During the past decades, extensive studies have evaluated the diabetes risk effect of long-term exposure to metals. However, few studies excluding other chronic disease comorbidity factors and investigated the joint effects of pollutant mixture in diabetes risk.
In this study, multiple statistical strategies were implemented to comprehensively assess the effects of individual and mixed heavy metal exposures on diabetes risk and to identify the metal elements in mixture that contribute significantly to the positive association. We found that urine Co, Zn, Cu, As, Mo, Cd, Te, Tl and Pb were positively related to diabetes risk, individually and as a mixture.The dose-response relationships for the above metals were also validated in the RCS model, and the results were also stable for mixed exposure and dose-response relationships.The major contributors to the diabetes associations of the mixture, however in WQS and BKMR models, were Zn and Tl. A potential interaction effect between Zn and Tl was also observed in participants.
4.1 Zinc
Zinc is known to be an essential trace element for human growth and development, with important catalytic and regulatory functions(Mammadova-Bach & Braun, 2019). Zinc cannot be stored in the body, so it requires daily intake to maintain basal levels and support all its functions(Bonaventura, Benedetti, Albarède, & Miossec, 2015). The highest zinc content is in the islets, and zinc accumulation in the cellular granules is regulated by Zn T8, the most highly expressed zinc transporter in pancreatic tissue and the product of the SLC30A8 gene, which is responsible for transporting zinc from the cytoplasm to the insulin secretory granules(Cruz et al., 2018). It was found that Zn T8-deficient mice had impaired pancreatic β-cell function, reduced insulin secretion, low circulating insulin levels, and impaired glucose tolerance(Wijesekara et al., 2010). Therefore, diabetes, insulin and zinc have a complex relationship. Zn deficiency leads to decreased insulin stability, thereby affecting the body’s plasma glucose level(Cruz et al., 2018). Therefore, diabetic patients may take zinc-containing drugs in order to improve insulin levels and control blood glucose, resulting in high urinary zinc levels in the body(Nazem, Asadi, Jabbari, & Allameh, 2019).
The results of a cohort study of middle-aged women also showed that women with excessive urinary zinc may have an increased risk of diabetes(X. Wang, Karvonen-Gutierrez, et al., 2020). In the present study, a nonlinear positive correlation was observed between urinary Zn and diabetes risk, which was consistent with the results of previous studies(Kazi et al., 2008; A. Yang, Liu, Cheng, Pu, Cheng, et al., 2017). This mechanism might be related to the loss of zinc in pancreatic beta cells, resulting in decreased insulin secretion(Soleimanpour et al., 2010). Second, studies have confirmed that zinc excretion is significantly increased in diabetic patients, and oral zinc supplementation was expected to provide adequate benefit and protection in diabetic patients in this condition, and zinc supplementation in type 2 diabetic patients increases serum zinc levels, improved blood glucose control and antioxidant capacity had beneficial effects, reduced the concentration of glycated hemoglobin, increased superoxide dismutase (superoxide dismutase, SOD) expression level(Nazem et al., 2019).
4.2 Thallium
Thallium is a well-known highly toxic heavy metal. Because of its odourless, tasteless and water-soluble properties, the general population is under the low-dose exposure through the consumption of contaminated water and food, as well as skin or respiratory inhalation of polluted air chronically in their daily life(Kemnic & Coleman, 2022). As one of the most harmful heavy metals to mammals and the priority pollutant determined by the United States Environmental Protection Agency, Tl is considered to be more toxic than arsenic, nickel, mercury, lead or cadmium(Rodríguez-Mercado & Altamirano-Lozano, 2013).
However, there is some research evidence that thallium toxicity could induce reaction oxygen species (ROS) formation, and the increased oxidative stress could cause tissue damage and organ dysfunction(Wu et al., 2019). Oxidative stress has been widely proposed to be one of the underlying pathogenic mechanisms for insulin resistance and dysfunction of β-cell(Jiang et al., 2018). The results of animal experiments indicate that the pancreas may be the target organ of thallium toxicity, because the pancreas is one of the organs with the highest thallium content(Jiang et al., 2018). In addition, hyperglycemia was also noted in the case reports of acute thallium poisoning(Zhu et al., 2019).
