In the current study, we reported levels of DEHP metabolites were positively correlated with parameters of body measures (weight, BMI, and waist) and parameters of BIA (fat mass, estimated percent body fat, ECF, and ECF/ICF ratio). Although the relationship between DEHP exposure and obesity parameters has been extensively studied, this is the first report identifying a positive association between urine DEHP metabolites with ECF, and ECF/ICF ratio in a nationally representative survey of U.S. adults. The representativeness of the study population is the main advantage of this study and we were able to control many covariates in the comprehensive NHANES database.
Obesity increases the incidence of cardiovascular disease, diabetes mellitus, and certain cancers [39]. In the United States, the prevalence of obesity was 42.4% in adults [40]. Its related health issues have a significant economic impact on the U.S. medical system, estimated to be approximately US$147 billion in 2008. Traditionally, the causes of obesity is resulting from a combination of behavior, genetics, diseases and medications [41]. Recently, it has been discovered that the effects of environmental toxins, especially those defined as endocrine-disrupting chemicals (EDCs), can promote obesity and are called " obesogens " [25, 42]. In vitro studies, the mono- and dicarboxylic acid metabolites of DEHP have affinity with the receptors of peroxisome proliferator-activated receptor (PPAR)α and PPARγ [43]. PPARα is mainly involved in the regulation of gluconeogenesis, lipogenesis, and fatty acid metabolism. Activation of PPARγcauses differentiation of pre-adipocyte cells into fat tissue. In the 3T3-L1 cell model, DEHP metabolites induce adipogenesis through this mechanism [44, 45]. In animal studies, female C3H/N mice exposed to DEHP doses similar to environmental exposures increased food intake, body weight, and visceral fat deposits [46]. In male C3H/He mice, chronic exposure to DEHP may induce obesity through changes in energy homeostasis. The synergistic effect of hypothyroidism and hypothalamic leptin resistance is a possible mechanism [47].
Several recent epidemiological studies have surveyed the association between DEHP exposure and obesity in adults. All the previous studies used BMI/waist/weight gain as markers of obesity. In cross-sectional studies, one study observed higher MEHP levels were associated with increased obesity prevalence in NHANES 1999–2004 [26]. Another study using data from the 2007–2010 NHANES, the authors reported higher levels of MECPP, MEHHP, and ΣDEHP were associated with increased prevalence of obesity [25]. However, other studies found MEHP levels have inverse associations with obesity in both 1999–2002 NHANES data [29] and the Nurses’ Health Study (NHS) and NHS2 cohorts [28]. By using physiologically-based pharmacokinetic model, a study reported the relationship between BMI and DEHP may be explained by higher energy intake and the consequent higher DEHP exposure [48]. In prospective analyses, MEOHP levels have positive trends with weight gain between baseline and year 3. However, no statistically significant association was observed after 6 years [27]. In the current study, we found higher levels of DEHP metabolites were correlated with higher body measures parameters (weight, BMI, and waist). Differences between studies may be due to a variety of factors, such as race, measurement method, lifestyle, and food.
BMI and waist are generally used as screening tools for overweight or obesity with high specificity [21]. However, these two parameters are less sensitive in identifying adiposity, because they could not distinguish half of people with high body fat [49]. Some experts suggest the value of assessing body-fat percentage, which is more directly to measure adiposity [49]. In recent years, it has been proven that BIA is a more accurate tool for evaluate body composition and fluid status [50]. Previous studies on DEHP exposure and fat mass were limited to children. Some studies reported there was no correlation between DEHP exposure and adiposity [30, 32, 33]. Another report showed the associations of DEHP exposure with body fat depended on the timing of exposure [31]. In adults, there were few studies studying the association between DEHP exposure and body fat percentage. One study found no correlation between urine MEHP levels and adiposity measures in elderly women [35]. Another study enrolled participants from NHANES 1999–2006, the authors reported there was no association between MEHP and body-fat percentage but a negative correlation with lean mass among women [34]. The difference between our study and the above two studies is that we use many metabolites of DEHP as biological indicators instead of just MEHP. In this study, we found levels of MEOHP, MEHHP, MECPP, and ΣDEHP was positively correlated with estimated fat mass, estimated percent body fat. The results provided the evidence of DEHP exposure might increase fat mass/body fat percentage in adults.
The fluid volume in human depends on age, gender, and body size and is also associated with human health. Extracellular fluid (ECF) is controlled by sodium balance and total body sodium content. Osmolality of the ECF is regulated by water intake and vasopressin secretion while the ECF volume is maintained by RAAS and some natriuretic factors, including atrial/brain natriuretic peptide [10]. In addition, growth hormone and sex hormones have been found to have a role in ECF volume [23, 51]. Environmental chemicals which have capacity to inhibit 11β-HSD-2. would be predicted to elevate blood pressure and expand ECF [17]. In mice, DEHP exposure have been reported to elevate blood pressure through activation of RAAS [19]. In premature infants, higher levels of postnatal DEHP exposure are correlated with increased blood pressure and hypertension. Statistical analysis showed this relationship was mediated by the cortisol-to-cortisone ratio, suggesting the mechanism may be because DEHP inhibits the activity of 11β-HSD-2 and then activates the mineralocorticoid receptor. In this study, increased expression of ENaC and phosphorylated Na+–Cl − cotransporter were also found in hypertensive infants [20]. Moreover, adipocytes produced 30% of the total blood angiotensinogen [52] and are also an important source of extra-adrenal aldosterone [22]. It is possible that DEHP induced activation of RAAS, and then contributes to sodium retention and ECF expansion [18]. Another possible mechanism is that DEHP, as an endocrine disruptive agent, have been reported to have estrogenic activity both in vitro and in vivo [53]. The high estrogen status will then increase ENaC activity via protein kinase C δ signaling in renal cortical collecting duct cells and expands ECF volume [23]. In the current study, we provide the first evidence that levels of DEHP metabolites were positively correlated with ECF, and ECF/ICF ratio. Since increased ECF may cause stress on the cardiovascular system [12], which may be one of the possible mechanisms of DEHP causing cardiovascular disease. However, this hypothesis still needs further study to clarify.
In the subgroup analysis, the correlation between ΣDEHP and ECF/ICF ratio were more evident in subjects with women. Women are exposed to a higher concentration of phthalates than men because of the personal care products they used [54]. Furthermore, report has shown there were sex differences in the regulation of RAAS. The adrenal response to exogenous angiotensin II was significantly higher in women. It is possible that the effect of DEHP induced RAAS may be more evident in women than in men [55]. We also found that the association between ΣDEHP and the ECF/ICF ratio was more pronounced among subjects who were not current smokers. Studies show that body composition may be altered by smoking habits [56, 57]. A possible explanation is that DEHP has a much weaker effect on body fluids than tobacco smoke. When considering the impact of DEHP on active smokers, the trend is too small to be statistically significant.
Our research has several limitations. First, causal inference is not suitable for cross-sectional research. Secondly, when DEHP is exposed, other undetected chemicals may be exposed at the same time, and the correlation found in the research is not caused by DEHP itself. Third, our study population is mainly composed of adults, so we cannot infer that this association also exists in children. Lastly, other restriction includes over-adjustments because of the relatively small number of cases.