The substance, called genistein, behaves like a watered-down version of the female sex hormone estrogen in its effects on the reproductive system. However, the researchers say they were surprised to see it could have an even greater impact than the hormone in its ability to suppress important immune functions in rodents.
Paul Cooke, a University of Illinois researcher and co-author of the study, says he and his colleagues don't believe their study proves soy formulas are dangerous for infants. However, he considers the results cause for alarm.
"We're not at all saying this is a danger to human infants," he says. "I hope there aren't problems. But there's that possibility, and I think it should be a concern."
Cooke's group reports the findings in the May 28 issue of the Proceedings of the National Academy of Sciences.
Mardi Mountford, of the International Formula Council, an industry group, calls the findings "old news," and says there's no reason for parents to be concerned.
"If you look in the scientific literature for evidence of hormonal effects when human infants consume soy formula, you don't find them," Mountford says.
Soy does have estrogen-like chemicals, called isoflavones, she adds, but breast milk also contains estrogen. The plant estrogens are many times weaker than the human hormone.
An estimated 15 percent of all infants in this country, or about 750,000 babies, drink soy formula. The milk substitute is a reasonable source of protein for children who don't nurse and don't have an easy time with cow's milk.
Some evidence has hinted that babies who drink soy formula are more vulnerable to allergies and asthma. However, recent work funded by the industry concluded that infants fed the beverage had normal immune system development.
In the latest study, Cooke, a graduate student, Srikanth Yellayi and colleagues studied the effects of genistein in mice who were either injected with or fed the substance. Female mice had their ovaries removed and the males were castrated, to simulate the hormonal environment of infancy.
Shots of the soy substance caused the animals' thymus glands -- a key immune system organ -- to shrink substantially in females. The effect increased with dose.
Not surprisingly, Cooke says, mice with withered thymuses had significantly fewer infection-fighting cells in their blood and spleen. At the highest dose of genistein -- far greater than what a baby would ingest -- the researchers saw an 80 percent reduction in these cells. The animals "would be immune-suppressed. They would be less likely to be able to amount an immune response" in the event of an infection, Cooke says.
Genistein had a similar effect on the thymus in male mice, though not quite as large, Cooke adds.
The researchers then fed mice enough genistein by weight to reach blood levels comparable to those seen in babies that drink soy-based formula. When they measured the animals' thymuses, "it looked like we were showing a very similar response" to what happened when the substance was injected.
Other experts defended soy formula.
"Generally, soy formulas are considered safe, albeit not the first choice of formulas for babies," says Dr. Melvin B. Heyman, a pediatrician at the University of California, San Francisco, and a member of the American Academy of Pediatrics' nutrition committee. "We have been reviewing much of the literature, and don't find particular reason to avoid these formulas."
The American Academy of Pediatrics recommends that babies breast-feed exclusively for the first six months of life.