Glycine includes 18 species native to East Asia and Australia, most of which are perennial legumes. The domesticated soybean, which has been cultivated in China for millennia, is derived from an East Asian annual species. The plant has leaves with three leaflets; its flowers, clustered in the leaf axils, give rise to flattened pods with only a few seeds per pod. This seed has become one of the world?s most important crops, as its protein content is superior to that of any other legume. Immature soybeans (edamame) are cooked and eaten as snacks, and soybeans can be used in recipes such as bean salad. Soy is more commonly transformed into a great variety of products such as tofu, soy sauce, soy milk, miso, soybean oil, and textured vegetable protein. Many processed foods contain soy products. Over half the world?s total production is American, although the American crop is used primarily for animal feed. Biodiesel can be manufactured from soybean, and in some areas of the world, increased soybean production is causing environmental damage.

Populations that regularly eat soy have much lower rates of some diseases, such as breast and prostate cancer. Soy contains phytoestrogenic compounds called isoflavones, such as genistein and daidzein. Phytoestrogens interact with some of the same receptor molecules on human cells that human estrogen interacts with, though their activity is far weaker. In lab studies, both soy phytoestrogens and soy protein have a variety of bioactivities that might help to prevent several diseases, including certain cancers. However, many benefits (or risks) suggested by lab studies do not exist in real life, and the question of whether soy really provides significant health benefits is controversial. Some evidence suggests that soy might only prevent breast cancer if it is consumed during youth while the breasts are developing. There is evidence that soy foods can modestly lower cholesterol.

Some have claimed that eating soy foods could promote the growth of breast cancer or even cause it, but the latter accusation in particular offers hardly any supporting evidence. However, the safety of isoflavone-enriched soy supplements is less certain. While humans have consumed plants containing phytoestrogens throughout their evolutionary history, the doses in dietary supplements can be far higher than one would ever get from eating plant foods. Compounds that are beneficial at low doses might be harmful at higher doses. Soy supplements are used to relieve hot flashes and prevent osteoporosis, as a putatively safe alternative to hormone replacement therapy, but most human studies have found that their effects are relatively weak. Soy foods may possibly interfere with absorption of artificial thyroid hormone, meaning that people who are hypothyroid and eat a lot of soy should get plenty of iodine and might need higher doses of thyroid hormone. Soy milk and soy-based formulas should not normally be given to babies, as they are nutritionally inadequate.

Selected References

Ahmad, Q. N., E. J. Britten, and D. E. Byth. 1984. The karyotype of Glycine soja and its relationship to that of the soybean, Glycine max. Cytologia (Japan) 49: 645-658.

Cooke, G. M. 2006. A review of the animal models used to investigate the health benefits of soy isoflavones. J. AOAC Int. 89: 1215-1227.

Doyle, J. J., J. L. Doyle, J. T. Rauscher, and A. H. D. Brown. 2004. Diploid and polyploid reticulate evolution throughout the history of the perennial soybeans (Glycine subgenus Glycine). New Phytol. 161: 121-132.

Goetzl, M. A., P. J. Van Veldhuizen, and J. B. Thrasher. 2007. Effects of soy phytoestrogens on the prostate. Prostate Cancer Prostatic Dis. 10: 216-223.

Hymowitz, T., and C. A. Newell. 1981. Taxonomy of the genus Glycine, domestication and uses of soybeans. Econ. Bot. 35: 272-288.

Messina, M. J., and C. L. Loprinzi. 2001. Soy for breast cancer survivors: a critical review of the literature. J. Nutr. 131: 3095S-3108S.

Messina, M., and G. Redmond. 2006. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid 16: 249-258.

Messina, M., O. Kucuk, and J. W. Lampe. 2006. An overview of the health effects of isoflavones with an emphasis on prostate cancer risk and prostate-specific antigen levels. J. AOAC Int. 89: 1121-1134.

Messina, M., W. McCaskill-Stevens, and J. W. Lampe. 2006. Addressing the soy and breast cancer relationship: review, commentary, and workshop proceedings. J. Natl. Cancer Inst. 98: 1275-1284.

Tempfer, C. B., E. K. Bentz, S. Leodolter, G. Tscherne, F. Reuss, H. S. Cross, and J. C. Huber. 2007. Phytoestrogens in clinical practice: a review of the literature. Fertil. Steril. 87: 1243-1249.

Thseng F. S, T. K. Lin, and S. T. Wu. 2000. The relations of genus Glycine subgenus Soja and Glycine formosana Hosok. collected from Taiwan: revealed by RAPD analysis. J. Jap. Bot. 75: 270-279.

Whitsett, T. G., Jr., and C. A. Lamartiniere. 2006. Genistein and resveratrol: mammary cancer chemoprevention and mechanisms of action in the rat. Expert Rev. Anticancer Ther. 6: 1699-1706.