Overactive bladder (OAB) is a common urologic disorder that affects individuals of both men and women (1, 2). In 2014, the internationally recognized definition of OAB was established by the International Continence Society, which described OAB as a condition characterized by urgency of urination, with or without urge incontinence, usually with frequent daytime urination and nocturia, but without confirmed urinary tract infection or other known pathological abnormality (3). The prevalence of OAB differs across regions or countries and may be influenced by factors such as ethnicity and lifestyle practices. OAB was reported to affect 17% of males and 30% of females in the United States (4). In February 2025, Zhang et al. (5) investigated the global prevalence of OAB and reported that the prevalence was 20%. Among males, the prevalence is 16.1%, while it is 21.9% among females (5). Individuals may experience a heavy burden from the syndrome, which could result in sleep disturbances, anxiety or depression, an unpleasant sexual life, and decreased participation in social and physical activities, thus significantly influencing their quality of life (6). In the United States, OAB contributes to annual healthcare expenses amounting to billions of dollars (7). Despite continuous research efforts, the complicated pathophysiological mechanisms responsible for the onset and progression of OAB are not well illuminated.

Heavy metals, commonly identified as environmental pollutants, seriously threaten human health. Heavy metals generally refer to metallic elements with a density equal to or exceeding 5 g/cm³ (8). As industrialization and urbanization progress, heavy metal pollution has become increasingly widespread. These pollutants find their way into the external environment via multiple channels, including pesticides, exhaust gases, and industrial wastewater (9). Consequently, people are exposed to heavy metals via multiple pathways, such as air, food, water, and dermal contact. Over time, heavy metals can accumulate in tissues and organs, particularly in the heart, urinary system, and nervous system (10). Accumulating in this way may lead to several health concerns like cardiovascular disease, urinary disorder, and neurotoxicity (11–13). To date, few studies have focused on the association between exposure to heavy metals and the risk of OAB. Gao et al. (14) investigated the association between exposure to single metal cadmium (Cd) and OAB risk in a U.S. population aged 40 years and older and found that blood Cd levels were weakly positively related to the risk of OAB. Given the coexistence of heavy metals in the environment, real-world exposure to heavy metals is complex, involving potential synergistic, antagonistic, additive, or other interactions among various heavy metals. Thus, exposures to individual heavy metals might not fully account for the pathogenesis of OAB. Exploration of the joint effects of co-exposure to heavy metal mixtures on the risk of developing OAB is essential. The health effects of heavy metal mixtures in the environment are complex due to the potential interactions between components. Two critical mechanisms underlie mixture toxicity: synergism (where combined effects exceed the sum of individual effects) and antagonism (where one component reduces the toxicity of another). For example, Cd and lead (Pb) may synergistically enhance oxidative stress, while selenium (Se) might antagonize mercury (Hg) toxicity by forming Hg-Se particles with low bioavailability (15, 16). Traditional regression models (e.g., linear regression) often fail to capture these interactions because they assume additive effects and independence between exposures, leading to biased effect estimates. Advanced statistical approaches such as Bayesian kernel machine regression (BKMR) and weighted quantile sum (WQS) regression have been developed to address these limitations. BKMR flexibly models non-linear exposure-response relationships and component interactions through kernel functions, enabling the detection of synergy or antagonism even in high-dimensional mixtures (17). In contrast, WQS regression quantifies the overall mixture effect by assigning weights to components under a pre-specified direction (e.g., all toxic or protective), making it robust to multicollinearity (18). However, WQS assumes unidirectional effects and may underestimate antagonistic interactions, whereas BKMR allows bidirectional effects. By integrating both models, we aimed to comprehensively assess both the joint mixture effect and potential interaction mechanisms, thereby advancing beyond traditional approaches. Emerging evidence suggests that co-exposure to Cd and Hg may induce synergistic toxicity through shared biological pathways. Both metals are known to disrupt redox homeostasis by generating reactive oxygen species and depleting glutathione, a critical antioxidant (19). Experimental studies have shown that combined exposure to Cd and Hg exacerbates oxidative damage in renal and hepatic tissues compared to individual exposures, likely due to their additive or supra-additive effects on mitochondrial dysfunction and lipid peroxidation (20). For example, Cd may impair Hg excretion by competing for metallothionein binding, while Hg can enhance Cd-induced inflammatory responses via NF-κB activation (19). In the context of bladder dysfunction, these metals may synergistically target urothelial cells through oxidative stress and sensory nerve sensitization, potentially amplifying the risk of OAB symptoms. Therefore, we hypothesized that the Cd-Hg mixture would exhibit synergistic effects on OAB pathogenesis.

