In this study, we explored the relationship between 11 metals and the risk of osteoporosis and osteopenia, examining the impact of concurrent exposure to these trace elements on bone health. Our findings indicate that in cases of single-metal exposure, elevated urinary concentrations of As, Cd, Cr, Fe, Hg, and Mn were linked to an increased risk of osteopenia, while concentrations of Co and Zn were associated with reduced risk. Additionally, higher urinary concentrations of As, Cd, Cr, and Mn were positively associated with the risk of prevalence of osteoporosis, while V exhibited a negative association with the risk of prevalence of osteopenia. Co-exposure to multiple metals was found to elevate the risks of both osteopenia and osteoporosis, with detrimental effects potentially stemming from Fe, Mn, Hg, and Cd.
The results of previously conducted epidemiologic studies suggest that heavy metal exposure is associated with an increased risk of developing osteoporosis, which is consistent with our findings. In the Dong and Miao populations of Guizhou Province, the risk of osteopenia and osteoporosis were associated with Cd and Mn exposures, a finding that was consistent across single-metal exposures, Quantile g-computation, and PCA. Cd is a toxic heavy metal commonly found in nature in a chemosynthetic state. It is primarily released into the environment through sources such as waste gas, waste water, and waste residue, leading to environmental pollution(Yuan et al., 2019). With a biological half-life that can extend from 10 to 30 years, Cd tends to accumulate in the body following exposure(Wang & Chen et al., 2021). Bone is identified as one of the primary target organs for Cd(Charkiewicz et al., 2023). In a Chinese cohort study, urinary cadmium levels were shown to be an independent risk factor for osteoporosis in women aged 50–79 years, and the risk of osteoporosis in the high UCd group (> 10µg/gcr) was 2.24 times higher than that in the low UCd group (< 5µg/gcr)(Wang & Wang et al., 2021). Another Meta-analysis showed that UCd was an independent risk factor for osteoporosis and osteopenia(Li et al., 2021). Numerous studies have demonstrated that cadmium induces abnormalities in calcium and phosphorus metabolism within bone tissue. This is achieved through mechanisms such as renal damage, particularly tubular damage, impaired calcium reabsorption, reduced vitamin D synthesis, and subsequent disturbances in calcium-phosphorus metabolism. These effects lead to the loss of calcium and phosphorus in bone tissue, ultimately resulting in abnormal bone mass(X. Chen et al., 2019). Cd impacts bone marrow mesenchymal stem cells (BMSCs), hindering their differentiation into osteoblasts and directly triggering apoptosis in BMSCs(Zhang et al., 2021). Moreover, Cd directly affects osteoclast activation, fostering bone resorption, and causing damage to osteoblasts along with oxidative stress. This cascade leads to DNA damage, mitochondrial dysfunction, and endoplasmic reticulum stress, ultimately culminating in apoptosis(Wang & Li et al., 2022).
Mn is an essential nutrient for intracellular activity, serving as a cofactor for various enzymes such as arginase, glutamine synthetase (GS), pyruvate carboxylase, and Mn superoxide dismutase (Mn-SOD). Its involvement in these metalloproteins is crucial for various physiological processes, including digestion, reproduction, antioxidant defense, energy production, immune response, and the regulation of neuronal activity(Erikson & Aschner, 2019). One of the major depositional sites for Mn is bone tissue(Rondanelli & Faliva et al., 2021). However, the results of the studies are not consistent. Some research suggests that women with osteoporosis exhibit lower serum Mn levels compared to those with normal bone density. Conversely, two studies based on NHANES data show a negative correlation between blood Mn levels and bone mineral density, both in adolescents and adults aged over 18(Liu et al., 2023; Wang & Zhu et al., 2022). Another cross-sectional study of involving occupational groups exposed to Mn similarly identifies a negative correlation between blood Mn levels and bone density(Li et al., 2020). Our study results indicate that Mn contributes to osteopenia and an increased risk of osteoporosis, with monometallic exposure outcomes aligning with those observed in polymetallic co-exposure.
