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Study Suggests Effects of Products in Soy-Based Formula on Thymus Size

WebMD Health News
Reviewed by Gary D. Vogin, MD

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.

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