14 September 2015

A study just published in JAMA Internal Medicine found that a Mediterranean diet (MeDiet) supplemented with extra virgin olive oil (EVOO) was associated with a lower rate of breast cancer.

Dr. Miguel Martinez-Gonzalez and co-authors were studying the effects of dietary interventions in men and women at high risk of cardiovascular disease. While the study was not initially designed to evaluate the effects of dietary interventions on the rates of breast cancer, the authors reported on some interesting findings.

Men (age 55-80) and women (age 60-80) at risk for cardiovascular disease (due to diabetes or at least 3 of the following: smoking, hypertension, elevated LDL, low HDL, overweight, obesity, family history of premature cardiovascular disease) in Spain were randomized to a MeDiet supplemented with EVOO (one liter per week for the family), a MeDiet supplemented with mixed nuts (30 grams / day), or a control diet (advice to lower overall fat intake).

During a median follow up of 4.8 years, 35 cases of malignant breast cancer were demonstrated in 4282 women. Breast cancer information was not available for 122 of the women. Women in the MeDiet + EVOO group had a 62% lower incidence of breast cancer compared to the control group (68% reduction when controlled for age, body mass index, exercise and alcohol intake). Women in the MeDiet + nuts group had a non-significant reduction in breast cancer incidence compared to the control group. The women who had the lowest incidence of breast cancer consumed at least 4 tablespoons of EVOO per day.

Screen Shot 2015-09-14 at 8.05.42 PM

There are several limitations to this study. Participants were white postmenopausal women, living in Spain, at risk for cardiovascular disease, so the study results may not apply to other populations. The rates of breast cancer observed in the study are very low, and we do not know if women had mammograms or if all were free from breast cancer at study enrollment. Also, while the relative risk reduction was significant, the actual risk reduction was less so: 2.9 women per 1000 / year in the control group developed breast cancer. 1.8 / 1000 / year developed cancer in the MedDiet + nuts group, and 1.1 / 1000 / year developed cancer in the MedDiet + EVOO group.

While the polyphenols and other compounds in EVOO have been shown to have certain anti-cancer effects in some cell culture studies, it is very premature to suggest that EVOO alone will prevent the development of breast cancer. The MeDiet is high in mono-unsaturated fats (EVOO, nuts, fish, and lean meats), low in saturated fats and dairy, and high in vegetables and fruits. While olive oil is one component of a healthy diet, total caloric intake, saturated fat intake, and quantity of vegetables and fruits consumed are all important. Simply adding olive oil to a diet that is otherwise unhealthy or adding olive oil to a diet that contains more calories than appropriate is unlikely to improve overall health or reduce the rate of breast cancer development.

Dr. Mitchell Katz, in an editorial accompanying the study, noted that despite the study limitations, the MeDiet “… is known to reduce the risk of cardiovascular disease and is safe. It may also prevent breast cancer.” My recommendation based on this and other studies is not to simply supplement your current diet with olive oil, but evaluate your entire diet. Look at your current sources of fat, your current intake of fruits and vegetables, as well as total caloric intake. Olive oil may or may not turn out to be a “magic” ingredient, but many of us can benefit from incorporating components of the MeDiet into our daily routine.

Additional information on diet and lifestyle