Academic journal article Environmental Health Perspectives

Urinary BPA and Phthalate Metabolite Concentrations and Plasma Vitamin D Levels in Pregnant Women: A Repeated Measures Analysis

Academic journal article Environmental Health Perspectives

Urinary BPA and Phthalate Metabolite Concentrations and Plasma Vitamin D Levels in Pregnant Women: A Repeated Measures Analysis

Article excerpt

Introduction

Vitamin D is a prohormone that plays an integral role in the regulation of bone metabolism and calcium and phosphorous absorption (Holick 2007; Norman 2008). The major source of vitamin D in humans is exposure to ultraviolet B (UVB) radiation from sunlight, although it can also be obtained through dietary food sources or supplements (Thacher and Clarke 2011). Vitamin D from the skin and diet (vitamin [D.sub.2] and [D.sub.3]) is biologically inactive and is transported to the liver where it is converted to 25-hydroxyvitamin D [25(OH)D], the circulating biomarker of vitamin D nutritional status (Norman 2008; Thacher and Clarke 2011). Further metabolism occurs in the kidneys, wherein 25 (OH)D is hydroxylated to its biologically active metabolite, 1-25-dihydroxyvitamin D [1,25[(OH).sub.2]D] (Norman 2008; Thacher and Clarke 2011); 1,25[(OH).sub.2]D is a secosteroid hormone that initiates biological actions by interacting with its nuclear receptor at target tissues (Bikle 2014; Carlberg 2014; Haussler et al. 2013). Although it is well established that vitamin D plays an essential role in the development and maintenance of skeletal health, the presence of its nuclear receptor and metabolic enzymes in reproductive tissues, such as the placenta, uterus, and ovaries, indicates that vitamin D may also have regulatory functions in female reproductive and pregnancy outcomes (Grundmann and von Versen 2011; Luk et al. 2012; Ma et al. 2012; Perez-Lopez 2007).

Maintaining maternal vitamin D homeostasis in pregnancy is necessary for placentation and the maintenance of the pregnancy state as well as for normal fetal growth and development (Luk et al. 2012; Murthi et al. 2016; Ponsonby et al. 2010). Human health studies have shown that reduced levels of 25(OH)D in pregnancy are associated with various maternal and fetal complications, such as preeclampsia, spontaneous preterm birth, and restricted fetal growth (Bodnar and Simhan 2010; Bodnar et al. 2015; Murthi et al. 2016; Robinson et al. 2011). Because pregnancy represents a period of susceptibility during which slight deviations in maternal hormone levels may have detrimental maternal and fetal health consequences, pregnant women are particularly vulnerable to the effects of endocrine-disrupting chemicals.

Phthalates and bisphenol A (BPA) are industrial chemicals found in a wide range of consumer products (Meeker et al. 2009b). Exposure to these agents has been reported in pregnant women worldwide (Cantonwine et al. 2014; Casas et al. 2011; Mortensen et al. 2014; Mu et al. 2015). Both phthalates and BPA may disrupt endocrine systems, and results from epidemiological studies suggest these environmental chemicals may alter sex and thyroid hormone levels in pregnant women (Huang et al. 2007; Johns et al. 2015, 2016a; Sathyanarayana et al. 2014). Given that the active vitamin D metabolite is similar in structure to that of classic sex steroid hormones (Norman 2008), and its nuclear receptor is in the same superfamily of sex steroid and thyroid hormone receptors (Pike and Meyer 2010), it is also plausible that phthalates and/or BPA might disrupt the vitamin D endocrine axis. In our recent investigation conducted among a representative sample of U.S. adults, we reported inverse associations between urinary metabolites of di(2-ethylhexyl) phthalate (DEHP) and total 25(OH)D (Johns et al. 2016b). Urinary BPA was inversely associated with total 25(OH)D among women in our sex-stratified analyses (Johns et al. 2016b). Although our previous study showed the potential for phthalates and BPA to alter circulating levels of total 25(OH)D in adult populations, it was limited by its cross-sectional design with single biomarker measurements collected at one time point. Moreover, we are not aware of any studies that have investigated these associations in pregnant women. In the present study, we assessed the associations between environmental exposure to phthalates and BPA and plasma total 25(OH)D levels in a large, prospective cohort of pregnant women. …

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