Mediterranean diet may prevent breast cancer, but there are other reasons to pour on the olive oil

This week, a preliminary study in JAMA Internal Medicine reported that older women in Spain who ate a traditional Mediterranean diet enhanced with extra-virgin olive oil were less likely to be diagnosed with breast cancer.

To be sure, it’s no shock that a tasty, wholesome diet that’s already been proven to sharply reduce the number of heart attacks can also help to fight breast cancer. “Am I surprised that the Mediterranean diet is beneficial for breast cancer? No, because it seems to be beneficial across the board,” says Dr. Beth Overmoyer, a breast cancer specialist at the Harvard-affiliated Dana-Farber Cancer Institute.

The new findings

The breast cancer study was bootstrapped onto a landmark clinical trial in Spain called Prevención con Dieta Mediterránea (PREDIMED).  In 2013, the first results from this study established that people who ate a Mediterranean diet — rich in extra-virgin olive oil, fruits and vegetables, nuts and legumes, and other superstars of healthy eating — were 30% less likely to have heart attacks or strokes or to die from heart-related causes, compared with people who were just told to eat less fat.

But the PREDIMED researchers were not done. They also kept track of how many women were diagnosed with breast cancer during the study period so they could see if the rates were different across three different groups—women who followed the diet plus extra servings of olive oil, women who followed the diet plus extra servings of nuts, and women who were simply advised to reduce fat intake. They followed about 4,300 women ages 60 to 80.

Out of a total of 35 breast cancers diagnosed during the study period, there were 62% fewer cancers in the women who ate the olive-oil–enhanced diet, compared with women just told to cut their fat intake. The rate of breast cancer in women who ate the Mediterranean diet plus extra servings of nuts was not statistically different from that in the women told to reduce fat intake.

Is it true?

That’s great news — but it isn’t definite proof that eating Mediterranean prevents breast cancer. The scientists acknowledge that their findings need to be confirmed in a study that catches a larger number of breast cancers. That could mean a larger study, or a longer study.

The study’s conclusions are based on just 35 cases of breast cancer. The small numbers leave the study more vulnerable to factors besides diet that could have skewed the math — such as how often the women had mammograms. The researchers did not keep track of which women were having mammograms — and fewer mammograms translates into fewer cancers diagnosed. But they argue that the process that randomly assigned study participants was so thorough — like shuffling a deck of cards over and over — that any pre-existing differences between participants would have averaged out.

How good is the good news?

Dr. Overmoyer stresses that women should keep in mind that a healthy diet is only one influence of lifestyle on the risk of breast cancer. “It says a healthy diet may be very important — plus you need to exercise, plus you need to lose weight,” she says.

If diet helps, how much does it help? Based on the study’s numbers, in a group of 1,000 women who eat a Mediterranean-style diet with extra olive oil for 10 years, 14 women would be diagnosed with breast cancer. A similar group of women who only cut fat from their diet without eating in the Mediterranean style would see 29 cases, meaning 15 additional breast cancers over a decade in every 1,000 women.

That sounds much less dramatic than “62% lower risk,” and might even make you wonder how big the benefit really is. But of course, each new case of breast cancer is an actual woman facing a serious disease. “If we are looking at an individual — if you are one of those women who gets breast cancer — then that’s important,” Dr. Overmoyer says.

This study has some important limitations. It could, like other “encouraging” preliminary studies, burn brightly like a meteor for a while before subsequent research with more sobering results causes it to peter out. Fortunately, we know that the Mediterranean eating pattern prevents heart disease, a leading killer. The evidence for whether it fights breast cancer may be preliminary, but women can still consider it a smart bet.

“What is the actual risk of choosing a Mediterranean diet high in olive oil? It’s not much,” Dr. Overmoyer says. “It may be a little more expensive, but it’s still a healthy choice. There might not be a huge upside for you personally, but the downside is very low.”