June 29, 2001 -- A small study in animals suggests that a substance found in soy and soy-based infant formula could have an adverse effect on growth of the thymus, which is critical to immune function.
But the study researchers -- as well as other nutritional experts -- are quick to say that the findings do not prove that soy-based formula is bad for babies.
"It's way too early to say if there is a smoking gun," says author Srikanth Yellayi, DVM, a doctoral student at the University of Illinois at Champaign. "Until we do much more research, it's too preliminary to say there is any danger."
But Yellayi's research suggests that a substance known as genistein -- an estrogen-like hormone that is found naturally in plants but not in humans -- affects the size and development of the thymus at doses that are comparable to what would be found in soy-based infant formula.
Yellayi tells WebMD it has previously been established that sex steroids, such as estrogen, have an effect on the thymus. "What we have shown is that the estrogen in soy is also having an effect on the thymus and could impair the immune system," he says.
In the study, five mice were injected with genistein, five were injected with estrogen, and five mice were given no treatment. After 21 days, the mice were compared by looking at the size of the thymus.
As expected, the animals that were treated with estrogen experienced a 50% reduction in the size of the thymus compared to mice who received no treatment. More unexpected were the results for the mice treated with genistein: They experienced a 20%-80% decrease in thymus size, depending on the dose.
The study was presented at a recent meeting of the Endocrine Society in Denver. Paul Cook, PhD, a professor of physiology at the University of Illinois, was a co-author of the study.
Yellayi says the effect was seen with doses as small as .02 mg a day -- comparable to the level of exposure for babies being fed with soy-based formula. "Thus, soy formula, which accounts for 25 percent of formula sales in the U.S., may be capable of inducing deleterious effects on the developing human infant thymus and immune system," say Yellayi and Cook.
But at least one expert on infant nutrition cast skepticism on the relevance of the study.
Susan Baker, MD, PhD, professor of pediatrics at the State University of New York at Buffalo Children's Hospital, says that there are important developmental differences between mice and other animals used in laboratory research, and humans. "The animal research is fascinating biologically, but you have to be very careful in applying it to humans," she says.
And Baker says that there has been years of experience with many thousands of babies who have been exclusively fed soy-based formula. There has been no documented evidence that the formula has any long-term adverse effects, she says.
"We have had generations of experience with soy and we have not seen a large population with adverse outcomes," Bakers says. "We haven't seen big swings in particular cancers or reproductive disorders that could be ascribed to a feeding pattern in infancy."
A 1998 statement by the American Academy of Pediatrics on the use of soy-based infant formula says that animal research on the effects of plant-derived estrogens in soy, like genistein, are not conclusive.
"In term infants whose nutritional needs are not being met from maternal breast milk or cow milk-based formulas, isolated soy protein-based formulas are safe and effective alternatives to provide appropriate nutrition for normal growth and development," the Academy says in its statement.