In summary, our study found that Fe and Hg were positively correlated with the risk of osteopenia and osteoporosis when exposed to a single metal. However, when exposed to multiple metals, PC1 characterized by Fe, and PC3 characterized by Hg were positively correlated with osteopenia and osteoporosis, which may be due to the influence of other metals. When exposed to multiple metals, PC1 characterized by Fe, and PC3 characterized by Hg were positively correlated with osteopenia and osteoporosis, which may be due to the influence of other metals. Hg, which exists in metallic form as well as inorganic and organic compounds, has a high affinity for the sulfhydryl groups of proteins, similar to cadmium(Ajsuvakova et al., 2020). Current research results on mercury and osteoporosis are inconsistent. Some studies have shown that mercury compounds can interfere with enzymatic and hormonal reactions in the human body. A study on adolescents aged 12–19 found that blood mercury levels were significantly related to bone density in adolescents. Inverse correlation(Xu et al., 2023), consistent with another study based on the US NHANES database, there is a negative correlation between blood mercury levels and spinal bone density in adults, with low blood mercury levels (< 3 ug/L) having increased osteoporosis risk(Tang et al., 2022). Another mate analysis found no association between mercury exposure and the risk of osteopenia or osteoporosis(Jalili et al., 2020). This is inconsistent with our research results and may be caused by differences in the selected biological samples and detection methods. Fe is one of the trace elements necessary for life activities and maintains various physiological functions of the human body. Cellular iron is mainly stored in cells in the form of Fe2+ or Fe3+. Fe2+ can bind to proteins and participate in various physiological reactions, and Fe3+ is the main form of iron transport in the human body(Vogt et al., 2021). Disorders of iron metabolism, including iron deficiency and iron overload, can lead to osteoporosis. Zhao et al. (Zhao et al., 2012)studied osteoblasts and found that excess iron can inhibit the activity of osteoblasts, mild iron deficiency can promote bone cell activity, but severe iron deficiency can inhibit bone levels. One study found that patients with a history of iron deficiency anemia had a higher incidence of osteoporosis. Patients with a history of iron deficiency anemia have almost twice the risk of osteoporosis compared with patients without iron deficiency anemia(Pan et al., 2017). Iron overload is one of the main characteristics of ferroptosis(Li & Huang, 2022). In 2012, Dixo et al. (Dixon et al., 2012)first reported a type of iron ion-dependent programmed cell death, which is morphologically different from apoptosis, autophagy, and necrosis. At present, the mechanisms and signaling pathways related to ferroptosis in osteoporosis are not fully understood. The results of a study show that iron overload can reduce cell viability, superoxide dismutase, and glutathione levels, and increase reactive oxygen species. production, lipid peroxidation, malondialdehyde levels and ferroptosis-related protein expression, and induced ultrastructural changes in mitochondria(Jiang et al., 2022), and another study found that iron overload can cause osteoblast apoptosis(Zhang et al., 2023).
In our study, As and Cr were positively correlated with osteopenia and osteoporosis when exposed to single metals, and this relationship also showed a positive weight in Quantile g-computation. Cr exists in the earth's crust and seawater and it is a naturally occurring heavy metal in industrial processes. Cr has multiple oxidation states from − 2 to + 6, of which Cr3+ and Cr6+ forms are the most common stable forms(Pechova & Pavlata, 2007). Cr6+ is related to a series of diseases and pathologies and is a strong carcinogen(Loomis et al., 2018), while Cr3+ is a trace amount necessary for natural lipid and protein metabolism, and is also an important component of glucose tolerance factor(Schwarz & Mertz, 1961), by enhancing the action of insulin. and exert its effect(Lipko & Debski, 2018). In vivo studies in rats by Sankaramanivel et al. showed that Cr6+ accumulated in the femur and caused a systemic decrease in ALP and TRAP, indicating an impact on both bone formation and resorption(Sankaramanivel et al., 2006). In vitro, studies have shown that Cr6+ is absorbed by osteoblasts through membrane transporters and rapidly reduced to Cr3+, leading to increased reactive oxygen species, oxidative stress, and DNA damage(Q. Y. Chen et al., 2019; Wise et al., 2008). In an animal experiment, it was found that the effect of Cr3+ on bone density was similar to that of low-dose estradiol. The effect of estradiol on bone density is dose-related and can be modified by Cr3+(Kong et al., 2019). As exposure increases the risk of various bone diseases. Chronic exposure to low levels of As can induce bone resorption by promoting osteoclast differentiation. After low-level As exposure, osteoclast precursor cells generate hydrogen peroxide, leading to their differentiation into cells that break down the bone matrix(Liu et al., 2019). Moreover, As induces apoptosis in osteoblast cell lines (including hFOB, MC3T3-E1, and MG-63) and mouse bone marrow stromal cells (M2-10B4), while also triggering endoplasmic reticulum (ER) stress(Tang et al., 2009). Ayla found that exposure may be associated with bone loss in men(Akbal et al., 2014). Consistent with our findings, exposure increases the risk of osteopenia and osteoporosis.
