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Epidemiological studies have shown that women of Asian cultures have a lower risk of breast cancer than their counterparts among Caucasians and Western Europeans. In fact, that risk is approximately six times lower (1). Many have suggested that this difference is due to a higher consumption of soy products among Asian women. Recent research on animals has shown that soy protein actually does reduce the risk of breast cancer. In addition, soy may also provide protection against other life-threatening diseases as well. However it should be noted most of the following studies were done on animals, not humans. Further research is required for conclusive, definitive evidence.

Breast Cancer

Of particular interest to researchers are the isoflavones found in soybeans. Isoflavones are a group of phytochemicals (chemical compounds manufactured by a plant) which are similar to the female hormone estrogen. One isoflavonoid, genistein, has been the focus of much research in the area of breast cancer. In one study, genistein was shown to inhibit the growth of a particular type of breast cancer cell in vitro (2). Another study confirmed this same effect in mice, and also reported that this isoflavonoid actually induced cell death among breast tumor cells (3). In yet a third study, rats were treated with a chemical to induce breast cancer and then fed one of several different diets. Rats receiving a diet high in soy protein had a 20% less incidence of tumors (4).

Prostate Cancer

Soy may also play a role in the inhibition of prostate cancer cell growth. In a study conducted by Byland et al., prostate cancer cells were transplanted into mice. The mice were then given one of three diets – a control diet, one containing rye bran, or one containing soy. The mice fed a soy diet developed fewer and smaller tumors than the control group mice. An increase in tumor cell death was also observed in the mice fed soy (5).

Colon Cancer

Although soy may be of benefit in other cancers, such as that of the breast and prostate, its effect on colon cancer has not yet been completely characterized. In some cases, genistein has been shown to inhibit the growth of colonic cancer cells. However, experiments conducted with rodents to determine the effects of soy have been conflicting. The relationship between soy and colon cancer has not been well established, and there is little support at this time for soy as a colon cancer weapon. (6).

Heart Disease

The positive effects on heart disease of a diet rich in soy have been well documented. Soy phytochemicals seem to lower the amount of LDL (low-density lipoprotein, or the “bad” cholesterol) and increase amounts of HDL (high-density lipoprotein, or the “good” cholesterol). Soy has also been shown to inhibit atherosclerosis in both the coronary and the carotid arteries (7, 8).

Diabetes

There has not been a great deal of research conducted regarding the effects of soy protein on diabetes. One study, indicating a possible positive effect, concluded that rats with type II diabetes might have a reduced insulin resistance when fed a diet of soy protein in combination with a diet low in polyunsaturated fatty acids (9). Another study addressed the potential problem in diabetics of a diet too high in protein leading to kidney damage. Patients were given a diet in which one half of the total protein was replaced with soy protein, with the hypothesis that soy protein would be less stressful to the kidneys. However, no significant difference was noted between the patients receiving soy and those receiving other protein (10). The lack of extensive research in this area makes it difficult to conclude what, if any, effect soy has on diabetes.

Osteoporosis

Some studies conducted in rodents suggest that soy can slow bone loss as effectively as estrogen. This may be due to the ability of soy protein to reduce calcium excretion from bones. Also, research suggests that isoflavones have a positive effect on bone density. However, most studies conducted in this area have been of short duration with too few research subjects. As a result, there are no firm conclusions at this time linking soy protein and the prevention of osteoporosis. (11)

Overall, the research indicates that soy protein does have a positive effect on certain life-threatening illnesses such as breast cancer and heart disease. However, further research needs to be conducted in the areas of osteoporosis, diabetes, and other cancers before definitive conclusions can be reached as to its effectiveness.