In news that may provide advocates of breast-feeding with yet more ammunition, researchers report today that soy-based infant formulas may impair the developing immune system. Infants drinking such formulas take in 10 times as much of an immune-suppressing, hormone-like compound as do adults eating a high-soy diet and 200 times as much as infants consuming breast milk or cows milk. The findings, published in the Proceedings of the National Academy of Sciences, raise questions about the suitability of soy products for children.

The immune system protects the body in two key ways: antibodies created by so-called B cells can attack bacteria and other toxic molecules (the humoral system), or T cells can interact directly with virus-infested cells (the cell-mediated system). In the new work, researchers at the University of Illinois found that genistein, an estrogenlike component of soybeans, compromises both of these vital branches of the immune system in mice. Mice injected with genistein exhibited an up to 80 percent decrease in the size of the thymus, the center of immune cell development, education and proliferation. In addition, genistein-injected mice possessed up to 86 percent fewer immune cells with which to fight foreign invaders, as well as significantly suppressed antibody production. Because infants receive genistein through their diet, researchers also administered the compound to mice through food instead of injection. Even then it reduced thymus size by 10 to 25 percent.

Used to feed up to 15 percent of infants in the U.S., soy-based formulas affect a significant proportion of children today. Adults taking high doses of soy supplements (as opposed to simply consuming a soy-rich diet) may also be at risk: serum levels of genistein in such adults resemble those of infants fed on soy-based formulas. "In light of our present results and other work suggesting potential immune, reproductive, and endocrine effects," the researchers conclude, "the use of soy formula for infant nutrition and high soy/isoflavone intake by adults through the use of supplements needs to be approached with caution."