Legumes play an important role in the traditional diets of many regions throughout the world. In contrast in Western countries beans tend to play only a minor dietary role despite the fact that they are low in fat and are excellent sources of protein, dietary fiber, and a variety of micronutrients and phytochemicals. Soybeans are unique among the legumes because they are a concentrated source of isoflavones. Isoflavones have weak estrogenic properties and the isoflavone genistein influences signal transduction. Soyfoods and isoflavones have received considerable attention for their potential role in preventing and treating cancer and osteoporosis. The low breast cancer mortality rates in Asian countries and the putative antiestrogenic effects of isoflavones have fueled speculation that soyfood intake reduces breast cancer risk. The available epidemiologic data are limited and only weakly supportive of this hypothesis, however, particularly for postmenopausal breast cancer. The data suggesting that soy or isoflavones may reduce the risk of prostate cancer are more encouraging. The weak estrogenic effects of isoflavones and the similarity in chemical structure between soybean isoflavones and the synthetic isoflavone ipriflavone, which was shown to increase bone mineral density in postmenopausal women, suggest that soy or isoflavones may reduce the risk of osteoporosis. Rodent studies tend to support this hypothesis, as do the limited preliminary data from humans. Given the nutrient profile and phytochemical contribution of beans, nutritionists should make a concerted effort to encourage the public to consume more beans in general and more soyfoods in particular.

There is substantial interest in the possible anticancer effects of soy foods. In part, this is because of the historically low incidence rates of breast and prostate cancer in Asia. Of the several putative soybean chemopreventive agents, isoflavones have received the most attention. Awareness of this research has led increasing numbers of consumers to use soy foods, isoflavone-fortified foods, and isoflavone supplements. Therefore, there is a need for guidance regarding appropriate isoflavone intake levels. To this end, this article analyzed soy protein (as a surrogate for isoflavones) and isoflavone intake of the major soy food-consuming countries using individual dietary surveys for the bulk of the information. In total, 24 surveys from 4 countries that met the inclusion criteria were identified: Japan (n = 11), China (n = 7), Hong Kong (n = 4), and Singapore (n = 2). The results indicate that older Japanese adults consume approximately 6-11 g of soy protein and 25-50 mg of isoflavones (expressed as aglycone equivalents) per day. Intake in Hong Kong and Singapore is lower than in Japan, whereas significant regional intake differences exist for China. Evidence suggests that < or =10% of the Asian population consumes as much as 25 g of soy protein or 100 mg of isoflavones per day. The applicability of these findings for making soy intake recommendations for non-Asians is discussed.

A variety of health benefits, including protection against breast cancer, have been attributed to soy food consumption, primarily because of the soybean isoflavones (genistein, daidzein, glycitein). Isoflavones are considered to be possible selective estrogen receptor modulators but possess nonhormonal properties that also may contribute to their effects. Concern has arisen over a possible detrimental effect of soy in breast cancer patients because of the estrogen-like effects of isoflavones. Genistein exhibits a biphasic effect on the growth of MCF-7 cells in vitro, stimulating proliferation at low concentrations but inhibiting it at high concentrations. In ovariectomized athymic mice implanted with MCF-7 cells, both genistein and soy protein stimulate tumor growth in a dose-dependent manner. In contrast, in intact mice fed estrogen, genistein inhibits tumor growth. Although two studies in premenopausal women suggested that soy exerts estrogenic-like effects on breast tissue, recently conducted year-long studies indicated that isoflavone supplements do not affect breast tissue density in premenopausal women and may decrease density in postmenopausal women. These latter effects are opposite to those of hormone replacement therapy (HRT). Importantly, substantial data suggest that the progestogen, not the estrogen, component of HRT increases risk of developing breast cancer. Furthermore, recently conducted studies have failed to find that even HRT reduces survival in breast cancer patients. Overall, the data are not impressive that the adult consumption of soy affects the risk of developing breast cancer or that soy consumption affects the survival of breast cancer patients. Consequently, if breast cancer patients enjoy soy products, it seems reasonable for them to continue to use them.

The impact of soyfood intake on breast cancer risk has been investigated extensively. Much of this focus can be attributed to the soybean being a dietary source that is uniquely rich in isoflavones. The chemical structure of isoflavones is similar to that of estrogen, and isoflavones bind to both estrogen receptors (ERα and ERβ) (although they preferentially bind to and activate ERβ) and exert estrogen-like effects under some experimental conditions. Isoflavones also possess nonhormonal properties that are associated with the inhibition of cancer cell growth. Thus, there are several possible mechanisms by which soy may reduce the risk of breast cancer. However, the role of isoflavones in breast cancer has become controversial because, in contrast to the possible beneficial effects, some data from in vitro and animal studies suggest that isoflavones, especially genistein, the aglycone of the main soybean isoflavone genistin, may stimulate the growth of estrogen-sensitive tumors. Limited human data directly address the tumor-promoting effects of isoflavones and soy. Because the use of soyfoods and isoflavone supplements is increasing, it is important from a public health perspective to understand the impact of these products on breast cancer risk in women at high risk of the disease and on the survival of breast cancer patients. To this end, a workshop was held in November 2005 to review the existing literature and to make research recommendations. This paper summarizes the workshop findings and recommendations. The primary research recommendation is that the impact of isoflavones on breast tissue needs to be evaluated at the cellular level in women at high risk for breast cancer.

Soy foods are a traditional staple of Asian diets but because of their purported health benefits they have become popular in recent years among non-Asians, especially postmenopausal women. There are many bioactive soybean components that may contribute to the hypothesized health benefits of soy but most attention has focused on the isoflavones, which have both hormonal and nonhormonal properties. However, despite the possible benefits concerns have been expressed that soy may be contraindicated for some subsets of the population. One concern is that soy may adversely affect thyroid function and interfere with the absorption of synthetic thyroid hormone. Thus, the purpose of this review is to evaluate the relevant literature and provide the clinician guidance for advising their patients about the effects of soy on thyroid function. In total, 14 trials (thyroid function was not the primary health outcome in any trial) were identified in which the effects of soy foods or isoflavones on at least one measure of thyroid function was assessed in presumably healthy subjects; eight involved women only, four involved men, and two both men and women. With only one exception, either no effects or only very modest changes were noted in these trials. Thus, collectively the findings provide little evidence that in euthyroid, iodine-replete individuals, soy foods, or isoflavones adversely affect thyroid function. In contrast, some evidence suggests that soy foods, by inhibiting absorption, may increase the dose of thyroid hormone required by hypothyroid patients. However, hypothyroid adults need not avoid soy foods. In addition, there remains a theoretical concern based on in vitro and animal data that in individuals with compromised thyroid function and/or whose iodine intake is marginal soy foods may increase risk of developing clinical hypothyroidism. Therefore, it is important for soy food consumers to make sure their intake of iodine is adequate.

During the past 20 years, a remarkable amount of research into the health effects of soy consumption has been conducted, which in large part can be a