In the current study, we reported that the levels of DEHP metabolites were positively correlated with parameters of body measures (weight, BMI, and waist circumference) 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 and ECF and the 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.
Recently, it has been discovered that the effects of environmental toxins, especially those defined as EDCs, can promote obesity and are called "obesogens" 27,41. In animal studies, female C3H/N mice exposed to DEHP doses similar to environmental exposures showed increased food intake, body weight, and visceral fat deposits 42. 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 43. 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 that higher MEHP levels were associated with increased obesity prevalence in NHANES 1999–2004 28. Another study using data from the 2007–2010 NHANES reported that higher levels of MECPP, MEHHP, and ΣDEHP were associated with an increased prevalence of obesity 27. However, other studies found that MEHP levels have inverse associations with obesity in both 1999–2002 NHANES data 31 and the Nurses’ Health Study (NHS) and NHS2 cohorts 30. By using a physiologically based pharmacokinetic model, a study reported that the relationship between BMI and DEHP may be explained by higher energy intake and consequent higher DEHP exposure 44. In prospective analyses, MEOHP levels had positive trends with weight gain between baseline and year 3. However, no statistically significant association was observed after 6 years 29. In the current study, we found that higher levels of DEHP metabolites were correlated with higher body measure parameters (weight, BMI, and waist circumference). Differences between studies may be due to a variety of factors, such as race, measurement method, lifestyle, and food.
BMI and waist circumference are generally used as screening tools for being overweight or obese with high specificity 21. However, these two parameters are less sensitive in identifying adiposity because they cannot distinguish half of people with high body fat 45. Some experts suggest the value of assessing body-fat percentage, which is more direct for measuring adiposity 45. In recent years, it has been proven that BIA is a more accurate tool for evaluating body composition and fluid status 46. Previous studies on DEHP exposure and fat mass were limited to children. Some studies reported that there was no correlation between DEHP exposure and adiposity 32,34,35. Another report showed that the associations of DEHP exposure with body fat depended on the timing of exposure 33. 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 37. Another study enrolled participants from NHANES 1999–2006, and the authors reported that there was no association between MEHP and body fat percentage but a negative correlation with lean mass among women 36. The difference between our study and the above two studies is that we used many metabolites of DEHP as biological indicators instead of MEHP alone. In this study, we found that the levels of MEOHP, MEHHP, MECPP, and ΣDEHP were positively correlated with estimated fat mass and estimated percent body fat. The results provided evidence that DEHP exposure might increase fat mass/body fat percentage in adults.
The fluid volume in humans depends on age, sex, 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 24,47. Environmental chemicals that have the capacity to inhibit 11β-HSD-2 would be predicted to elevate blood pressure and expand ECF 17. In mice, DEHP exposure has been reported to elevate blood pressure through activation of the RAAS 19. In premature infants, higher levels of postnatal DEHP exposure are correlated with increased blood pressure and hypertension. Statistical analysis showed that this relationship was mediated by the cortisol-to-cortisone ratio, suggesting that 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 − cotransporters were also found in hypertensive infants 20. Moreover, adipocytes produce 30% of the total blood angiotensinogen 48 and are also an important source of extra-adrenal aldosterone 22. It is possible that DEHP induces activation of RAAS and then contributes to sodium retention and ECF expansion 18. In addition, the mono- and dicarboxylic acid metabolites of DEHP have affinity with the receptors of peroxisome proliferator-activated receptor (PPAR)α and PPARγ in vitro studies 23. Activation of PPARγ results in enhanced sodium and water reabsorption by upregulation of sodium–hydrogen exchanger 3, the Na-K-2Cl cotransporter, water channels aquaporin-2, and ENaC 25. Another possible mechanism is that DEHP has been reported to have estrogenic activity both in vitro and in vivo 49. The high estrogen status will then increase ENaC activity via protein kinase C δ signaling in renal cortical collecting duct cells and expand the ECF volume 24.
In the current study, we provide the first evidence that the levels of DEHP metabolites are positively correlated with ECF and the ECF/ICF ratio. Increased ECF may cause stress on the cardiovascular system 12, which may be one of the possible mechanisms by which DEHP causes cardiovascular disease. However, this hypothesis still needs further study.
In the subgroup analysis, the correlation between ΣDEHP and the ECF/ICF ratio was more evident in women. Women are exposed to a higher concentration of phthalates than men because of the personal care products they use 50. Furthermore, a report showed that 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 51.
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 52,53. 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. Second, 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 was mainly composed of adults, so we cannot infer that this association also exists in children.