Olives (Olea europaea) and olive oil have been linked to reduced breast cancer risk. The oil consists primarily of oleic acid (an omega-9 monounsaturated fatty acid), linoleic acid (an omega-6 fatty acid), palmitic acid, and stearic acid. In addition, olive oil is a significant source of CoQ10, lupeol and oleanolic acid.
Extra-virgin olive oil contains at least 30 phenolic compounds. Included are some familiar flavonoids such as luteolin and phenolic acids such as caffeic acid, gallic acid, p-coumaric acid, vanillic acid, and ferulic acid. Luteolin has been shown to increase programmed cell death in triple negative (ER-/PR-/HER2-) breast cancer cells and also to inhibit angiogenesis (new blood vessel formation) and aromatase activity (in which androgens are converted into estrogens) in the laboratory.
However, the most important phenolic compounds include the less familiar oleuropein, oleocanthal, hydroxytyrosol, and tyrosol. Black olives also contain rutin, which may reduce the cardiotoxic effects of Adriamycin, as well as cyanidin-3-glucoside, which has been shown to possess both chemopreventive and chemotherapeutic activity and to enhance the treatment effects of Herceptin. 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 again 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.

Pure oleic acid can promote metastasis

Oleic acid might not account for olive oil's anti-cancer effects. In pure form, oleic acid has been shown to induce migration and proliferation of both hormone receptor positive (ER+/PR+) and triple negative breast cancer cells. These findings imply that the protective effect of olive oil may be due to other components of the oil and not the direct effect of oleic acid. The findings also argue against using highly refined olive oil.

Breast cancer-related effects of consuming olives and olive oil

Population studies have found link between olive oil and lower BC risk

A number of studies have found that olive oil consumption (and monounsaturated oil consumption generally) is associated with a lower risk of breast cancer. For example, a prospective Swedish study found a lower risk of breast cancer for women over 50 with a high intake of monounsaturated fat compared to consumption of other types of fat. A study of women in the Canary Islands found a lower risk of breast cancer for women in the highest quintile of monounsaturated fat consumption and even lower for those consuming mostly olive oil. Italian and Greek studies have also found that olive oil consumption is associated with a lower risk of breast cancer.
One 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. On the other hand, a large prospective study among women in Spain, Italy and Greece reported that women in the highest third of olive oil consumption had essentially the same risk of breast cancer as those in the lowest third.

Olive oil may reduce HER2+ risk preferentially

Numerous studies have found that extra virgin olive oil inhibits proliferation of and induces apoptosis of HER2 overexpressing (HER2+) breast cancer cells. These effects appear to be due to olive lignans and secoiridoids rather than olive phenols and phenolic acids. Some researchers have concluded that the protective effect of olive oil against breast cancer is mostly confined to the HER2+ breast cancer subtype, with no significant influence on the occurrence of HER2- disease.

Olive oil consumption may reduce aggressiveness of BC that does develop

Consumption of olive oil has been shown to reduce 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 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. Another study comparing olive oil to safflower oil reported similar results.
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.

Olive oil enhances effects of some treatments

Extra-virgin olive oil potentiates the effects of aromatase inhibitors in ER+/PR+ breast cancer cells by increasing programmed cell death. Oleuropein (found in green olives) has been shown to have synergistic treatment effects when combined with Adriamycin in a mouse model of triple negative breast cancer. Oleic acid has been shown to increase the effect of the chemotherapy drug Taxol (paclitaxel) chemotherapy against breast cancer. Gallic acid has also been shown to increase the effectiveness of Taxol in ER+/PR+ breast cancer cells. Olive oil has also been shown to increase Herceptin's effectiveness in treating HER2+ breast cancer and may counteract resistance to Herceptin. However, dietary olive oil was shown to reduce the effectiveness of radiotherapy in one 2024 study using an animal model of breast cancer.

Additional comments

Oleic acid has been shown to increase the bioavailability of beta-carotene in the diet, which may help explain why the high raw vegetable/high olive oil dietary pattern appears to be protective against breast cancer.
Olive oil packaged in plastic bottles has the potential to contain leached bisphenol A (BPA). Generally speaking, glass bottles are preferable, especially in the case of imported olive oil.

Sources of information provided in this webpage

The information above, which is updated continually as new research becomes available, has been developed based solely on the results of academic studies. Clicking on any of the underlined terms will take you to its tag or webpage, which contain more extensive information.
Below are links to 20 recent studies concerning olives, olive oil and their components. For a more complete list, including less recent studies, please click on olives.