Soy - Is it healthy or not?

Over the past decade, the research regarding the health benefits of soy foods and soy constituents has increased at an amazing rate. In many ways, it seems soy research has produced more questions then answers. The findings of many human studies are often inconsistent and contradictory.
Evidence does suggest that Americans would benefit from replacing some animal protein in their diet with soy protein. Decreasing the percentage of animal protein in the diet is desirable. The link between animal products and many different diseases is strongly supported in the scientific literature. Soy products can be useful in moving toward a plant-centered diet with less saturated fat, less animal protein, more plant protein and more fruits and vegetables. Soybeans are rich in protein and the amino acid content makes them a complete vegetable protein.

There is evidence to support the role of soy protein and isoflavones in lowering cholesterol levels and thus, reducing the risk of coronary heart disease. In October 1999, FDA approved the following health claim for foods containing soy protein: “A diet low in saturated fat and cholesterol that includes 25 gm of soy protein may help reduce the risk of heart disease. These soy components include trypsin inhibitors, phytic acid, saponins, isoflavones, and fiber. Since the FDA health claim was approved, ongoing studies have shown that the hypocholesterolemic effects of soy protein while relevant, are quite modest. The American Heart Association issued a statement in January this year saying that after analyzing 22 studies, an association committee found large amounts of soy in the diet reduced LDL cholesterol only 3% and had no effect on HDL cholesterol.(1)

Soybeans contain two groups of phytochemicals: saponins and isoflavones, that contain health benefits. Isoflavones are classified as phytoestrogens as they can have very mild estrogenic effects under some conditions or anti-estrogenic effects as they block the body’s hormonally active compounds. Because of this, soy isoflavones have been and are continuing to be studied to determine their relationship to conditions and diseases of particular concern to women. These include effect on menstrual cycle, menopause symptoms, osteoporosis, breast cancer, and mental decline associated with menopause and aging.

Eastern or Asian populations have a lower incidence of hormone-related diseases, such as breast cancer, uterine, and prostate cancers, cardiovascular disease and osteoporosis. It has been suggested that soy consumption is one reason for this difference in disease incidence. Women who were born in Asia, but migrated to the US had low risks of breast cancer; possibly due to their early exposure to soy. But obviously soy is only one of many factors that influence cancer risk and now we know it is the effect of many contributing factors that make a diet cancer-protective. At this time, consensus is building that soy food intake during adolescence, a time when breast tissue is most sensitive to environmental stimuli and carcinogenesis, may reduce the risk of breast cancer later in life. (2) Beyond adolescence, the results on soy and breast cancer are more difficult to discern, especially since dietary influence on cancer is greater before adulthood compared to after.

The intake of dietary and/or supplemental soy, particularly soy isoflavones, however is a concern in breast cancer survivors because there is fear that soy isoflavones may have detrimental effects in women with breast cancer or those with a history of breast cancer due to the estrogen-like effect of isoflavones. Concerns are based primarily on the estrogen-like effects of isoflavones and the results form one rodent study which showed that in immunocompromised mice with their overies surgically removed to abolish estrogen production, certain soy products and isoflavones stimulated the growth of exisiting estrogen-sensitive mammary tumors.(3)
To summarize, there are two contradictory claims being made about soy. One is that soy is protective against breast cancer and should be recommended for consumption by healthy women and breast cancer patients. The other is that soy is harmful for women with a history of or at high risk for breast cancer and because of this should be avoided by such women. There is actually a lack of convincing evidence to endorse either claim. Beans in general have dramatic benefits to protect against breast cancer. A healthy diet includes a variety of beans, and not a disproportionate share of calories from any one food. Some soy beans or tofu added to a healthy diet should not be expected to be a risk or offer significant benefit.
Extraneously to the issues above, there is internet chatter and opinions from health writers who have an agenda to vilify soy as a dangerous food. Soy may not be a super-food (such as broccoli) but the preponderance of evidence does not suggest that eating moderate amounts of unprocessed (edamame or soy beans) or lightly processed (tofu or soy milk) soy creates hypothyroidism or causes cancer. Processed foods, because of their low nutrient levels, high amount of salt, acrylamides and other toxic additives should not be considered healthy. Vegetarians and vegans who eat tofu-turkey, soy burgers, soy ice cream, soy hot dogs, soy cheese and other soy-derived processed foods on a regular basis are certainly not eating a healthy diet. Isolated soy protein is a heavily processed food with a low nutrient-per-calorie ratio. The key to good health is to eat unprocessed foods because their nutrient per calorie density is high.

Lastly, there are some legitimate health concerns from soy-based infant formulas.(4) Why should that be a surprise, since the beneficial health effects from breast milk is not even closely approximated by infant formulas. The fact that soy formulas may be worse than cow’s milk based formulas because of higher aluminum content or high isoflavone content does not criminalize the soy bean.

References

1. Erdman JW. Soy Protein and cardiovascular disease. 2000; Circulation, 102:2555. Soy Protein Shows Little Effect on “Bad” Cholesterol. American Heart Association scientific statement. January 17, 2006. Available at: http://www.americanheart.org/presenter.jhtml?identifier=3037031

2. Shu XO, Jin F, Wen W, et al. Soybean intake during adolescence and subsequent risk of breast cancer among Chinese Women. Ca Epidemiology Biomarkers and Prevention 2001; 10:483-488.

3. Hsieh, C., Santell, RC, Hasleam SZ, Helferich WG. Estogenic effects of genistein on the growth of estrogen receptor-positive human breast cancer cells in vitro and in vivo.

4. Setchell KD, Zimmer-Nechemias L. Exposure of infants to phyto-oestrogens from soy-based infant formula. Lancet 1997; 350(9070):23-7. Miniello VL, Moro GE, Tarantino M, Natile M, Granieri L, Armenio L. Soy based formulas and phyto-oestrogens: a safety profile. Acta Paediatr Suppl. 2003; 91(441): 93-100. Badger TM, Ronis MJ, Hakkak R, Rowlands JC, Korourian S. The Health Consequences of early soy consumption. J. Nutr. 2002; 132(3): 559S-559S.