Health Effects of Soy Protein and Isoflavones in Humans
Summary: The Nutrition Committee of the American Heart Association has assessed 22 randomized trials conducted since 1999 and found that isolated soy protein with isoflavones (ISF) slightly decreased LDL cholesterol but had no effect on HDL cholesterol, triglycerides, lipoprotein(a), or blood pressure. The other effects of soy consumption were not evident. Although the contributing factors to these discrepancies are not fully understood, the source of soybeans and processing procedures of the protein or ISF are believed to be important because of their effects on the content and intactness of certain bioactive protein subunits.
Keywords: soy protein, isoflavones, health, food
Introduction
Soy foods have been consumed for centuries in Asian countries. Many potential benefits have been linked to intake of soy products according to epidemiological investigations. For instance, consumption of soy foods may contribute to lower incidences of coronary heart diseases, atherosclerosis, type 2 diabetes, and decreased risk of certain types of carcinogenesis such as breast and prostate cancers as well as better bone health and relief of menopausal symptoms. Animal and human studies have also shown that consumption of soy protein or associated isoflavones (ISF)4 has beneficial impacts on the risk factors for cardiovascular disease including lowering liver or blood triglyceride, total and LDL cholesterol levels, increasing HDL cholesterol and the ratio of HDL/LDL cholesterol. Cellular and molecular biology studies have demonstrated that soy components modulate the key transcription factors involved in the regulation of lipid metabolism and their regulated downstream gene expression in animals and in vitro cultured human cells at transcriptional or posttranslational levels.
Production and consumption of soy foods within Western countries have increased dramatically in the last decade, especially after the approval of a food-labeling health claim for soy proteins in the prevention of coronary heart disease by the U.S. FDA in 1999. To date, similar petitions have also been approved in 8 other countries including Japan in 1996, the United Kingdom in 2002, South Africa in 2002, the Philippines in 2004, Brazil in 2005, Indonesia in 2005, Korea in 2005, and Malaysia in 2006 (Table 1). However, the most recent human results obtained since 1999 show some inconsistencies in the lipid-lowering functions of soy, especially the magnitude of the effects. Moreover, studies on the other potential health benefits of soy such as prevention of postmenopausal bone loss, certain types of cancers, and diabetes and relief of menopausal symptoms remain inconclusive. Meanwhile, the potential adverse effects of certain soy components observed in animal and human studies such as antithyroid actions, endocrine disruption, and carcinogenesis enhancement potential are not well understood but are increasingly becoming a concern for soy consumers, health professionals, and policy makers. The purpose of this article is to overview current knowledge concerning health benefits and potential adverse effects of consuming products containing soy proteins and associated ISF.
TABLE 1 Health claims for soy protein in different countries
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