This is the first study to investigate the associations of urinary thiocyanate, nitrate, and perchlorate concentrations with continuous and binary serum lipid variables (to explore associations with dyslipidemia) in a sample of 15,563 adults from the general population of the United States. Our two-tiered strategy first involved analyzing individual chemicals (thiocyanate, nitrate, and perchlorate) and then a mixture of the three. The latter analysis allowed us to determine the associations of co-exposure to thiocyanate, nitrate, and perchlorate with continuous and binary serum lipid variables. We discovered the following: (I) multivariate linear regression revealed that urinary thiocyanate was significantly positively associated with multiple continuous serum lipid variables (TG, TC, LDL-C, non-HDL-C, and Apo B) but not HDL-C; (II) multivariate logistic regression revealed that urinary thiocyanate was significantly positively associated with multiple binary serum lipid variables (elevated TG, TC, LDL-C, and non-HDL-C) but not Apo B or HDL-C, whereas urinary perchlorate was significantly negatively associated with elevated LDL-C; (III) multivariate RCS logistic regression revealed a linear dose–response relationship between the continuous urinary thiocyanate variable and multiple binary serum lipid variables (elevated TG, TC, LDL-C, non-HDL-C, and Apo B) and a nonlinear dose–response relationship with lowered HDL-C (IP = 1.622 mg/L); and (Ⅳ) WQS regression showed that the mixture of thiocyanate, nitrate, and perchlorate was significantly positively associated with all forms of dyslipidemia, except for elevated Apo B.
Environmental and dietary exposures to thiocyanate, nitrate, and perchlorate are ubiquitous, and the relationships between these three chemicals and human health has been widely reported. The effects of thiocyanate, nitrate, and perchlorate exposure on thyroid function in various populations have received widespread attention for decades (García Torres et al. 2022, King et al. 2022, Leung et al. 2014, Willemin &Lumen 2017). Observational studies have shown that thiocyanate, nitrate, and perchlorate are linked to higher cancer prevalence (Picetti et al. 2022, Said Abasse et al. 2022, Shiue 2015, Zhang et al. 2018), which is supported by several animal studies (Carvalho et al. 2021, Wang et al. 2022a, Wang et al. 2018). High urinary perchlorate was linked to a higher prevalence of diabetes and elevated levels of risk factors (fasting glucose, HbA1c, insulin, and homeostatic model assessment of insulin resistance) in a sample of 11,443 individuals (Liu et al. 2017). A cross-sectional study of adults in the United States showed that urinary nitrate was negatively linked to the prevalence of general and abdominal obesity, but urinary thiocyanate was positively associated with the prevalence of obesity (Zhu et al. 2019). In addition, there is strong evidence that nitrates (both pharmacological and dietary) increase CVD (heart failure (Ferguson et al. 2021), hypertension (Kapil et al. 2020), myocardial infarction (Jackson et al. 2017), and stroke (Jackson et al. 2017, Lundberg et al. 2008)) and metabolic syndrome (Kapil et al. 2020, Liu et al. 2020). However, there are few general population studies on the associations of thiocyanate, nitrate, and perchlorate exposure with continuous and binary serum lipid variables (the latter reflecting dyslipidemia). As several of the above studies suggest that long-term exposure to these three chemicals is a significant environmental risk factor for human health, they deserve further investigation.
Our analysis revealed a positive linear relationship between thiocyanate and dyslipidemia. Thiocyanate is the primary indirect marker of cyanide exposure (Bhandari et al. 2014). Myeloperoxidase (MPO)-catalyzed thiocyanate oxidation is an important endogenous source of cyanate (Bhandari et al. 2014). Cyanate has been shown to modify LDL and HDL through carbamylation (Holzer et al. 2011), which contributes to the development of AS, vascular endothelial dysfunction, and CVD (Verbrugge et al. 2015). In addition, cyanate exposure can affect liver function and lipid metabolism (Okafor et al. 2002, Sokołowska et al. 2011). Cyanate induces oxidative stress damage by inhibiting the NF-E2-related factor 2 (Nrf2)/heme oxygenase-1 (HO-1) pathway, increases reactive oxygen species (ROS) levels, impairs liver function, and causes dyslipidemia (Hu et al. 2019). In a study of mice model, the cyanate group (compared to the control group) had significantly elevated liver function biomarkers (alanine aminotransferase [ALT], aspartate aminotransferase [AST], and alkaline phosphatase [ALP]), TC, and LDL-C and significantly decreased HDL-C (Hu et al. 2019). It is believed that oxidative stress may be a significant mechanism involved in lipid metabolism (Chen et al. 2003, Tangvarasittichai 2015), and impaired antioxidant capacity can accelerate the development of hyperlipidemia. The liver's primary physiological function is to metabolize glucose and lipids, and it plays a crucial role in the regulation of insulin sensitivity, inflammatory responses, and oxidative stress (Alfaradhi et al. 2014). Moreover, thiocyanate exposure typically induces a hyperinflammatory response (White et al. 2018, Whitehouse &Jones 2009, Zhu et al. 2021). Population studies have shown that thiocyanate exposure induces allergic inflammation and is associated with allergy-related symptoms (Zhu et al. 2021). In addition, elevated thiocyanate in smokers has been associated with impaired innate immune responses (White et al. 2018). In an animal study, thiocyanate supplementation increased inflammatory responses to multiple factors that cause arthritis/fibrotic inflammation (Whitehouse &Jones 2009). Increased inflammatory responses are thought to be associated with lipid metabolism (Esteve et al. 2005, van Diepen et al. 2013), which provides insight into the potential causes of dyslipidemia associated with thiocyanate exposure. Nevertheless, further research is required to investigate the pathways between thiocyanate exposure and dyslipidemia.
Our analysis also revealed that urinary perchlorate was negatively associated with elevated LDL-C, but not other dyslipidemia indicators, which is interesting and somewhat unexpected. This is because in a recent animal experiment, mice fed a high-fat diet and exposed to perchlorate exhibited no significant changes in LDL-C, though HDL-C was significantly increased (Wang et al. 2022c). Additionally, a study of zebrafish found that perchlorate exposure (relative to control treatment) failed to alter liver or systemic lipid accumulation (Minicozzi et al. 2021). The exact mechanism by which perchlorate affected lipid metabolism in our study remains to be determined. In addition, consistent with previous findings (DesOrmeaux et al. 2021, Khalifi et al. 2015), we found that creatinine-corrected nitrate was negatively associated with dyslipidemia (elevated LDL-C and non-HDL-C).
Our study also has some limitations. First, the causality regarding the associations of urinary thiocyanate, nitrate, and perchlorate with dyslipidemia were not explored because the temporal relationship between these factors could not be determined. Second, missing serum lipid data, especially TG, LDL-C, and Apo B data, reduced the sample size and decreased the statistical power. Third, although we adjusted our models for many confounders, residual confounders may affect the results. Fourth, the NHANES data on thiocyanate, nitrate, and perchlorate were collected from a single urine spot sample per participant, which may not accurately reflect long-term exposure status.