Obesity, defined as excessive body fat (>25% for men; >30% for women), is a significant human health problem that has reached epidemic proportions in the United States over the last 2–3 decades (Oken and Gillman, 2003). Obesity and overweight are not confined to the United States. The World Health Organization has declared excessive weight as 1 of the top 10 health risks in the world, and 1 of the top 5 in developed nations. Obesity and overweight are associated with increased risk of a number of diseases and metabolic disorders including type 2 diabetes, hyperinsulinemia, insulin resistance, coronary heart disease, high blood pressure, stroke, gout, liver disease, asthma and pulmonary problems, gall bladder disease, kidney disease, reproductive problems, psychological and social problems, osteoarthritis, and certain cancers. Obesity is a significant health risk for adults, but it is a far more serious problem for children because the incidence of type 2 diabetes, usually considered an adult-onset disease, is dramatically increasing in children and adolescents along with the rise in obesity. Further, because overweight children have a greater chance of becoming overweight adults, their prospect for good future health is not promising.
Obesity, like many other chronic health problems, is caused by a complex interaction between genetic, behavioral, and environmental factors. Commonly held causes of obesity are overeating and a sedentary lifestyle, imposed on a background of genetic predisposition for the disease. Although much interest has focused on these factors, uncertainty still remains as to the etiology of obesity. Another hypothesis that has recently emerged is that in utero and early developmental exposures to environmental chemicals may play a role in the development of obesity later in life. An increasing number of studies report that exposure to chemicals during critical periods of differentiation, at low environmentally relevant doses, alters developmental programming but does not result in malformation or low birth weight, yet still causes increased susceptibility to disease later in life (National Toxicology Program, 2001; Newbold et al., 2004). With regard to obesity, Baillie-Hamilton (2002) postulated a role for chemical toxins in the etiology of obesity and reviewed data showing that the current epidemic of obesity coincides with the marked increase of chemical use in the environment. She cited numerous studies in which chemicals such as pesticides, organophosphates, carbamates, polychlorinated biphenyls, polybrominated biphenyls, phthalates, bisphenol A, heavy metals, and solvents caused weight gain, possibly by interfering with weight homeostasis, such as alterations in weight-controlling hormones, altered sensitivity to neurotransmitters, or altered activity of the sympathetic nervous system (Baillie-Hamilton, 2002). Of interest, some of the chemicals cited in the review were actually intended to increase weight, especially some of those chemicals with “hormone-like” activity.
For >20 years, research in our laboratory has focused on the effects of estrogenic compounds on development and differentiation. Our working premise has been that the developing organism is extremely sensitive to perturbation by chemicals with estrogenic or endocrine disrupting activity and that exposure to these chemicals during critical stages of differentiation may have permanent long-lasting consequences, some of which may not be expressed or detected until later in life. Diethylstilbestrol (DES) is a well-known example of such a chemical; thus, we have used DES as a model chemical to study environmental estrogens. DES, a synthetic estrogen, was widely prescribed from the 1940s through the 1970s for the prevention of threatened miscarriage. A range of 2–8 million treated pregnancies worldwide has been estimated. Today it is well recognized that prenatal DES treatment results in a low incidence of neoplasia in the female offspring and a high incidence of benign abnormalities in both the male and female offspring (NIH, 1999). To study the mechanisms involved in the toxicity of DES, we developed an animal model using outbred CD-1 mice treated with DES by subcutaneous injections on GD 9–16 (the period of major organogenesis in the mouse) or days 1–5 of neonatal life (a period of cellular differentiation of the reproductive tract and a critical period of immune and behavioral differentiation). The prenatal DES animal model has successfully duplicated and, in some cases, predicted many of the alterations (structural, function, cellular, and molecular) observed in similarly DES-exposed humans (Newbold, 1995). Although our major focus has been on reproductive tract abnormalities, we also examined the effects of DES on body weight over a wide dose range of exposure. High prenatal DES doses (10–100 μg/kg of maternal body weight) caused a decrease in the offspring's adult body weight; likewise, high neonatal DES doses (1000 μg/kg/day on days 1–5 [1 mg/kg/day]) caused a decrease in body weight later in life. However, low doses of DES (either prenatal or neonatal) caused an increase in body weight; Figure 1 illustrates control and neonatal DES 0.001 mg/kg/day treatment (DES-0.001). Note that body weight was not different between DES-exposed and unexposed controls during the time of treatment and shortly thereafter, but it gradually reached significance by 6 weeks of age. Further, data from our laboratory indicate that this increase in body weight in DES-exposed mice is associated with an increase in the percentage of body fat. Using Lunar PIXImus mouse densitometry (Lunar PIXImus, GE Healthcare, Waukesha, WI), we measured the percentage of fat in untreated controls and neonatal DES-treated mice at 16 weeks of age. A representative image generated from the mouse densitometry is shown in Figure 2. As seen in the image, mice treated neonatally with DES are markedly larger than controls. Measurements obtained from densitometry show a significant increase in the estimated body weight, estimated fat weight, and percent fat compared to controls (Table 1). Neonatal exposure to other estrogens such as 2OH estradiol (20 mg/kg/day) and 4OH estradiol (0.1 mg/kg/day), which are approximately equal estrogenic doses to DES-0.001, also caused an increase in body weight at 4 months of age (Fig. 3), suggesting that DES is not a unique estrogenic chemical in causing this increased obesity. Further, neonatal exposure to the naturally occurring phytoestrogen genistein at 50 mg/kg/day, an approximately equal estrogenic dose to DES, caused a significant increase in body weight at 3 and 4 months of age compared to untreated controls (Fig. 4). We are currently comparing the weight of fat depots from mice exposed neonatally to various environmental estrogens to determine possible alterations in adipose tissue, including size of specific fat pads and hormone levels (e.g., leptin, adiponectin). By 18 months age, differences in body weight between genistein-treated and untreated controls are difficult to determine due to large individual animal variability within groups.
|Treatmenta||Estimated body weightb||Estimated fat weight (g)||% Fat|
|Control||30.7 ± 1.3||6.5 ± 0.8||20.9 ± 1.6|
|DES 0.001||40.3 ± 2.3c||11.4 ± 1.3c||27.6 ± 1.8c|
Taken together, our data support the idea that brief exposure to low levels of environmental estrogens early in life increases body weight as the mice age. Whether our results can be extrapolated to humans, as in the reproductive abnormalities from the DES mouse model, remains to be determined, but this is a fruitful area for further research. In addition, the use of this mouse model to study mechanisms involved in altered weight homeostasis (direct and/or endocrine feedback loops, e.g., ghrelin, leptin) by environmental endocrine disrupting chemicals is an important basic research area that may shed light on the future prevention and treatment of obesity.