There is some research epidemiological studies have shown that thallium exposure in pregnant women's urine may be a risk factor for gestational diabetes(QQ Zhang et al., 2021; Zhu et al., 2019). Based on the evidences above, we think there might be an association between thallium exposure and diabetes risk. This is in agreement with the research results of ours. But, current epidemiological data concerning the health effects from human Tl exposure is still insufficient.
In our study used the BKMR model to explore the interaction between urinary metals and diabetes risk. Our results showed that urinary zinc and thallium levels played a positive interactive role in the development of diabetes. Some studies have shown that trace thallium can be detected in zinc sulfide and other mixtures and can lead to bioaccumulation(Pavoni et al., 2017). However, the epidemiological evidence on the impact of zinc and thallium interaction on human health is still insufficient. Therefore, more in vivo and in vitro experiments are needed to verify this result. In view of this limited and epidemiological evidence, as well as the high variability and heterogeneity of zinc and thallium exposure levels in different studies, further studies are still needed to clarify the true individual and interactive effects of zinc and thallium in urine on diabetes.
4.3 Lead
Lead is a common environmental toxic metal(B. Liu et al., 2016). As a ubiquitous heavy metal, lead is widely present in the atmosphere, soil, water and food, and easily enters the human body through the digestive tract, respiratory tract, and skin, and has health effects on the human body(Ravipati, Mahajan, Sharma, Hatware, & Patil, 2021). Some researchers have investigated the relationship between lead exposure and the prevalence of diabetes, and believe that lead exposure may promote the occurrence and development of diabetes(Leff, Stemmer, Tyrrell, & Jog, 2018).
A possible mechanism is that Pb can activate the expression of genes related to glucose metabolism, thereby increasing the activity of hepatic gluconeogenesis enzymes, interfere with insulin secretion, eventually lead to elevated blood glucose(Tyrrell, Hafida, Stemmer, Adhami, & Leff, 2017). There is strong evidence that lead can also cause oxidative stress, thereby promoting insulin resistance and blood glucose(Rehman, Fatima, Waheed, & Akash, 2018). This is consistent with our findings. However, in the present study, we found through the univariate effect of the BKMR model that when Pb increased to a certain level, there would be a hypoglycemic effect, which may be related to the co-exposure of Pb and other metals, which is similar to Jing Zhang's study(J. Zhang et al., 2022). However, the specific reasons for this phenomenon remain unclear.
4.4 Other metals
Copper is the active component of many enzymes in human body and participates in various physiological activities and metabolic processes (B. Liu et al., 2016). However, excessive copper may catalyze the production of toxic reactive oxygen species, thereby damaging cells(Scheiber, Dringen, & Mercer, 2013). The results of Feng et al. showed that the increase of urinary copper level was significantly associated with the increased risk of diabetes (OR = 1.770, 95%CI: 1.107–2.831)(Feng et al., 2015). This was consistent with the positive correlation results obtained by our univariate regression model. The restricted cubic splines of this study also reflect the linear relationship. A large number of studies on diabetes have found that copper concentration was related to the occurrence and development of diabetes(Qiu, Zhang, Zhu, Wu, & Liang, 2017). Previous studies have also found that copper was positively correlated with insulin resistance, which may be the cause of copper-induced abnormal blood glucose(Kim & Song, 2014).In addition, some studies have suggested that copper excess could produce oxidative stress and become a risk factor for the onset and progression of type 2 diabetes(Bjørklund et al., 2020).
As a toxic element, cadmium exposure can cause a variety of metabolic disorders, accompanied by an imbalance of glycolipid homeostasis(Sabir et al., 2019). In the study of Hong Huihui et al., it was found that drinking water cadmium exposure increased blood glucose levels in C57/6J mice, thereby reducing serum insulin levels, causing glucose intolerance, and inhibiting insulin expression. This study demonstrated the metabolic toxicity of cadmium exposure to pancreatic beta cells at the metabolomic level, and provided new clues for the occurrence and development of cadmium exposure and diabetes(Hong et al., 2022). It also provides mechanistic evidence for our analysis results.
