Soybeans (Glycine max) have a fat content of approximately 20%. To produce soybean oil, the beans are crushed, after which the oil is solvent-extracted and refined. The remaining soybean husks are used primarily as animal feed. Soybean oil is the second-most widely produced food oil in the world, after palm oil.
Soybean oil is a common ingredient in mayonnaise and salad dressings. Much of the oil sold in U.S. supermarkets as "vegetable oil" is soybean oil or a soybean oil blend. Soybean oil is a dietary source of CoQ10, vitamin E and vitamin K, as well as several phytosterols such as campesterol. The major fatty acids in soybean oil include linoleic acid (54%), oleic acid (23%), palmitic acid (10%), alpha-linolenic acid (7%), and stearic acid (4%). Linoleic acid, an omega-6 fatty acid, is the most prevalent polyunsaturated fatty acid in the U.S. diet.
This webpage focuses on soybean oil — separate webpages cover tofu, soybean paste, soy protein isolate, and soybeans. We attempt to untangle the conflicting findings regarding breast cancer risk and soybean isoflavones in the genistein and daidzein webpage.
Breast cancer-related effects of consuming soybean oil
Soybean oil and linoleic acid have been linked to increased breast cancer risk in human, animal and cell studies. The "not recommended" rating we have given soybean oil is based on these findings, not on any estrogen content since soybean oil is not a source of phytoestrogens.
Human studies
Women with breast cancer have been found to have higher levels of omega-6 fats in their breast tissue than similar women without breast cancer. Several studies have found that lower dietary omega-6/omega-3 ratios are associated with reduced risk of breast cancer. Consuming soybean oil would tend to increase the ratio for most women because of its high omega-6 content.
Women with a specific genotype (ALOX5AP −4900 A > G polymorphism) who consume a significant amount of linoleic acid in their diets have been shown to have an increased risk of breast cancer.
Soybean oil forms 4-hydroxy-2-trans-nonenal, a mutagenic and cytotoxic product of linoleic acid, when heated to 365 degrees Fahrenheit, suggesting that the oil should not be used for deep frying. One U.S. study that examined the association between the type of fat used in cooking and the risk of breast cancer reported that women cooking primarily with vegetable or corn oil had a 30% higher risk of breast cancer than women normally using olive or canola oil.
Animal studies
Several studies have shown that a diet high in soybean oil stimulates the formation and growth of breast tumors in laboratory rats. For example, one study reported that the incidence of carcinogen-induced mammary tumors was higher in rats on a diet supplemented with 10% soybean oil than those on a 10% perilla oil diet. Several experiments also have shown that offspring of rat mothers fed high linoleic acid (corn oil) diets are more likely to develop mammary tumors than offspring of mothers fed canola oil diets.
Cell studies
Where soybean oil is used in cell studies, it is often compared another, more promising oil since soybean oil is not suspected to have a beneficial effect on breast cancer. One such study found that, unlike fish oil, soybean oil did not inhibit the growth of human breast cancer cells.
Linoleic acid has been reported to promote migration, invasion and angiogenesis of triple negative (ER-/PR-/HER2-) breast cancer cells in several studies.
Insulin is a hormone well known to be involved in the metabolism of carbohydrates. However, insulin is also a growth factor that can stimulate cell proliferation and migration. High circulating insulin levels such as those typical of type 2 diabetes have been shown to increase the risk of breast cancer and its recurrence. Linoleic acid has been shown to amplify the deleterious responses to insulin in both ER+/PR+ and triple negative breast cancer cells.
Partially-hydrogenated soybean oil should be avoided
Partially-hydrogenated soybean oil is found in many processed foods (e.g., nondairy creamers, margarine, baked goods) and restaurant meals. Hydrogenation increases hardness and stabilizes fats, increasing product shelf life and decreasing refrigeration requirements. Partial hydrogenation results in the formation of trans fatty acids ("trans fats"), which are known to increase LDL cholesterol (the bad cholesterol) and lower HDL cholesterol (the good cholesterol), thereby increasing risk of cardiovascular disease.
Consumption of trans fats has been found to be associated with increased breast cancer risk. One 2024 study reported a correlation between intake of trans fat and HER2+ breast cancer risk in particular.
Dietary trans fat has also been shown to increase heart damage caused by Adriamycin (doxorubicin) chemotherapy in a mouse model of breast cancer.
Soybean oil is not directly estrogenic
Unlike other soy foods, refined soybean oil does not contain isoflavones genistein and daidzein.
However, soybean oil repeatedly heated and reused for frying food has been found to have endocrine disrupting properties. One study using an animal model of late-stage breast cancer reported that mice fed soybean oil deteriorated by repeated reheating (such as occurs in fast food restaurants) had a sharply higher frequency of metastatic tumor formation compared to mice fed fresh soybean oil.
Additional comments
Note that the breast cancer-linoleic acid connection is not straightforward since omega-6 fats are essential to health. The ratio of omega-6 to omega-3 fats (such as the docosahexaenoic acid (DHA) found in fatty fish) appears to be more important than the absolute amounts of these fatty acids in the diet.
Dietary sources of linoleic acid
Below are common cooking oils with high levels of linoleic acid as a percentage of total fat content:
| Cooking oil | Linoleic acid |
|---|---|
| Safflower oil | 75% to 82% |
| Sunflower oil | 59% to 71% |
| Corn oil | 52% to 62% |
| Soybean oil | 51% to 56% |
Better choices are olive oil (7% to 15% linoleic acid) or canola oil (19% to 26%).
Sources of information 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 soybean oil and its components. For a more complete list, including less recent studies, please click on soybean oil.