Breast cancer is the leading cancer for women worldwide, and its incidence has increased by more than 20% worldwide since 2008. It has been suggested that the Mediterranean diet, rich in plant foods, fish and olive oil, may reduce the risk of breast cancer. A study carried out in Spain supports this theory.
Statistically, breast cancer is less common among Mediterranean women than those living in the US or the rest of Europe, and diet has been studied as a modifiable risk factor.
The Lyon Diet Heart Study, the first clinical trial to demonstrate the beneficial effects of the Mediterranean diet in reducing heart disease, concluded that it lowered the risk of cancer generally by 61%. Overall, however, research into the influence of diet has been limited, with conflicting results.
The problem with assessing one food type is that foods are not consumed in isolation but as part of a larger diet. The effect of a single nutrient may vary when consumed in conjunction with another nutrient.
On the other hand, combinations of foods with particular nutrient constituents may interact synergistically to influence biological pathways leading to or protecting from cancer.
For this reason, the authors of the new study, published in JAMA Internal Medicine, suggest that looking at diet overall may be the best approach.
The research team, led by Dr. Miguel A. Martinez-Gonzalez of the University of Navarra in Pamplona and Centro de Investigación Biomédica en Red de la Fisiopatología de la Obesidad y Nutrición (CIBEROBN) in Madrid, aimed to evaluate the effect of the three following diet types, with participants randomly allocated in a 1:1:1 ratio:
- A Mediterranean diet supplemented with extra-virgin olive oil
- A Mediterranean diet supplemented with nuts
- A low-fat diet as prescribed by the doctor.
Participants were chosen who were already involved in the Prevención con Diet Mediterránea (PREDIMED) study, which was investigating how diet influences risk for cardiovascular disease (CVD).
The researchers included 4,282 participants from Spain aged 60-80 who were free of CVD at study baseline. However, they had either type 2 diabetes or at least three of the following major CVD risk factors: smoking, hypertension, elevated low-density lipoprotein cholesterol level, low high-density lipoprotein cholesterol level, overweight or obesity, or family history of premature coronary heart disease.
- In Belgium, there were nearly 112 cases of breast cancer per 100,000 women in 2012
- In the US, there were 92 cases per 100,000 and 80 per 100,000 in Canada
- North America and Oceania have the highest rates, while Africa and Asia have the lowest.
The group taking the Mediterranean diet with olive oil were given 1 liter a week of extra-virgin olive oil, for themselves and their families, while those taking the Mediterranean diet plus mixed nuts received 30 grams of mixed nuts per day, composed of 15 g of walnuts, 7.5 g of hazelnuts and 7.5 g of almonds.
After nearly 5 years, 35 cases of breast cancer were confirmed for the whole group.
Results showed that women who took the Mediterranean diet with extra-virgin olive oil had a 62% relatively lower risk of malignant breast cancer. The risk reduction for the Mediterranean diet with nuts was “nonsignificant,” but the risk was still less than with the low-fat diet.
Overall, the results showed an inverse association between the Mediterranean diet plus olive oil and the incidence of breast cancer, suggesting that extra-virgin olive oil can help prevent breast cancer.
A number of limitations to the research have been expressed by the research team. One is that the results come from a secondary analysis of a previous trial (the original PREDIMED research), meaning that the women were all white, menopausal and at risk of CVD. Results were also based on few (only 35) incident cases.
Being the first randomized trial to find an effect of a long-term dietary intervention on breast cancer incidence, the authors call for the results to be confirmed by further research in longer-term and larger studies.
Overall, the results suggest a beneficial effect of a Mediterranean diet supplemented with extra-virgin olive oil in the primary prevention of breast cancer.
JAMA Internal Medicine deputy editor Dr. Mitchell H. Katz echoes the team’s reservations but concludes that:
“Consumption of a Mediterranean diet […] is known to reduce the risk of cardiovascular disease and is safe. It may also prevent breast cancer. We hope to see more emphasis on Mediterranean diet to reduce cancer and cardiovascular disease and improve health and wellbeing.”
A Knowledge Center article from Medical News Today further explores the health benefits of olive oil.