In a study on the relationship between urinary arsenic and insulin resistance, it was found that total urinary arsenic exposure may be related to insulin resistance(Zhou, Zhao, & Huang, 2022). Qiang Zhang et al found that efficient arsenic metabolism was associated with higher odds of diabetes in the results of a baseline survey of the Chinese Arsenic and Non-Infectious Diseases Cohort (AsNCD)(Q Zhang et al., 2020). Urinary dimethylarsenic acid interacts with individual factors to synergistically affect the occurrence of diabetes in Chinese population. In conclusion, this study also verified that our analysis results were consistent with them.
Our study found that Co and Mo levels were strongly correlated with increased diabetes risk in participants. Jingli Yang et al. also found that Significant sex-specific and dose-response relationships were observed between urinary metals (Co and Mo) and diabetes-related indicators (J. Yang, Lu, Bai, & Cheng, 2023). According to Lai et al. Co could lead to insulin resistance and diabetes at low levels(Lai et al., 2018). Moreover, studies have also found that Co toxicity may lead to mitochondrial dysfunction, which also plays a key role in the development of diabetes(Rovira-Llopis et al., 2017). On the other hand, some studies have found that cobalt has a potential hypoglycemic effect and can prevent the development of diabetes(Nomura, Okamoto, Sakamoto, Feng, & Nakamura, 2005).Therefore, patients with diabetes may take cobalt containing drugs to increase the solid content in urine.
Molybdenum is a transition metal element, which is a necessary trace element for human body, animals and plants. Xiao et al. found that high urinary molybdenum concentration will increase the risk of diabetes(Xiao et al., 2018).Rotter et al. found that molybdenum concentration was positively correlated with insulin level(Rotter et al., 2015). Molybdenum is also one of the basic components of xanthine oxidase and aldehyde oxidase in the liver and intestine of animals. But the enhancement of xanthine oxidase activity may lead to uric acid accumulation and reactive-oxygen-species-related diseases, such as hyperuricemia and diabetes(Ichida, Amaya, Okamoto, & Nishino, 2012; J. Yang et al., 2023).In this study, the risk of diabetes is also positively correlated with the concentration of molybdenum in urine.
Our study found that high Te levels were strongly correlated with increased diabetes risk in participants. Qing Liu et al study showed that cadmium telluride quantum dots(CdTe QDs) can increase reactive oxygen species (ROS) in hepatocytes after being taken up by hepatocytes, which triggers a significant mitochondrial-dependent apoptotic pathway, leading to hepatocyte apoptosis(Q. Liu et al., 2022).In the previous review, we also found that the regulators of apoptosis signaling events in hepatocytes can modulate insulin signaling pathways and that mediators of insulin resistance in turn influence liver cell apoptosis.The liver is a central regulator of glucose homeostasis and stores or releases glucose according to metabolic demands(Gjorgjieva, Mithieux, & Rajas, 2019; Schattenberg & Schuchmann, 2009). Therefore, liver injury may lead to glucose homeostasis imbalance and increase the risk of diabetes. In conclusion, this proves that telluride may cause glucose homeostasis imbalance through hepatocyte apoptosis, thus potentially leading to an increased risk of diabetes.However, more epidemiological evidence is still lacking.
In this study, first, to better controlling interference of confounding factors(especially chronic diseases comorbidity), we conducted a 1:2 matched case-control study after excluding the subjects of other chronic diseases except for diabetes. Second, we assessed the impact of metal mixtures, single metals, and metal-to-metal interactions on diabetes risk levels when exposed to multiple metals, and used BKMR models to analyze the exposure-response relationship between each metal and risk of disease relation. Finally, this study investigated the association of multiple metal exposure exposure with diabetes in urban areas of northwestern China. It could serve as preliminary evidence for the effect of multiple metal exposure exposure on diabetes in the population in the region. However, our current study also has many limitations. First, we were unable to establish a causal relationship between metal exposure and diabetes outcomes due to the limitations of the cross-sectional study. Secondly, due to the inclusion and exclusion factors, the sample size of this study was relatively small. The results would be more reliable if more samples could be obtained.