Preliminary results suggest Mediterranean diet reduces breast cancer risk

A new study suggests there may be a way for women to dramatically reduce their risk of breast cancer, without cutting calories, losing weight or taking medication.

Women who were randomly assigned to follow a Mediterranean diet – one with lots of fruits, vegetables, olive oil and fish, but not much red meat, dairy or sugar – had a 68% lower risk of breast cancer after 4.8 years, compared with women told to follow a low-fat diet, according to the study, published Monday in JAMA Internal Medicine.

But the study's results  may be too good to be true, some experts said.

The new findings come from the same Spanish study that, in 2013, found that a Mediterranean diet reduced the risk of heart attack, stroke or death from heart disease by 30% over 4.8 years. One-third of participants in the study were randomly assigned to follow a low-fat diet; one-third were told to follow a Mediterranean diet that was heavy on extra virgin olive oil; and one-third followed a Mediterranean diet with extra nuts.

Doctors immediately took notice of the 2013 study, which was rigorously designed and allowed them to draw definitive conclusions about the Mediterranean diet and heart disease.

Independent experts say the breast cancer results are far less solid.

Breast surgeon Susan Love notes that researchers didn't ask whether women were getting mammograms, which can find breast tumors years before they're large enough for women or their doctors to feel a lump. Researchers also didn't note whether women in the study had a family history of breast cancer, which can sharply increase a person's risk, said Love, author of Dr. Susan Love's Breast Book, who wasn't involved in the new study. 

Women in the study weren't asked to eat less or exercise more. There was no significant difference in weight loss or gain among the three diet groups, said study coauthor Miguel Martinez-Gonzalez at the University of Navarra in Pamplona, Spain.

Only 35 women of 4,200 women in the study developed breast cancer, said Barnett Kramer, director of cancer prevention at the National Cancer Institute, who wasn't involved with the new study. So the whopping drop in breast cancer risk came from just a handful of breast cancers. 

Seventeen women on the low-fat diet developed breast cancer, compared with 10 on the Mediterranean diet with nuts and eight on the Mediterranean diet with olive oil.

A statistical analysis showed that the differences between the low-fat and nut-rich diets could have been due to chance.

Many women may see the study as the answer to their prayers, finally giving them an easy way to protect themselves from breast cancer, which the American Cancer Society estimates will be diagnosed in 231,840 women this year, killing more than 40,000.

"We're all looking for a magic bullet, like adding extra virgin olive oil to our diets," said Marji McCullough, a nutritional epidemiologist with the cancer society who wasn't involved in the new study.

The truth, she said, is likely to be more complicated.

Even if the dramatic drop in breast cancer rates really was caused by the olive oil-rich diet, that doesn't mean that other women will benefit equally from the same diet, McCullough said. That's because the study targeted a very specific population of women at high risk of heart disease.

Women in the study had an average age of 67 and an average body mass index of 30, making them obese. Women either had diabetes or at least three other major risk factors, such as smoking, high blood pressure, bad cholesterol levels or a family history of early heart disease. The women were primarily white.

To truly resolve questions about breast cancer and a Mediterranean diet, researchers will need to conduct a rigorous clinical trial designed to look specifically at breast tumors, Love said.

Research suggests there are several ways that women can reduce their breast cancer risk, including exercising, maintaining a healthy weight and avoiding hormone replacement therapy, said Jennifer Ligibel, a breast cancer researcher at Boston's Dana-Farber Cancer Institute. Avoiding alcohol is also linked to a lower risk of breast cancer.

Women who are at high risk of breast cancer, such as those who have pre-cancerous changes in the breast, can opt to take drugs such as raloxifene, tamoxifen or aromatase inhibitors, said Ligibel, who wasn't involved in the new study. These drugs aren't recommend for average-risk women, however, because they can cause serious side effects.

None of these strategies can guarantee that women won't develop breast cancer, said Ligibel, who noted that her patients have included vegans and marathon runners.

Still, Love said there's no risk to following a Mediterranean diet, given its proven benefits for the heart.

"It's not enough to say that putting olive oil on your food is enough to prevent breast cancer," Love said. "It's not going to hurt you. And it will be tasty."