Soyfoods, soybean isoflavones, and bone health: a brief overview

Soyfoods have received considerable attention during the past 5 years for their role in disease prevention, especially in relation to heart disease, osteoporosis, and cancer. However, limited research also suggests that soy protein favorably affects renal function. Much of the research interest in soy is aimed at establishing the physiological effects of isoflavones. Isoflavones are diphenolic compounds that have a very limited distribution in nature. Soybeans and soyfoods are, for practical purposes, the only nutritionally relevant dietary sources of isoflavones. Isoflavones are weak estrogens in that they bind to estrogen receptors, but they also have important nonhormonal properties as well. Initial speculation that soyfoods, and in particular isoflavones, might promote bone health was based on the estrogenic properties of isoflavones and the similarity in structure between isoflavones and the osteoporosis drug, ipriflavone, which is a synthetic isoflavone. In ovariectomized rodents, isoflavones retard bone loss almost as effectively as estrogen. Most research, but not all, also indicates that soyfoods rich in isoflavones favorably affect bone turnover and spinal bone mineral density in perimenopausal and postmenopausal women. However, studies conducted thus far have been of short duration and involved small numbers of subjects. Furthermore, no studies have actually examined the effect of soy feeding on fracture risk. Thus, although the data in general are encouraging, no firm conclusions can be drawn about the relationship between soy consumption and bone health. In addition to a possible direct effect of isoflavones on bone tissue, soy protein when substituted for animal protein may indirectly enhance bone strength. Several studies have found that in comparison with animal protein, soy protein decreases calcium excretion, a result of the lower sulfur amino acid content of soy protein. Although the high potassium content of soy is a consideration, the evidence clearly indicates that clinicians should consider recommending that their renal patients incorporate soyfoods into their diet.