Eating the Mediterranean diet – which is rich in plant foods, fish and olive oil – reduces the risk of developing breast cancer, a new Spanish study suggests.

The authors of the study describe it as “the first randomised trial finding an effect of a long-term dietary intervention on breast cancer incidence”. The PREDIMED study analysed 4,152 women aged 60 to 80 years, who were at high risk of developing cardiovascular disease and did not have a history of breast cancer. The women ate one of three diets:

• A Mediterranean diet supplemented with extravirgin olive oil
• A Mediterranean diet supplemented with mixed nuts
• A control diet in which researchers advised women to reduce dietary fat.

Healthy diet could cut breast cancer risk – is olive oil the key?

After a median of 4.8 years, 35 women had developed breast cancer. Women who ate the Mediterranean diet with extravirgin olive oil were 68 per cent less likely to develop breast cancer than controls, after adjusting for confounders. Women who ate the Mediterranean diet with nuts were 41 per cent less likely to develop breast cancer, although this was not statistically significant.

Olive oil seemed to account for much of the reduction in breast cancer risk. Each additional 5 per cent of calories that women obtained from extra-virgin olive oil was associated with a 28 per cent reduction in breast cancer risk. Women in the quintile that consumed the most olive oil were 82 per cent less likely to develop breast cancer than the quintile that ate the least.

The findings derive from an analysis of a previous trial and include relatively small numbers of breast cancers. Longer-term and larger studies are needed to confirm the results.

_{(JAMA Intern Med doi:10.1001/rnmed.2015.4838)}