HEALTH BENEFITS OF SOYBEANS

For about 5,000 years, soy has been considered one of the five sacred plants—the others being rice, wheat, barley and millet. Historical accounts document that soybean curd (tofu) was used by the Chinese as a household staple regardless of their rank in society. Soy is high in nutritional value; not only because of its supply of essential amino acids but also because it serves as a good source of fiber, B vitamins, and omega-3 fatty acids. Regardless, soy’s overall impact on health continues to be a subject of lively debate. Increasingly, the widely accepted belief in its cardiovascular benefits appears to be exaggerated, although a modest response continues to surface at least at high intakes. Likewise, the multiple clinical trials examining the effects of isoflavones or isoflavone-rich soy protein on bone mineral health have provided mixed results. Finally, evidence for the preventive or therapeutic effects of soy or its isoflavones for breast, endometrium, or prostate cancers continue to be inadequate to draw firm conclusions and are sometimes coupled with threats about adverse effects. Maybe one of the major fallacies in the literature is the assumption that soy is equivalent to soy protein or its associated isolated isoflavonoids. Undeniably, many other constituents including n-3 fatty acids, fiber or specific minerals and vitamins may be instrumental in determining an overall response to soy. Other compounds such as Lunasin, a cancer-preventive peptide found in soy and reported to reduce chemically induced tumors, may have relevance in humans. Likewise, it is illogical to assume that all individuals will respond identically to soy or its constituents. For example, its cardiovascular effects appear to depend on the level of hypercholesterolemia such that the most effect is seen with severe hypercholesterolemia, mild effectiveness for those with moderately elevated cholesterol, and little or no effect in those with mildly elevated or average cholesterol. A greater knowledge of nutrigenomics (gene-diet interactions including those associated with genomics, epigenetics and transcriptomics) should help with the identification of responders from non-responders. The biological response to soy and its isoflavones are exceedingly complex, and their ultimate cellular actions are determined by many factors including the relative levels of estrogen receptor (ER) α and β, and the diverse mixture of coactivators and corepressors present in any given cell type. Although the estrogenic potency of isoflavones is 100–1000 times less compared with 17-β-estradiol their biological potential should not be ignored since typical circulating levels may exceed endogenous estradiol concentrations by 10,000-fold following consumption of a soy-containing diet. The literature already provides evidence that estrogen receptor beta genotypes likely influence the ability of soy to modulate inflammatory biomarker (VCAM-1) as well as the ability to increase HDL cholesterol. Recent evidence points to the ability of genistein and other isoflavones in soy to reverse hypermethylation and reactivation of p16INK4a, RARbeta, and MGMT genes, again suggesting that some subpopulations may be particularly responsive to soy. It is also possible that inconsistencies in the literature may reflect the inability to achieve an exposure (threshold) needed to bring about a change in a biological process. Since the isoflavonoid content, as well as other constituents, can vary considerably, not all soy preparations should be considered equivalent. The use of metabolomic technologies have revealed that soy isoflavone ingestion in humans can significantly influence several metabolic pathways associated with osmotic fluctuation and energy metabolism. Interestingly, these responses were more pronounced following the ingestion of unconjugated soy isoflavone (miso) than a conjugated soy protein diet, suggesting that the chemical composition of the isoflavones may also influence the biological response. Additionally, increasing evidence suggests that bacterial formation of equol, which appears to be greater in Asians, may contribute to health benefits attributed to soy. Timing of exposure to soy may also be an additional factor contributing to literature inconsistencies. For example, preclinical and clinical evidence suggest that early and continual exposure may provide the greatest benefits and that introduction during adult periods may have minimal or no effects. While soy may not be the panacea once thought, it still can serve as source of essential nutrients and may offer special health benefits as well as complications to subpopulations. Future research must be devoted to identifying those individuals who will benefit most and those who might be placed at risk from exaggerated intake of soy and/or its components.

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