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NTP Tackles Soy Infant Formula Safety

By Mamta Behl
June 2010

Kristina  Thayer, Ph.D.
"The levels of concerns are determined on a case by case basis and integrate weight of evidence for developmental and reproductive effects in humans and laboratory animals with the extent of current human exposure levels as well as other factors that are often very specific to the chemical being evaluated," explained Thayer, above, in response to members' questions. (Photo courtesy of Steve McCaw)

Elaine  Faustman, Ph.D.
Early in the discussion, Faustman, above, expressed her uneasiness about voting on acceptance of the report. "I would encourage the NTP to re-visit the guidelines for the definition of levels of concern." (Photo courtesy of Steve McCaw)

Larry Williams, M.D.
Williams, who led off the public comment portion of the meeting, told the board, "SIF has been fed to more than 25 million human infants over the past 60 years with no known adverse effects." (Photo courtesy of Steve McCaw)

At its May 10 meeting(, the National Toxicology Program (NTP) Board of Scientific Counselors (BSC)( took up the issue of whether use of soy infant formula (SIF) may cause adverse developmental effects in humans due to the presence of estrogenic isoflavones, also known as phytoestrogens. After nearly four hours of discussion, the BSC voted six to three to support the NTP Draft Brief on Soy Infant Formula conclusion of minimal concern for adverse effects on development in infants consuming SIF.

Kristina Thayer, Ph.D., acting director of the NTP Center for the Evaluation of Risks to Human Reproduction (CERHR), opened her presentation by describing the use of SIF. "Soy infant formula has been used for the past 60 years to replace or supplement use of breast milk or cow milk-based formula and now accounts for approximately 12% of U.S. infant formula sales," explained Thayer. She further emphasized that infants fed SIF are exposed to significantly higher levels of genistein, the main phytoestrogen present in SIF, compared to those fed breast milk or cow-milk formula.

Basis for NTP's conclusion of minimal concern level of concern

The NTP assigned a level "2" to SIF on the five-level scale of concern used by the CERHR, with levels of concern ranging from "1" for negligible concern to "5" for serious concern. The basis for this conclusion stemmed from a combination of insufficient information from studies in humans to reach a conclusion on potential adversity, coupled with findings from laboratory studies demonstrating clear adverse effects on the female reproductive system in rodents exposed to genistein. Extrapolation of these results to human infants is complicated because the animals were treated with only one component of SIF.

"Although infants fed SIF can have total genistein blood levels exceeding those measured in neonatal or weanling rodents treated with genistein, the effects of genistein or its glucoside genistin may be very different when administered in SIF, which is a complex mixture of other isoflavones and non-isoflavone ingredients," Thayer cautioned. Some of these mixture issues could be addressed through a series of animal studies being proposed by the NTP, presented to the BSC later in the day by Kembra Howdeshell, Ph.D., of CERHR.

Risk communication of the NTP Brief

One of the points of reservation expressed by members of the BSC and public spokespersons was the use of what some considered confusing language by the NTP. Some of the reviewers and public speakers, including Larry Williams, M.D., who represented the International Formula Council, pointed out that expressing "minimal concern" while also stating that SIF could "possibly affect human development" might create unnecessary anxiety for parents and confuse the general public. During the discussion, several members said that they felt the need for NTP to develop a more objective, precise, and consistent evaluation scale.

Voting on the NTP Draft Brief on Soy Infant Formula

There was a lively discussion by the BSC on the extent to which the animal findings should be used to potentially raise concerns about the use of SIF, given its six decades of usage. Some panel members were reassured by the lack of reports of adverse effects in people who used soy formula during infancy. Others felt that the types of findings being observed in the animal studies had not been adequately assessed in humans.

The absence of reports of adverse effects does not necessarily mean they haven't or couldn't occur. Two prospective cohort studies of infants fed SIF are in early stages of development and will not generate results for several years - an NIEHS-funded Infant Feeding and Early Development Study (IFED) being conducted at the Children's Hospital of Philadelphia, and the USDA-funded Beginnings Study conducted by the Arkansas Children's Nutrition Center. 

In the end, two-thirds of the members voted in favor of NTP classifying the risk level as minimal concern for adverse health effects from consumption of SIF by infants. Two members, Elaine Faustman, Ph.D., and Ruthann Rudel, said they felt the level of concern was too low, while the other opponent, James Sherley, M.D., Ph.D., said he considered the level of concern too high.

(Mamta Behl, Ph.D., is a research fellow in the NTP Toxicology Branch)

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