In this study, we carried out a cross-sectional study based on U.S. Civilian data from the National Health and Nutrition Examination Survey (NHANES). After heavy metals enter the human body, they are first transported to various tissues and organs through the blood. The heavy metal concentration in the blood directly reflects the recent exposure and the current level of heavy metals in the circulation. However, heavy metals in urine are the part that is excreted through the kidneys after metabolism. They may be affected by short-term excretion rates, renal function, and hydration status (such as water intake), and fluctuate greatly, making it challenging to represent the overall load. In addition, some heavy metals (such as Pb and Cd) have a relatively long half-life in the blood, which can more stably reflect chronic exposure and accumulation in the body. However, the concentration of heavy metals in urine may only reflect exposure or acute excretion within a few days, and has a relatively low sensitivity to long-term cumulative exposure (21, 22). Therefore, blood heavy metal concentrations (versus urine) are optimal for this topic. We conducted a multivariate logistic regression to investigate the effects of a single metal on the risk of OAB. In addition, we employed WQS, quantile-based g computation (qgcomp), and BKMR models to explore the joint effects of heavy metal mixtures on the risk of OAB. Our study provided new epidemiological insights into the correlations between heavy metal exposure and the risk of OAB, aiding in the identification of hazardous factors associated with OAB.

As far as we know, this cross-sectional study is the first to explore the joint effects of co-exposure to blood heavy metal mixtures on the risk of OAB. Participants with OAB had significantly higher blood concentrations of Cd and Pb and lower concentrations of Se and Mn than those without the condition. Multivariate logistic regression models for single-metal and multi-metal exposures identified that exposure to Cd significantly increased OAB risk. At the same time, RCS analysis further validated the linear dose–response relationship between Cd and OAB risk. Furthermore, the WQS, qgcomp, and BKMR models consistently revealed a significant positive association between co-exposure to heavy metal mixtures and the risk of OAB. In the age-stratified subgroup analysis, we found a similar significant association among the young/middle-aged individuals but not in the older adult group. According to the WQS and BKMR model results, Cd was the main positive factors driving the overall effect. The qgcomp model results consistently validated the positive effects of heavy metal Cd.

Cd, one of the most hazardous metals, is extensively present in food, soil, industrial products, and water supplies. Long-term exposure to Cd can lead to serious health concerns, such as carcinogenicity, endocrine disruption, nephrotoxicity, and reproductive toxicity (35). Similar to the findings from Gao et al. (14), we identified a significant positive association between exposure to single metal Cd and the risk of OAB. There are multiple possible causes accounting for the pathophysiology of OAB, which are not entirely illuminated. According to Chu and colleagues (36), OAB can be categorized into idiopathic OAB, myogenic OAB (e.g., bladder outflow obstruction), neurogenic OAB (e.g., spinal cord injury), and inflammatory OAB (e.g., interstitial cystitis). Griffiths et al. (37) proposed that decreased central nervous system inhibition of the urination reflex may be a potential pathophysiological mechanism of OAB. Their findings showed that injury to the frontal cortex may disrupt bladder control. In vitro and in vivo studies have validated that Cd has harmful effects on the nervous systems, both peripheral (38) and central (39), which result in extensive clinical symptoms. The phenomenon of oxidative stress is due to an imbalance in the production and clearance of reactive oxygen species within cells. Prior research revealed that excessive oxidative stress, hypoxia, and loss of blood supply play a crucial role in OAB (40–42). Heavy metals like Cd, Hg, and Pb can trigger the production of reactive oxygen species, which depletes the antioxidant capacity of cells and causes oxidative stress (43, 44). Oxidative stress induced by heavy metals, indicated by abnormal expression of SOD, MDA, GSH, and 8-OHdG, can lead to inflammation and subsequent health issues (45). In addition, it may be regulated by multiple signaling pathways, such as the nuclear translocation of NF-κB, Keap1-Nrf2-ARE, and high expression of HIF. We speculate that these three heavy metals may be involved in the occurrence and progression of OAB by promoting oxidative stress.

