A critical review of methods for comparing estrogenic activity of endogenous and exogenous chemicals in human milk and infant formula.

The two primary sources of nutrition for infants are human milk and infant formula. Both contain an array of endogenous and exogenous chemicals that may act through many separate hormonal mechanisms. The safety of infant nutrition sources has been questioned based on the possibility that exogenous chemicals may exert adverse effects on nursing or formula-fed infants through estrogen-mediated mechanisms. In response to these and other concerns, the National Research Council recommended assessing the estrogenic potency of natural and anthropogenic hormonally active agents. Furthermore, the Endocrine Disruptor Screening and Testing Advisory Committee of the U.S. Environmental Protection Agency specifically recommended testing chemicals present in human milk as a representative mixture to which large segments of the population are exposed. To date, no clinical or epidemiologic evidence demonstrates that levels of chemicals currently found in human milk or infant formulas cause adverse effects in infants. Nonetheless, the question is sufficiently important to warrant a consideration of how best to evaluate potential estrogenic risks. We reviewed the types of data available for measuring estrogenic potency as well as methods for estimating health risks from mixtures of chemicals in infant nutrition sources that act via estrogenic mechanisms. We conclude that the science is insufficiently developed at this time to allow a credible assessment of health risks to infants based on estimates of estrogenic potency or on an understanding of toxicologic effects mediated by estrogenic mechanisms. However, clinical and epidemiologic data for infant nutrition sources may provide insights about risks of such substances in human milk and infant formulas.