Low Phytoestrogen Levels in Feed Increase Fetal Serum Estradiol Resulting in the “Fetal Estrogenization Syndrome” and Obesity in CD-1 Mice

The typical view of estrogen is that it is associated with a reduction in food intake, fat deposition, and body weight in adults (Cooke and Naaz 2004). However, a maxim in developmental biology and pediatric medicine is that it is inappropriate to use effects in adults to predict effects during development (i.e., babies are not little adults) (National Research Council 1993). During critical periods in development, estrogenic drugs and chemicals can have unexpected stimulatory effects on the differentiation of adipocytes, as well as on postnatal growth and obesity (Cooke and Naaz 2004; Howdeshell et al. 1999; Newbold et al. 2005). There is also an extensive literature showing that administration of supplementary estrogen to rodents, even at very low doses, disrupts normal development of the reproductive system (Welshons et al. 2003, 2006). The administration of diethylstilbestrol (DES) to millions of women, and experiments in mice showing a high concordance in pathologic outcome, revealed that mice are appropriate sentinels for studies of the effects of estrogen on development in humans (Newbold 1995; Swan and vom Saal 2001). The prediction that obesity and other abnormalities are related to events (e.g., alterations in fetal hormone levels due to maternal stress, maternal nutrition, or exposure to environmental chemicals) that occur during fetal development is know as the “fetal basis of adult disease” hypothesis. For example, there is now considerable evidence that metabolic syndrome in humans is related to factors that influence growth in utero. Specifically, intrauterine growth restriction (IUGR) that is followed by a high velocity of postnatal growth results in adult obesity and associated metabolic diseases. The “thrifty phenotype” hypothesis predicts that this is due to the inability of the IUGR individual’s homeostatic systems that regulate body weight to function when there is a high level of energy intake common in developed countries (Wild and Byrne 2004; Yajnik 2001). Endocrine changes are associated with obesity (for example, there is an increase in the hormone leptin that is produced by adipocytes), and they not only impact the neuroendocrine control of metabolic systems and appetite (Kalra et al. 2005; Schwartz 2001), but they also impact reproductive tissues and processes (Baratta 2002). Although estrogenic drugs such as DES and estrogenic environmental chemicals such as bisphenol A (BPA) can profoundly disrupt normal development at very low doses in rats and mice (vom Saal and Welshons 2006; Welshons et al. 2006), there remains much controversy as to whether estrogenic chemicals that are synthesized in plants are harmful or beneficial. One basis for this controversy is related to the issue of selective estrogen receptor modulators—estrogens whose effects can differ markedly in different tissues and at different times in life (Welshons et al. 2006). Soy is the major source of plant-phytoestrogen exposure in humans and in laboratory rodents, which have been bred and maintained by commercial animal breeders for many generations on soy-containing feed. This suggests that laboratory rodents have been subjected to selection for traits that allow them to tolerate relatively high levels of phytoestrogens present in the most commonly used commercial soy-based feeds. In the present study we compared mice reared and maintained on a low- (non-soy) and high-phytoestrogen–containing soy-based feed. The objective was to determine the effects on phenotype of eliminating soy phytoestrogens from feed, because all of our prior research involving the effects of endocrine-disrupting chemicals has been conducted with animals maintained on soy-based Purina-Mills (PMI; St. Louis, MO) feed. The basis for our interest in this issue was a desire to avoid the potential disruption of experiments concerning the effects of estrogenic endocrine-disrupting chemicals because of the marked variability in phytoestrogen levels known to exist from one batch of soy to another (Heindel and vom Saal 2008). Many environmental factors influence phytoestrogen levels per gram of soy protein, and diets with a constant level of soy protein can show variable levels of phytoestrogens that are great enough to disrupt studies of the effects of DES in mice (Thigpen et al. 2003). One logical assumption would thus be to eliminate this problem by using a soy-free feed. In contrast to our prediction, we found that there is a paradoxical estrogenization of fetuses that occurs when pregnant female mice are fed a low-phytoestrogen, soy-free feed because of an elevation in endogenous fetal serum estradiol; this is related to adverse postnatal reproductive traits similar to those produced by exposing fetuses to estradiol, DES, and other man-made estrogenic chemicals such as BPA. In addition, the low-phytoestrogen feed produced a phenotype consistent with metabolic syndrome. Our findings suggest a dramatic difference in phenotype of mice exposed to plant phytoestrogens compared with estradiol or man-made estrogenic drugs and chemicals, which is consistent with other recent findings (Dolinoy et al. 2007).