Previous studies have indicated a relationship between inflammation and the onset and development of OAB. Long-term inflammation can alter bladder function and increase sensitivity, causing OAB symptoms to emerge (46, 47). Molecular imbalances in inflammatory proteins may cause the progression of OAB. Ma and colleagues (48) demonstrated that patients with OAB exhibited increased levels of inflammatory factors (e.g., TNF-α) and decreased levels of anti-inflammatory factors (e.g., IL-4). It is commonly accepted that either single or combined exposures to heavy metals, such as Cd, Pb, and Hg, can result in long-lasting inflammation (increased levels of TNF, IL-6, and IL-8), which may be involved in the development of OAB (49, 50). Chen et al. (51) conducted animal studies and reported that exposure to Cd could elevate IL-1β, COX2, iNOS, and HO-1 levels, activate the TNF-α/NF-κB signaling pathway, and trigger an inflammatory response. Furthermore, according to Taha et al. (52), the findings from animal experiments demonstrated that subacute Cd poisoning reduced detrusor muscle contractions and disrupted bladder function by blocking the Ca²⁺/calmodulin signaling pathway. These findings suggest that Cd affects bladder function via multiple mechanisms and pathways.

In addition to oxidative stress and inflammation, growing research indicates that heavy metals may increase the risk of OAB by altering the gut microbiota composition. Relevant studies have reported that alterations in the gut microbiota are related to OAB (53, 54). Okamoto et al. (54) reported that compared to non-OAB groups, individuals with OAB-daily urgency exhibited a significant decrease in levels of the genus Bifidobacterium, while levels of the genus Faecalibacterium were significantly increased. Increasing evidence from animal studies indicates that exposure to heavy metals disrupts gut microbiota homeostasis, causing inflammation (elevated levels of IL-1β and TNF), possibly through interference with protein synthesis (55, 56). According to a mouse model study, Cd exposure significantly reduced the abundance of gut microbiota and decreased the number of microorganisms that produce short-chain fatty acids (57). Another investigation conducted by Nielsen and colleagues (58) focused on the effects of exposure to Hg on the metabolic profiles and gut microbiota in mice and fish. They demonstrated that exposure to Hg disrupted gut microbiota homeostasis in both species, significantly reducing the relative abundance of several long-chain fatty acids, such as oleic acid and stearic acid. It is generally believed that oleic acid can improve insulin sensitivity, increase antioxidant capacity, and reduce levels of pro-inflammatory factors (59). In summary, despite the need for further exploration of the exact mechanisms involved, we have grounds to speculate that gut microbiota dysbiosis plays a crucial role in the impact of heavy metals on OAB.

