediterranean diet had a 61% lower risk of any type of cancer than those on a standard ‘healthy’ one.

However, there have until now been no randomised controlled trials of the effect of the diet on breast cancer incidence.

The large PREDIMED (Prevención con Dieta Mediterránea) study was originally designed to assess the effect of Mediterranean diets on the incidence of cardiovascular disease in middle-aged and elderly people with risk factors for these conditions.

A total of 7447 people, of whom 4282 were women, were enrolled into this trial between 2003 and 2009 and randomised to receive either simple dietary advice (the control arm) or advice to keep to a Mediterranean diet supplemented with either extra virgin olive oil or nuts.

Follow-up ended in December 2010 after the Mediterranean diets had been shown to confer significant protection against cardiovascular disease; the median follow-up at that point was 4.8 years.

The incidence of five common cancer types including breast cancer in women were pre-specified as secondary outcomes for all participants without prior cancer diagnoses.

A group of researchers led by Miguel Martínez-González from the University of Navarra School of Medicine, Navarra, Spain has now re-analysed the data from this trial to investigate the effect of the Mediterranean diets on breast cancer incidence.

They reviewed the medical records and (where relevant) death certificates of all female participants in the trial for pathologically confirmed diagnoses of invasive breast cancer.

The statistical analysis used Cox and Poisson regression analyses with initial models adjusted for age, many health-related factors and adhesion to the prescribed diet.

Subgroup analysis was performed based on many of these groupings, although statistical models could not be fitted for all subgroups because of the small numbers involved.

Finally, the analyses were repeated merging the two Mediterranean diet groups together and comparing with the control group.

A total of 35 confirmed incident cases of breast cancer were observed during the follow-up period in the 4160 women participants with data available.

Out of the cancer cases where receptor status was recorded, 31 of 33 were oestrogen receptor positive, 21 of 27 were progesterone receptor positive and 12 of 21 were ERRB2 positive.

Women who had been allocated to the Mediterranean diet supplemented with olive oil had a significant reduction of 62% in breast cancer risk with a hazard ratio of 0.32 (95% confidence interval 0.13-0.79).

The risk reduction in women allocated to the Mediterranean diet supplemented with nuts was non-significant with a hazard ratio of 0.59 (95% CI 0.26-1.35).

When both Mediterranean diet groups were merged together the risk reduction was smaller than for the olive oil group but still statistically significant at 51%.

Removing cases during the first year after trial enrolment (which were unlikely to have been affected by recent dietary changes); including non-confirmed breast cancer cases as either cases or non-cases; or only including oestrogen positive breast cancers hardly changed these results.

Furthermore, the women who consumed the most olive oil had the highest reduction in breast cancer risk; the hazard ratio for each additional 5% of calories from extra virgin olive oil was 0.72 (95% CI, 0.57-0.90).

Taken together, these results suggest that the consumption of a Mediterranean diet rich in olive oil is protective against breast cancer.

This is the first published study using data from a randomised controlled trial to assess the effect of a long-term dietary intervention in preventing breast cancer.

It is, however, a secondary analysis of a trial designed initially to investigate the risk of a different disease, and few breast cancer cases were observed.

Martínez-González and his colleagues concluded by suggesting that larger studies of similar dietary interventions in breast cancer are needed to confirm their results.

Reference

Toledo, E., Salas-Salvadó, J., Donat-Vargas, C. and 18 others (2015). Mediterranean diet and invasive breast cancer risk among women at high cardiovascular risk in the PREDIMED trial: A randomized clinical trial. JAMA Internal Medicine, published ahead of print 14 September 2015.