A Spanish study generated worldwide headlines when its results suggested that eating a Mediterranean diet supplemented with extra virgin olive oil was linked to a lower risk of breast cancer.
Still, before you run out and buy a gallon of olive oil, there are some important points about the study you should know.
First, the study was designed to see how effective the Mediterranean diet was in preventing cardiovascular diseases, such as heart attacks and strokes, in people at high risk for cardiovascular disease. Studying breast cancer was not part of the study design. The published results were a secondary analysis from the larger PREDIMED (Prevención con Dieta Mediterránea) study which is being conducted at 16 locations in Spain.
The study was published online on Sept. 14, 2015 by JAMA Internal Medicine. Read the abstract of “Mediterranean Diet and Invasive Breast Cancer Risk Among Women at High Cardiovascular Risk in the PREDIMED Trial.”
Following a Mediterranean diet means eating a diet full of vegetables, fruits, whole grains, fish, nuts, and olive oil, and eating few dairy products and red meats.
From 2003 to 2009, the researchers randomly assigned 4,282 Spanish women ages 60 to 80 at high risk for cardiovascular disease to one of three diets:
- a Mediterranean diet supplemented with extra virgin olive oil (1,476 women)
- a Mediterranean diet supplemented with mixed nuts (walnuts, almonds, and hazelnuts) (1,285 women)
- no special diet, but the women in this group were given advice to reduce the amount of fat in their diets (1,391 women); this was the control group
Extra virgin olive oil comes from the first pressing of olives. Lower grades of olive oil come from processing the olive pulp that’s left after the first pressing.
The average age of the women was 68 years and they had an average body mass index (BMI) of 30.4 (a BMI of 30 or higher is considered obese). Most of the women had gone through menopause and less than 3% of the women had used hormone replacement therapy.
The women were to follow their prescribed diet for about 6 years. To ensure the women followed their diet, they participated in small group sessions run by a dietician every 15 weeks throughout the study. At the group sessions, women in the:
- olive oil supplementation group were given 15 liters of extra virgin olive oil (they were to consume one extra liter per week)
- nut supplementation group were given enough nuts for the next 15 weeks (they were to eat 15 grams per day of walnuts, 7.5 grams per day of hazelnuts, and 7.5 grams per day of almonds)
- control group were given canned low-fat foods, books on how to eat a low-fat diet, and other information materials
All three groups also received recipes, shopping lists, and suggestions for foods to add to their diets. All the women also filled out a 14-question survey to ensure they were following their prescribed diets.
After a follow-up of about 5 years, 35 of the women developed breast cancer.
The researchers said that women who ate the Mediterranean diet supplemented with extra virgin olive oil had a 68% relatively lower risk of developing breast cancer compared to women in the control group. Women who ate the Mediterranean diet supplemented with nuts had a slightly lower risk of breast cancer compared to the control group.
While this sounds exciting, it’s also important to know these things:
- The 35 diagnosed cases of breast cancer in 4,282 women is a very low number of cases. This could make the relative reduction in risk seem high.
- Because the researchers weren’t studying breast cancer, they don’t know how many of the women had regular screening mammograms. It could be that few women following the Mediterranean diet supplemented with olive oil were getting mammograms, so weren’t diagnosed with breast cancer. It also could be that the women in the control group had an overall higher risk of breast cancer than the women in the olive oil group. We just don’t know.
- It’s not clear if any risk reduction benefits are from the extra virgin olive oil alone or because it was being consumed along with a Mediterranean diet.
- Whether or not the women followed their prescribed diet was based on their answers to a 14-question food survey that they filled out roughly every 3 months. It’s possible that some or many of the women didn’t clearly remember what they ate for the previous 3 months or didn’t stick to their prescribed diets.
- The women in the study were all white, Spanish, between 60 and 80 years old, and at high risk of cardiovascular disease. So the results apply only to that group of women -- it’s not good science to apply these findings to all women.
Much, much more research is needed before we know if olive oil can help reduce breast cancer risk and for whom and how.
Doing all that you can do to keep your breast cancer risk as low as it can be makes good sense, including:
- eating a healthy diet that’s low in processed foods, sugar, and trans fats
- maintaining a healthy weight
- avoiding alcohol
- exercising every day
- not smoking
You can learn much more about breast cancer risk and other steps you can take to minimize your risk in the Breastcancer.org Lower Your Risk section.