Although previous studies have reported that exposure to single metal Cd is associated with an increased risk of OAB, there is a lack of information regarding the joint effects of metal mixtures. In this study, we employed newly established statistical methods to unveil the combined effects of heavy metal mixtures on OAB. Furthermore, increasing evidence indicates that the pathogenesis, clinical patterns, and prevalence of OAB differ between young/middle-aged individuals and the older adult (2, 46). Therefore, we carried out subgroup analyses by age. The findings from the WQS regression were essentially consistent with those from the BKMR model used in this research. We found a significant positive association between co-exposure to heavy metal mixtures and the risk of OAB, with Cd being primarily responsible for the overall effect. However, the findings from the subgroup analysis do not completely align with the overall results. A similar significant positive association was identified among the young/middle-aged individuals but not in the older adult group. Aging might be a primary factor contributing to this phenomenon. An essential feature of aging is the increase in oxidative stress (60), which suggests that older adult individuals have higher baseline levels of oxidative stress compared to young/middle-aged individuals. Previously, we introduced possible explanations for the association between heavy metals and OAB, beginning with an elevation in oxidative stress triggered by heavy metals. For the older adult group, the increased baseline levels of oxidative stress might mask the impact of oxidative stress elevation triggered by heavy metals, thus obscuring the association between exposure to heavy metal mixtures and OAB, consequently contributing to discrepancies in the subgroup analysis findings. Additionally, we found that Cd was the top-weighted metal for young/middle-aged individuals, whereas Hg held the highest weight among older adult individuals. Cd exposure in young/middle-aged individuals is primarily due to unhealthy lifestyle habits (e.g., smoking) and occupational exposures. When smoking, Cd enters the lungs with the smoke, and the absorption rate is as high as 50%. Young/middle-aged smokers may accumulate Cd due to long-term smoking. Some cheap whitening products may be illegally added to contain Cd compounds. Moreover, young people are more likely to experience occupational exposure to the heavy metal Cd, such as battery manufacturing (nickel-Cd batteries), electroplating industry, alloy production, pigment/dye processing, construction industry (welding Cd-containing metals or processing Cd-containing coatings). In contrast, Hg exposure in the older adult is mainly related to traditional medicines and dental materials. In some cultures, Hg-containing Chinese medicine or folk remedies (e.g., cinnabar) are used to calm the mind, detoxify the body, or treat chronic diseases. The older adult may be chronically exposed to inorganic Hg (e.g., Hg sulfide) due to reliance on traditional Chinese medicine. Moreover, amalgam was once widely used in dental fillings, and its elemental Hg may slowly release trace amounts of Hg vapor. The older adult may be chronically exposed to low doses of Hg from early use of amalgam fillings, especially decades ago. The detoxification function of the liver and kidneys is weakened in the older adult, which is more likely to lead to Hg accumulation. Interestingly, we observed an underlying interaction between Cd and Hg in the BKMR model. Although the exact biological mechanisms are poorly understood, they may be associated with oxidative stress and an imbalance in metal homeostasis (61). We also carried out sensitivity analyses, such as qgcomp and RCS models, to ensure the robustness and reliability of the findings. The qgcomp regression analysis consistently unveiled that blood metal mixtures significantly increased the OAB risk, identifying Cd as the main positive driver. According to RCS analysis, we identified a positive linear dose–response relationship between the blood metal Cd and the risk of OAB.

There are several advantages to our study. First, this study thoroughly evaluated the influence of single heavy metal exposure on the risk of OAB, bridging previous research gaps. Second, we employed several new and comprehensive statistical methods to explore the association between co-exposure to heavy metal mixtures and the risk of OAB in a large population from various perspectives. We obtained basically consistent results, which made us confident in their robustness and reliability. Nevertheless, we must acknowledge that our study has some limitations. First, recall bias could not be entirely avoided since we determined the diagnosis of OAB through a self-reported questionnaire. Second, we could not establish an inference of a causal relationship between co-exposure to heavy metal mixtures and OAB risk because of the cross-sectional nature of the study. Third, NHANES data are collected through mobile examination centers (MECs) following standardized protocols to minimize inter-center variability. However, specific MEC identifiers or operational details (e.g., technician training, equipment calibration) are not publicly available in NHANES datasets, precluding direct adjustment for field-center effects. Additionally, seasonal variation in blood metal concentrations (e.g., due to diet, air pollution, or sample stability) was not explicitly adjusted for in this study, as NHANES does not release detailed examination dates (only survey cycles). These are the inherent limitations of the NHANES study. Finally, blood metal concentration measurements were single-point and may not reflect long-term exposure. Future studies should consider serial measurements or hair-keratin analysis, which better capture metal accumulation over months. Additionally, OAB itself (e.g., frequent voiding) might alter dietary or physical activity patterns and thereby affect metal retention. OAB-related behaviors such as increased fluid intake, altered dietary habits (e.g., reduced consumption of metal-contaminated foods), or diuretic use may influence metal absorption, distribution, or excretion.

In summary, our study identified that exposure to single metal Cd significantly increased the risk of OAB. Furthermore, the mixed-exposure analyses consistently unveiled a significant positive association between co-exposure to blood heavy metal mixtures and the risk of OAB, identifying Cd as the main positive driver. The young/middle-aged group exhibited similar significant associations. Given the limitations of the NHANES study, well-designed prospective studies are required to confirm our findings in the future.