WASHINGTON (AP) _ An ingredient in soy-based infant formula tends to weaken the immune system of laboratory mice, but researchers said they were uncertain if the chemical would have the same effect in humans.
Paul S. Cooke, a researcher at the University of Illinois, Urbana, said the chemical, genistein, is found in soy-based baby formula. A study being released Tuesday says genistein caused the thymus gland in young mice to shrink and the animals to lose white blood cells.
``What we have done does not prove there is a human problem,'' said Cooke, senior author of the study. ``But when you are in a situation where all of the nutrition comes from something like soy formula, the potential for something to happen that is deleterious is there.''
The study by Cooke and his colleagues appears Tuesday in the Proceedings of the National Academy of Sciences.
Soy-based infant formula is used by about 25 percent of the formula-fed babies in the United States.
Researchers at Illinois fed genistein to young lab mice and found that it shrank their thymus glands by 50 percent when compared with animals not fed the chemical.
The test mice also had a dramatic drop in their T-cell count. The T-cell is a white blood cell that plays a key role in protecting the body from infection. It is processed in the thymus gland.
Dr. Brian L. Strom of the University of Pennsylvania Center for Clinical Epidemiology & Biostatistics said the Illinois findings in mice have not been confirmed in a related study in humans.
Strom was senior author of a study that looked at the health of women, 20 to 34, who had been raised on soy-based formula. He said the study gathered data on their reproductive health and the frequency of colds and other infections.
``We did not see any particular problem,'' said Strom. ``There are no human data suggesting immune system problems from soy formula.''
Dr. Frank R. Greer, professor of pediatrics at University of Wisconsin and a member of the nutrition panel of the American Academy of Pediatrics, said that soy milk has been used commonly since the 1960s for formula-fed babies who do not thrive on cow's milk formulas.
He said the biggest concern, historically, has been that genistein, an estrogen-like compound, could have some effect on the female reproductive system. But studies, including the one by Strom and his colleagues, have found nothing to raise concern.
``We don't have enough evidence to say that we shouldn't feed soy formula,'' said Greer. When babies do not do well on cow's milk, ``you put them on soy formula and they tend to do better.''
Greer said there are alternative formulas that do not contain soy or cow's milk ``but these are expensive and hard to find.''
``I don't recommend that people stop using soy-based formula,'' he said.
Dr. R. Michael Roberts, a biochemist at the University of Missouri, said the science in Cooke's paper ``is solid and sound,'' but he emphasized that results are entirely in lab mice.
``Mice certainly aren't humans,'' said Roberts. ``The mouse immune system and the human immune system differ quite a bit.''
Although Roberts said the Cooke study could lead to more research on the effects of soy milk on babies, ``it is unclear that we should be concerned.
``Millions of children, including my own, have used soy formula,'' said Roberts.