Olive oil consumption has been associated with reduced risk of breast cancer in numerous population studies. Olive oil reduces the risk of HER2/neu overexpressing (HER2+) breast cancer in particular and has been shown to reduce the aggressiveness of breast cancer in cell and animal studies. It appears to be the phenolic compounds in virgin olive oil, not oleic acid (the main fat), that are responsible for its anti-cancer effects. Therefore, olive oil from the first pressing (extra virgin olive oil) is preferable to later pressings and highly refined olive oil might not be beneficial. However, a new population study has found almost no evidence for reduced breast cancer risk from olive oil consumption.

Olive oil consumption is associated with reduced risk of breast cancer

Olive oil is a mostly unsaturated oil consisting primarily of oleic acid (an omega-9 monounsaturated fatty acid), linoleic acid (an omega-6 fatty acid), palmitic acid, and stearic acid, as well as a variety of polyphenols. Consumption of olive oil has been reported to be associated with reduced risk of breast cancer in numerous population studies, although not all studies are in agreement. A 2011 meta-analysis of data from 19 previous case-control studies found that the highest level of olive oil consumption was associated with approximately half the risk of developing breast cancer compared with the lowest intake level of intake.

Olive oil reduces risk of HER2 overexpressing breast cancer in particular

Olive oil appears to preferentially reduce the risk of HER2+ breast cancer. A prospective study that included 8,861 Italian women identified four typical dietary patterns (1) salad vegetables (high consumption of raw vegetables and olive oil); (2) prudent (cooked vegetables, poultry, fish); (3) western (potatoes, meat, eggs, butter); and (4) canteen (pasta, tomato sauce, wine). The salad vegetables dietary pattern was found to have a pronounced protective effect against HER2+ breast cancer, which was much stronger than for HER2- cancers.

Other population studies have found a weaker effect for largely vegetable diets and have not reported a protective effect specifically for HER2+ breast cancer. It appears to be the combination of raw vegetables and olive oil that is responsible for this effect. Oleic acid has been shown to increase the bioavailability of beta-carotene in the diet, which may help explain the association with reduced breast cancer risk, but not with HER2+ breast cancer in particular.

Note that an inverse relationship between ER expression and HER2 overexpression has been found (in other words, many HER2+ tumors are estrogen receptor negative (ER-)). Olive oil has also been shown to increase Herceptin's effectiveness in treating HER2+ breast cancer.

Phenolic compounds in virgin olive oil, not oleic acid, have anti-cancer effects

Oleic acid, the main type of fat in olive oil, does not appear to account for its anti-cancer effects. While oleic acid has anti-inflammatory properties, it has also been shown to induce migration and proliferation of both ER+/PR+ and triple negative breast cancer cells. These findings imply that the protective effect of olive oil are due to other components of the oil and not the direct effect of oleic acid uptake.

Extra-virgin olive oil contains at least 30 phenolic compounds (a diverse group of aromatic compounds). Included are some familiar flavonoids (apigenin, luteolin) and phenolic acids (caffeic acid, p-coumaric acid, vanillic acid, ferulic acid). However, the main phenolic compounds are oleuropein, hydroxytyrosol and tyrosol. The phenolic compounds present in olive oil are strong antioxidants and free radical scavengers and have been shown to have anti-cancer effects in breast cancer cells. This highlights the importance of using extra-virgin olive oil (from the first pressing) rather than later pressings or more processed and refined olive oil, which tend to have less or very little phenolic content.

Olive oil consumption reduces aggressiveness of breast cancer that does develop

Consumption of olive oil reduces the aggressiveness of mammary tumors in animal models of breast cancer. One study compared induced tumors in rats that were fed high-fat diets — either olive oil or corn oil. The high corn oil diet clearly stimulated cancer development, resulting in the earliest appearance of tumors and the highest tumor incidence and yield, whereas the high extra-virgin olive oil diet appeared to have only a weak enhancing effect, primarily by increasing tumor yield compared to control animals fed a low-fat diet.

Laboratory studies have documented specific olive oil-induced molecular changes in cancer cells and tumors, including changes in the composition of cell membranes, signaling proteins and gene expression. These changes are likely to result in lower proliferation and more apoptosis (programmed cell death) of breast cancer cells, as well as enhanced protection against DNA damage.

Latest study finds little association between olive oil and breast cancer risk

The prospective study referenced at the beginning of this news article was designed to investigate the association between olive oil consumption and breast cancer risk by hormone receptor type. Population studies have been somewhat inconsistent and have primarily been case-control studies. The study included 62,284 postmenopausal women recruited from Spain, Italy and Greece into the European Prospective Investigation into Cancer and Nutrition (EPIC). The women, who had completed country-specific dietary questionnaires at baseline, were followed for an average of nine years. Results were adjusted for known breast cancer risk factors. A total of 1,256 women were diagnosed with invasive breast cancer during follow-up.

Women in the highest third of olive oil consumption were found to have essentially the same risk of breast cancer as those in the lowest third. In addition, no association was found between olive oil and risk of ER+ or progesterone receptor positive (PR+) tumors. However, there was a suggestion of a reduction in risk of ER- and PR- tumors. The authors conclude that the study results show that olive oil consumption during adult life is not associated with risk of breast cancer. However, larger prospective studies are needed to explore possible differences related to hormone receptor status, according to the authors.

Comments regarding the study

This was a large, well-designed, long-term study, with the advantage of having been prospective (data regarding diet was collected before any diagnosis of breast cancer). However, there is a great deal of evidence from other studies suggesting that olive oil consumption reduces breast cancer risk. Also, given the evidence of a protective effect for HER2+ breast cancer in particular, it would have been useful to include HER2 status, in addition to ER and PR status, in the analysis.

It is possible that the benefit of olive oil is in part a substitution effect. In other words, food that is prepared using olive oil is by definition not prepared with harmful fats. Also, olive oil users might have more healthy diets overall. In addition, the protective effect of olive oil might be more relevant for premenopausal women, who tend to have more aggressive forms of breast cancer. In any case, a single study that contradicts previous findings generally must be confirmed before the results are fully accepted.

Please see our article on olives and olive oil for more information.