Most fast food restaurants use soybean oil, or a mixture of soybean oil and other vegetable oils, to prepare French fries, chicken nuggets, hash browns and other deep-fried foods. The oil is repeatedly reheated to relatively high temperatures (typically 330 to 360 degrees Fahrenheit) and used for many hours over a period of days, although normally it is filtered daily to remove debris. This pattern of use causes "thermal abuse" of frying oil, resulting in chemical changes that appear to promote breast cancer.

Soybean oil repeatedly reused for frying food has been shown to have endocrine disrupting properties. Soybean oil has also been found to form 4-hydroxy-2-trans-nonenal, a mutagenic and cytotoxic product of the peroxidation of linoleic acid, when heated to 365 degrees, suggesting that soybean oil should not be used for deep frying. In addition, soybean oil generates carcinogenic polycyclic aromatic hydrocarbons (PAHs) in the smoke during heating.

Previous studies using animal models of breast cancer have reported that dietary corn oil and soybean oil promote mammary tumor development whereas olive oil, flaxseed oil and walnut oil inhibit it. For example, mice fed a high-fat soybean oil diet in one study experienced sharply higher levels of mammary tumor development and lung metastases than mice on a standard diet, despite gaining only modest amounts of extra weight. Now a new study has reported that mice consuming thermally abused soybean oil experienced a marked increase in lung metastasis compared to mice fed fresh soybean oil in an animal model of late-stage breast cancer.

Latest research finds reheated soybean oil promotes metastasis

The study referenced above was designed to investigate the effects of consuming thermally abused soybean oil by mice in an animal model of late-stage breast cancer. When vegetable oils are heated for use in deep frying, they undergo chemical alterations that result in new lipid compounds, including include triglyceride dimers, polymers, oxidized triglycerides, and cyclic monomers.

To conduct the study, the authors used a mouse model of stage IV breast cancer, using bioluminescent imaging to monitor metastasis of tumor cells. Mice were fed either thermally abused soybean oil or fresh soybean oil. The oils were used in preparing otherwise similar relatively low-fat diets. The thermally abused frying oil diet was found to be associated with sharply higher lung metastasis compared to the fresh soybean oil diet. In addition, mice on the thermally abused soybean oil diet had lung mets with a 1.4-fold increase in proliferation (as measured by the Ki-67 proliferation marker). The authors conclude that consumption of a low-fat diet with deteriorated oil heightened the frequency of metastatic tumor formation in this late-stage breast cancer model.