Bioavailability of soy isoflavones through placental/lactational transfer and soy food

Isoflavones are non-nutritive components of soy responsible for estrogenic responses observed in vitro and in experimental animals. Possible beneficial effects (e.g., reduction of serum lipids, increased bone mineral density, relief of hot flashes and other menopausal symptoms, mammary and prostate cancer chemoprevention) in humans have been attributed to consumption of isoflavones but evidence for potential adverse effects (e.g., stimulation of estrogen-dependent mammary tumors and aberrant perinatal development) has also been reported in experimental animal models. Bioavailability from appropriate food matrices and exposure during different life stages are both critical determinants of isoflavone effects. For these reasons, it is important to compare isoflavone bioavailability in adults to that in fetal and neonatal animals for a more complete understanding of potential susceptibility issues. Studies of the major soy isoflavone genistein were conducted in pregnant and lactating Sprague-Dawley rats to quantify placental and lactational transfer to plasma and brain to understand better biological effects observed in multigenerational studies. In addition, studies were conducted with genistein in adult Balb/c mice to define absolute bioavailability from both gavage and soy protein isolate (SPI)-containing food. The information derived from these studies makes it possible to predict internal exposures of children to genistein from soy infant formula, which is manufactured using SPI.