Mediterranean Diet/EVOO and breast cancer risk
Jacob Schor, ND, FABNO
The famous PREDIMED trial from Spain reminds me of planting olive trees. One plants and cares for the trees and keeps harvesting the fruit of one’s labor for generations. The PREDIMED study was a randomized, single-blind, controlled field trial conducted at primary health care centers in Spain initially looking at the impact of diet on cardiovascular disease risk. The study was designed so that a great deal of additional information could be harvested from the initial data. The newest harvest relates to breast cancer risk and suggests a strong benefit from eating extra virgin olive oil.
This study analyzed data collected from 4282 women aged 60 to 80 years who at enrollment were free of CVD but who had either type 2 diabetes mellitus or at least 3 of the following major cardiovascular risk factors: smoking, hypertension, elevated low-density lipoprotein cholesterol level, low high-density lipoprotein cholesterol level, overweight or obesity, or family history of premature coronary heart disease.
These women were on average 67.7 years old, had an average body mass index of 30.4, most of them had undergone menopause before the age of 55 and less than 3 percent used hormone therapy.
These women were randomly allocated to a Mediterranean diet supplemented with extra-virgin olive oil, a Mediterranean diet supplemented with mixed nuts, or a control diet (n=1,391) in which they only received advice to reduce dietary fat. Participants in the 2 intervention groups were given supplementary foods for free: extra virgin olive oil (EVOO) (n=1,476), (1 Liter/wk for the participant and their families) or mixed nuts (n=1,285) (30 g/d: 15 g walnuts, 7.5 g hazelnuts, and 7.5 g almonds) according to their randomization group.
Recall that this study was initially about heart attacks and stroke. Breast cancer incidence was a secondary outcome measure that was tracked in addition to this in the women in the study who didn’t have a prior history of breast cancer (n = 4152).
The results are significant After a median follow-up of 4.8 years, 35 cases of breast cancer were identified. Observed rates (per 1000 person-years) were 1.1 for the Mediterranean diet with extra-virgin olive oil group, 1.8 for the Mediterranean diet with nuts group, and 2.9 for the control group. The multivariable-adjusted hazard ratios vs. the control group were 0.32 for the Mediterranean diet with extra-virgin olive oil group and 0.59 for the Mediterranean diet with nuts group. In analyses with yearly cumulative updated dietary exposures, the hazard ratio for each additional 5% of calories from extra-virgin olive oil was 0.72. 
There are lots of good reasons for women to consume a Mediterranean style diet This study gives further reason to encourage consumption of extra virgin olive oil (EVOO).
Basically women who ate a Mediterranean diet supplemented with EVOO showed a 68 % lower risk of getting diagnosed with breast cancer than who followed the control diet. The women eating a Mediterranean diet supplemented with nuts showed a non-significant risk reduction compared with women in the control group. Keep in mind that all the participants in this trial were for the most part following a Mediterranean diet despite their allocation to a ‘low fat diet’. The study participants changed their diets only slightly, even with quarterly counseling sessions and encouragement from dieticians. Over the nearly five years of the trial, the only significant changes made by the two Mediterranean diet groups were in fish and legume consumption; fish consumption increased by 0.3 servings/week and legumes by 0.4 servings/week in comparison to the control group. No other changes reached statistical significance, except for extra virgin olive oil or nut consumption.
Although the control group was counseled to follow a low fat diet, they were hardly compliant. The percentage of Total Energy in the diet from fat dropped non-significantly from 39 to 37%, a non-significant decrease of 1.96%, over the course of the trial. [2,3] No conclusions can be drawn about the impact of switching to a ‘low fat diet’ from the initial study’s data analysis, as no one appears to have really done so. One year into the study, nearly 92% of the “low fat diet” control group reported that olive oil was the primary culinary fat in their diets. At five years this consumption level had increased to more than 96%. More than 58% of the control group on the low fat diet reported consuming more than 4 Tablespoons of olive oil per day. It seems as if this low fat group had figured out that they were the controls and were doing their best to mimic the experimental diet.
There have been several large case control studies published in the last few years that also suggest benefit from the Mediterranean diet in reducing breast cancer incidence.
