Breast cancer is the most common cancer in women. About 12.3% of women will experience breast cancer at some point in their life (1).
Even though modern medical treatments work well in many cases, it still causes about half a million deaths per year.
Breast cancer has become increasingly more common in the past few decades, which may be attributed to our modern lifestyles (2).
Today’s study is the first randomized controlled trial that looks at the effect of the Mediterranean diet on breast cancer risk.
It is an analysis based on data from the massive PREDIMED study, the largest study ever done on the Mediterranean diet.
Estefania Toledo, et al. Mediterranean Diet and Invasive Breast Cancer Risk Among Women at High Cardiovascular Risk in the PREDIMED Trial. JAMA Internal Medicine, 2015.
Aim of The Study
The aim of the study was to evaluate the effects of a Mediterranean diet, supplemented with either extra virgin olive oil or nuts, on the risk of breast cancer.
How The Study Was Designed
This paper is based on the famous PREDIMED study, a large multicenter randomized controlled trial on the Mediterranean diet.
The study was originally designed to test whether a Mediterranean diet reduced the risk of cardiovascular disease, which it did (7).
This part of the study only evaluated women, not men. The women were between 60 to 80 years of age, had no heart disease, but had either type 2 diabetes or at least 3 major risk factors for heart disease.
The trial included a total of 4282 women, which were randomized into 3 different diet groups:
- A Mediterranean diet supplemented with extra virgin olive oil (MED+EVOO).
- A Mediterranean diet supplemented with nuts (MED+Nuts).
- A “low-fat” diet (control group).
Those in the olive oil group received a liter (34 oz) of extra virgin olive oil per week, while those in the nut group received 30 grams (1 oz) per day of mixed nuts. These foods were provided to the participants for free.
Both group and individual sessions with dietitians were included to ensure compliance to the prescribed diets. Compliance was evaluated using dietary screening questionnaires.
There were no recommendations to cut calories or increase physical activity in any group.
The women in the study were followed for an average of 4.8 years.
After a 4.8 year study period, 35 cases of malignant breast cancer were confirmed.
Relative to the “low-fat” control group, the risk of breast cancer was lower in both of the Mediterranean diet groups:
- Med+Nuts: 38% lower risk, not statistically significant.
- Med+EVOO: 62% lower risk, statistically significant.
This graph shows the differences between the groups (click for a larger version):
When all participants were analyzed, the 20% who consumed the most extra virgin olive oil had a whopping 82% lower risk of breast cancer compared to the 20% who consumed the least.
Additionally, each 5% increase in percentage of calories from extra virgin olive oil was linked to a 28% lower risk of breast cancer. Women who consumed over 15-20% of calories from extra virgin olive oil had the lowest risk.
According to this study, a Mediterranean diet can lead to a reduced risk of breast cancer if supplemented with extra virgin olive oil.
Additionally, a dose-response relationship was found. As in, the more extra virgin olive oil the women consumed, the lower their risk of breast cancer was.
The study did not find a statistically significant effect in the Mediterranean diet group that supplemented with nuts.
The exact mechanism behind the breast cancer-lowering effects is not fully known at this point.
However, there are several possible explanations:
- The mechanism may have something to do with reduced oxidative stress on a Mediterranean diet (8, 9).
- Extra virgin olive oil also contains many beneficial plant compounds, including powerful polyphenol antioxidants (10, 11).
- Oleic acid, the main fatty acid in olive oil, has also been shown to have anti-cancer effects in some studies (12).
- Other compounds in extra virgin olive oil that may have anti-cancer effects include squalene and lignans (13, 14).
The study did have some notable limitations:
- The study was designed with the end point of cardiovascular disease in mind, not breast cancer.
- Some data on breast cancer, either before or during the study, may be missing because it wasn’t tracked as well as heart disease.
- The cases of breast cancer in the study were relatively few, only 35.
- This study included white postmenopausal women at high cardiovascular risk. The results may not apply to other age groups or ethnicities.
- The Mediterranean diet groups got better social support than the control group in the beginning of the study.
- The women in the “low-fat” control group did not reduce their fat intake substantially, so they were not really eating a low-fat diet.
Overall, however, the study was very well designed. It was a long-term randomized controlled trial looking at hard end points instead of just risk factors. This is about as good as it gets in studies on nutrition.
What do Other Studies Say?
As mentioned above, results from observational studies have been inconsistent.
One other large randomized controlled trial has been done on the Mediterranean diet and cancer risk.
This study was called the Lyon Diet Heart study, and it showed that a Mediterranean diet reduced the risk of overall cancer (not just breast cancer) by 61% (15).
Summary and Real Life Applications
According to this study, a Mediterranean diet reduces breast cancer risk in women, especially when supplemented with extra virgin olive oil.
This study also shows that the women who eat the most extra virgin olive oil have the lowest risk of breast cancer, by far.
Combined with the cardiovascular benefits (7) and overall cancer risk reduction (15) of the Mediterranean diet, these are compelling reasons to follow a Mediterranean diet if your goal is to live a long life and reduce your risk of chronic disease.
This study also provides a compelling reason to eat more extra virgin olive oil. It also has numerous other benefits, which you can read about in this article here from 2014.
Extra virgin olive oil really may be the healthiest fat you can eat. It seems like a good idea to eat it in place of other cooking fats. It is a myth that olive oil is a poor choice for cooking.