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Donovan said it's an important question to ask because almost 25
percent of formula-fed babies in the United States consume soy
formula. Although babies on soy formula appear to grow normally,
these formulas contain very high concentrations of genistein, from
32 to 45 milligrams, which is higher than the amount found to affect
menstrual cycles in women, she said.
"I'm struck by the fact that these babies are receiving isoflavones
at such high concentrations," Donovan said. "Formula is the sole
source of nutrition for these infants for the first four to six
months of life, when so many important organ systems are
developing."
In the first study, published in the Journal of Nutrition in June
2004, Donovan treated intestinal cells in culture with genistein in
the amount present in soy infant formula and found that the cells
"basically stopped proliferating." However, actions seen in cells in
culture may not be seen in infants, Donovan said.
In a second study, she fed one group of newborn piglets a cow's
milk-based formula, while feeding other piglets formula supplemented
with genistein at the level found in soy formula. Newborn pigs are
an excellent model for human infants because they have a similar
metabolism and physiology, she said.
In the piglets fed genistein, the number of proliferating cells in
the intestine was 50 percent lower than piglets fed the cow's milk
formula alone. Concentrations of genistein in the piglets' blood
were similar to those of babies fed soy formula, so these data may
be applicable to human infants, the researcher said.
No differences in the activities of digestive enzymes or uptake of
nutrients were observed, which suggests that genistein had little
effect on mature intestinal cells that were no longer proliferating.
The piglet study will be published in the February 2005 issue of
Pediatric Research, but an electronic version was released Dec. 7.
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Although these data are the first to show that genistein is
bioactive in the neonatal intestine, Donovan said these results do
not mean soy formulas should not be fed to babies. "Clinical data
suggest that soy formulas are safe," she said, "but even though they
appear to be safe, they may not be without effect."
So far, Donovan has studied genistein alone, but soy formulas
contain other soy isoflavones that likely affect genistein's actions
in the intestine. The researcher plans to study those interactions.
"Soy isoflavones may have both positive and negative effects that
need to be better understood," she said.
Donovan pointed to recent research that showed adding long-chain
polyunsaturated fatty acids to infant formula improves brain and
retinal development and even IQ scores. Recent research has also
shown that adding nucleotides to baby formula causes babies to react
better to vaccinations and make more antibodies, she said.
"And although that research showed differences that were measurable,
the babies fed formula with added fatty acids or nucleotides
appeared to grow the same as other babies do," she said. "So I don't
think you can use growth as the sole parameter for normal
development."
An-Chian Chen contributed to the cell-culture study. Researchers
contributing to the piglet study were An-Chian Chen, Mark A. Berhow
and Kelly A. Tappenden. Both studies were funded by the Illinois
Council on Food and Agricultural Research and USDA.
[University of Illinois news
release]
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