For women, breast can­cer is the most fre­quently diag­nosed malig­nant tumor and is the lead­ing cause of death among women. Since 2008, breast can­cer inci­dence world­wide has increased by more than 20 per­cent and mor­tal­ity from breast can­cer has increased by 14 per­cent.

Historically breast can­cer rates have been lower in Northern or Central European coun­tries where the Mediterranean diet (MedDiet) is com­monly con­sumed. The MedDiet is char­ac­ter­ized by an abun­dance of plant foods, fish, and the con­sump­tion of extra vir­gin olive oil (EVOO). Numerous prospec­tive cohort stud­ies have eval­u­ated the asso­ci­a­tion between the MedDiet and breast can­cer risk, but results have often been lim­ited and con­flict­ing. Until recently (Sept 2015) there has never been a ran­dom­ized con­trol trial to help estab­lish more solid evi­dence.

The study by Toledo and Colleagues, pub­lished in JAMA Internal Medicine, was con­ducted in the frame­work of the Predimed trial. The Predimed trial was a Spanish mul­ti­cen­ter ran­dom­ized sin­gle blind con­trolled trial.

The sub­analy­sis done by Toledo and Colleagues took 4,282 post­menopausal women out of the 7,447 par­tic­i­pants in the Predimed trial. Women were aged 60 to 80 years and 4,152 of the 4,282 had no pre­vi­ous his­tory of breast can­cer. There were three lines of ran­dom­ized dietary rec­om­men­da­tions, a MedDiet sup­ple­mented with EVOO, a MedDiet sup­ple­mented with nuts and a con­trol group fol­low­ing gen­eral low-fat dietary advice.

According to Hoffman and Schwingshackl, pro­vid­ing com­men­tary on the Toledo trial in Evidence Based Medicine, “Invasive breast can­cer was pre­spec­i­fied as a sec­ondary out­come.” There was an aver­age fol­low-up of 4.8 years with 35 new cases of malig­nant breast can­cer iden­ti­fied by med­ical records or death cer­tifi­cates.

Incredibly, the results of the sec­ondary analy­sis revealed that the risk of inva­sive breast can­cer was reduced by 68 per­cent in those that con­sumed the MedDiet sup­ple­mented with EVOO. Whereas, there was no such reported effect for the MedDiet sup­ple­mented with nuts or the con­trol group. It was estab­lished that every addi­tional 5 per­cent of calo­ries con­sumed from EVOO reduced risk of inva­sive breast can­cer by approx­i­mately 28 per­cent.

According to Hoffman and Schwingshackl, although there is ample evi­dence around the ben­e­fi­cial effects of olive oil for can­cer pre­ven­tion, much of the research is obser­va­tional evi­dence. Hoffman says, “a dis­tin­guish­ing fea­ture of the study by Toledo and co-work­ers is the fact that it is a ran­dom­ized con­trolled trial (RCT). More pre­cisely, it is the first RCT demon­strat­ing the ben­e­fit of a MedD pat­tern on breast can­cer inci­dence.”

One lim­i­ta­tion is that breast can­cer was not a pri­mary out­come of the Predimed trial. However, given the large sam­ple size, the ran­dom­iza­tion was able to pro­vide well-bal­anced and com­pa­ra­ble groups, even larger in num­ber than many other tri­als based on mea­sur­ing spe­cific pri­mary out­comes. If any­thing the authors sug­gest that the results of their sub­analy­sis are under­es­ti­mated, sug­gest­ing that the low rate of breast can­cer inci­dence in the Predimed trial over­all could, in fact, be attrib­uted to the con­sump­tion of a MedDiet.

One other thing that is not clear, which would be ben­e­fi­cial to explore in fur­ther research, is whether “the observed ben­e­fi­cial effect was attrib­ut­able mainly to extra vir­gin olive oil or to its con­sump­tion within the con­text of the tra­di­tional MeDiet.”

Given the nature of this being a ran­dom­ized trial, one of the most pow­er­ful forms of evi­dence, what this research sug­gests is that con­sum­ing a MedDiet sup­ple­mented with EVOO could pro­vide a favor­able effect, not just in reduc­ing risk as other stud­ies have shown, but in the pri­mary pre­ven­tion of can­cer, espe­cially breast can­cer.



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