Our study showed that V, Zn, and Co were positively correlated with BMD, aligning with previous studies highlighting their correlative roles in bone formation. V, an element belonging to group VB, The compounds of V can emulate the biological functions of insulin and growth factors, exhibiting various osteogenic effects related to the formation of extracellular matrix and collagen in bone cells, thus promoting osteogenic activity(Barrio & Etcheverry, 2006). An animal study found that V not only possesses hypoglycemic properties but also improves bone condition in diabetic patients(Sanchez-Gonzalez et al., 2017). Zn is an essential trace element that is essential for the normal growth of human and animal bones. In a study of elderly people with proximal femur fractures, it was found that zinc concentration in bones is most closely related to osteoporosis and plays a key role in bone development and maintenance of bone mass(Lin et al., 2022). Zn can stimulate runt-related transcription factor 2 (Runx2) and promote osteoblast differentiation(Fu et al., 2018). On the other hand, excess zinc induces osteoclast apoptosis(Ciosek et al., 2023). In our study, zinc played a protective role in osteopenia, but the protective factor was not significant in the osteoporosis stage. This may be due to differences in urinary zinc concentrations. Co, commonly used in bone and joint implants, may exert its potential mechanism on bone formation by acting as an inducer of HIF-1α (Skalny et al., 2023). However, it is noteworthy that in Quantile g-computation, Co exhibited different directional weights in association with osteopenia and osteoporosis. This suggests that metals may play distinct roles at different stages of the disease, which needs to be verified by subsequent studies.
In the multi-metal mixed exposure model, our research results further showed a significant positive association between 11 metal mixtures and the risk of osteopenia and osteoporosis in the Dong and Miao populations. The link between metal mixtures and bone health has been reported previously. A study on elderly Chinese women found that exposure to a mixture of nine metals (Ca, Cd, Co, Pb, Mg, Rb, Sr, V, and Zn) had a significant negative trend in the risk of osteoporosis, with Rb and V being the primary contributors (Feng et al., 2023). The NHANES study, which collected data from adults aged ≥ 20 years, indicated that simultaneous exposure to a mixture of Cd, Pb, Hg, Mn, Cu, Se, and Zn was associated with reduced bone density, with Pb, Mn, and Se being the main contributors(Wei et al., 2021). Partially consistent with our research results, we observed the protective effect of V on osteoporosis and the harmful effect of Mn. Nevertheless, discrepancies among various studies persist, primarily stemming from variations in the study population, the choice of metals, the biological samples employed for testing, and the statistical methods applied. It is noteworthy that in Quantile g-computation, the same metal exhibits different directional weights at different stages of the disease. For example, Pb exhibits a positive weight in association with osteopenia but a negative weight in osteoporosis, suggesting that the metal may exert differing effects at different stages of the disease. Therefore, further research is essential to comprehensively the relationship between exposure to metal mixtures and bone health.
This study presents several strengths. Firstly, CMEC baseline survey data were utilized, ensuring the collection of information by trained investigators with rigorous quality control measures in place during the questionnaire survey, physical examination, and laboratory testing, thus enhancing the reliability of the data. Secondly, this study used two methods, Quantile g-computation, and PCA, to collectively explore the effects of polymetallic exposure on BMD and identify the primary metals contributing to the effects. However, there are some limitations in this study. Firstly, this study is a cross-sectional study, which makes it difficult to draw a causal relationship between urinary metal concentration and BMD. Secondly, this study solely examined metal concentrations in spot urine samples, neglecting external exposure to metals in the environment and other biological samples such as plasma and hair. This limitation will be addressed in more detail during subsequent follow-up investigations.