Soybeans have an oil content of approximately 20%. To produce soybean oil, the beans are crushed, the oil is solvent-extracted with hexane, and then the oil is 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). Much of the oil sold in U.S. supermarkets as "vegetable oil" is soybean oil. Soybean oil is a dietary source of vitamin E and vitamin K. It does not have the phytoestrogen content of other soy foods, however soybean oil repeatedly reused for frying food has been found to have endocrine disrupting properties. This web page focuses on soybean oil; separate web pages 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 web page.
Breast cancer-related effects of consuming soybean oil
Soybean oil is mainly comprised of fat. Major fatty acids in soybean oil include linoleic acid (51%), 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. It is found in most foods we eat, including meats, dairy foods, vegetables, vegetable oils, fruits, legumes, nuts, seeds, cereals and breads. Since linoleic acid is the metabolic precursor of arachidonic acid (and eicosanoids derived from arachidonic acid), concern has been raised that dietary linoleic acid could increase arachidonic acid content and eicosanoid formation in the body and thereby promote cancers, cardiovascular disease, inflammation, and related disorders. Arachidonic acid also has been shown to increase aromatase activity, thus increasing circulating estrogen. Linoleic acid has been shown to encourage the growth of mammary tumors in mice.
Oleic acid is a monounsaturated fatty acid also found in olive oil. The evidence is mixed regarding oleic acid and breast cancer: oleic acid has been found both to promote and to inhibit breast cancer, depending on the mixture of other fatty acids and other components of the foods in which it is found. Alpha-linolenic acid is an omega-3 fatty acid. The protective effect on breast cancer risk of omega-3 fatty acids depends in part on the relative level of omega-6 fatty acids. The ratio of omega-6 to omega-3 fatty acids in soybean oil is 7-to-1, so that consumption is likely to increase the ratio in the direction considered harmful.
Although the fatty acid profile of soybean oil is suggestive of an unfavorable impact on breast cancer risk, fatty acid analysis must be combined with more direct evidence since different combinations of the same fatty acids can have different effects. Studies evaluating the impact of soybean oil on breast cancer are rare since this oil is not suspected to have a beneficial effect. Where soybean oil is used, it is often compared another, more promising oil. One such study found that, unlike fish oil, soybean oil did not inhibit the growth of human breast cancer cells. Another study found 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.
Population studies specifically examining the impact of soybean oil on risk of breast cancer are also relatively rare. Several Asian studies have focused on the carcinogenic effects of the fumes and smoke generated by cooking oils. Soybean oil has 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 Fahrenheit, suggesting that soybean oil should not be used for deep frying. Another study found that soybean oil generated a greater amount of carcinogenic polycyclic aromatic hydrocarbons in the smoke during heating than canola oil or sunflower oil. Breathing fumes and smoke from cooking oil has been found to be associated with higher risk of lung cancer in several Chinese studies.
One U.S. study that examined the association between the type of fat used in cooking and the risk of breast cancer found that women cooking primarily with vegetable/corn oil (both rich in linoleic acid) had a 30% higher risk of breast cancer than women normally using olive/canola oil (both rich in oleic acid). Another study 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 in an animal model of late-stage breast cancer.
Partially-hydrogenated soybean oil, found in many processed foods and restaurant meals, is to be avoided. Hydrogenation increases hardness and stabilizes fats, increasing product shelf life and decreasing refrigeration requirements. Partial hydrogenation results in the formation of trans fatty acids, which are known to increase LDL cholesterol (the "bad" cholesterol) and lower HDL cholesterol (the "good" cholesterol), thereby increasing cardiovascular disease risk factors. Consumption of trans fatty acids has also been found to be associated with increased breast cancer risk.
Below are links to recent studies concerning this food. For a more complete list, including less recent studies, please click on soybean oil.