A 2014 Greek study reported that in comparing 250 newly diagnosed breast cancers with 250, age-matched controls, one unit greater adherence to the Mediterranean diet was associated with 9% lower likelihood of having breast cancer. Their data suggested the most important components and with beneficial effect were non-refined cereals, vegetables, fruits, and alcohol, followed by red meat, but with unfavorable effect. Adherence was evaluated using the 11-components MedDietScore. 
A Spanish study published in September compared 1017 breast cancer cases and 1017 matched controls examining the association between dietary patterns and BC in general and also by menopausal status and tumour subtypes. Women adhering to a Western diet had a 46% higher overall risk of breast cancer. This risk was even higher in premenopausal women, a 75% greater overall risk. Following a Mediterranean diet was associated with a 44% lower risk.
The protective effect of our Mediterranean pattern was stronger for triple-negative tumors, dropping 68%. 
A year earlier, in June 2013, data from a large cohort appeared in the International Journal of Cancer. Mediterranean diet adherence and breast cancer risk was calculated from 335,062 women recruited from 1992 to 2000, in ten European countries, and followed for an average of 11 years. A total of 9,009 postmenopausal and 1,216 premenopausal breast cancers were identified among the cohort. Mediterranean diet adherence was inversely associated with the risk of BC overall and in postmenopausal women (hazard ratio [HR] = 0.94 and HR = 0.93 respectively). The association was more pronounced in ER-/PR- tumors (HR = 0.80). Adherence was not associated with breast cancer risk in premenopausal women in this study. 
It is interesting to note that a study analyzing data on Swedish women published in 2013 failed to find a similar associations. The Swedish Women's Lifestyle and Health cohort study included 49,258 women aged 30 to 49 years at recruitment in 1991-1992. During that period, 1,278 incident breast cancers were diagnosed. Adherence to a Mediterranean dietary pattern was not statistically significantly associated with reduced risk of breast cancer overall, or with specific breast tumor characteristics.  The women in this cohort were younger and likely to have been premenopausal so these findings are similar to the findings of the previous study mentioned. It may be that the Mediterranean diet has greater impact on older, menopausal woman. [Or perhaps it only works in sunny parts of the world?]
The findings of this most recent report from the PREDIMED trial give further support to the idea that the Mediterranean diet is protective against breast cancer and that in particular EVOO may offer even greater benefit.
1. Toledo E, Salas-Salvadó J, Donat-Vargas C, Buil-Cosiales P, Estruch R, Ros E, Corella D, et al. Mediterranean Diet and Invasive Breast Cancer Risk Among Women at High Cardiovascular Risk in the PREDIMED Trial: A Randomized Clinical Trial. JAMA Intern Med. 2015 Sep 14:1-9.
2. Estruch et al: Table S7. Intake of Energy, Nutrients and Supplemental Foods at Baseline and the end of the Trial by Study Group.
3. Estruch et al: Table S8. Mean Baseline Values and Changes in Energy, Nutrient and Supplemental Food Intake by Study Arm.
4. Mourouti N, Kontogianni MD, Papavagelis C, Plytzanopoulou P, Vassilakou T, Malamos N, Linos A, Panagiotakos DB. Adherence to the Mediterranean diet is associated with lower likelihood of breast cancer: a case-control study. Nutr Cancer. 2014;66(5):810-7.
5. Castelló A, Pollán M, Buijsse B, Ruiz A, Casas AM, Baena-Cañada JM, Lope V, et al. Spanish Mediterranean diet and other dietary patterns and breast cancerrisk: case-control EpiGEICAM study. Br J Cancer. 2014 Sep 23;111(7):1454-62
Free PMC Article: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4183855/
6. Buckland G, Travier N, Cottet V, González CA, Luján-Barroso L, Agudo A, Trichopoulou A, et al. Adherence to the mediterranean diet and risk of breast cancer in the European prospective investigation into cancer and nutrition cohort study. Int J Cancer. 2013 Jun 15;132(12):2918-27.
7. Couto E, Sandin S, Löf M, Ursin G, Adami HO, Weiderpass E. Mediterranean dietary pattern and risk of breast cancer. PLoS One. 2013;8(2):e55374. doi: 10.1371/journal.pone.0055374. Epub 2013 Feb 4.
Free PMC Article: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3563544/