Existing studies have primarily focused on investigating the associations between urinary metal mixtures and the risks of NAFLD and advanced liver fibrosis. However, to the best of our knowledge, there is currently a lack of epidemiological evidence regarding the potential links between individual urinary heavy metals and the occurrence of NAFLD or advanced liver fibrosis15, 16. In order to address these gaps in knowledge, we conducted a comprehensive study among US participants utilizing data from NHANES 2003–2018 survey cycles. Our primary objective was to examine the associations between urinary heavy metals and the prevalence of NAFLD and advanced liver fibrosis. Notably, this is the first study to identify a significant positive correlation between the heavy metals Sb, Sn, Ur, and the occurrence of NAFLD or advanced liver fibrosis. Our findings contribute important insights to the field by highlighting the potential role of these specific heavy metals in the development of NAFLD and advanced liver fibrosis. In this study, we aimed to explore the relationship between urinary heavy metal concentration and the prevalence of NAFLD and advanced liver fibrosis within a population from the United States. Through our investigation, we identified a significant positive association between urinary Ba, Cd, Co, Pb, Sb, Sn, Tu, Ur concentration and NAFLD. Additionally, we observed a significant correlation between higher urinary levels of Cd, Pb, Sb, Sn and the occurrence of advanced liver fibrosis. These findings align with previous studies on the subject and hold true even after accounting for various confounding variables, including sociodemographic factors, lifestyle factors, hypertension, and T2DM15, 28, 29. By considering these factors, our study provides robust evidence supporting the link between urinary heavy metal concentration and both NAFLD and advanced liver fibrosis in the US population.
Xie Z et al.'s study discovered a significant correlation between urinary Cs and Mo levels and NAFLD15. Our analysis further revealed that individuals below the age of 60 exhibited a stronger positive association between urinary levels of Ba, Co, Pb, Sn, and Tu with NAFLD, compared to older participants. Additionally, we observed a significant link between Sb and Tu levels in urine and NAFLD specifically among older adults (aged ≥ 60 years). It is worth noting that the incidence of NAFLD and fibrosis generally tends to increase with age30. Our divergent findings might be attributed to variations in the sample size and the inclusion criteria across the studies. In addition, when we performed stratified analyses based on sex, we found distinct patterns. Specifically, among female individuals, we observed positive associations between urinary levels of Ba, Cd, Co, Sb, Sn, Tu, and Ur with NAFLD, as well as between Sb and advanced liver fibrosis. Conversely, in the male population, we identified a significant positive association between urinary levels of Ba, Cd, and Pb with NAFLD, and between Pb and advanced liver fibrosis. These findings align with previous studies that have also highlighted sex differences in the health risks associated with heavy metals16, 29. Interestingly, heavy metals demonstrated a stronger positive correlation with NAFLD and advanced liver fibrosis in women compared to men. This suggests that the impact of heavy metals on liver health may vary between the sexes, emphasizing the importance of considering sex-specific factors when assessing the relationship between heavy metals and these liver conditions.Several potential explanations may shed light on the observed sex differences in the associations between urinary heavy metals and NAFLD and advanced liver fibrosis. For instance, it is suggested that a deficiency of iron in women may contribute to a compensatory increase in the absorption of heavy metals31. Additionally, rat models have shown that exposure to heavy metals can result in decreased expression of organic anion transporter 3 (Oat) in hepatocyte membranes, leading to reduced intake of certain heavy metals such as mercury (Hg) and subsequent higher accumulation in the female liver32. However, it is noteworthy that Xie Z et al.'s study did not detect significant gender differences in the association between urinary heavy metals and NAFLD15. To gain a comprehensive understanding of the relationship between urinary heavy metals, age, and gender, further investigations are warranted. Additional studies exploring the impact of urinary heavy metal levels on age and gender are essential for enhancing our knowledge in this area.
The subgroup analysis conducted in our study revealed a noteworthy and positive association between NAFLD, advanced liver fibrosis, and nearly all urinary heavy metals, particularly among individuals of Non-Hispanic white when compared to other ethnic groups. Additionally, we identified a significant correlation between urinary levels of Cd and Sb with NAFLD, and urinary Sn with advanced liver fibrosis specifically among Mexican Americans. This may be attributed to the larger proportion of Non-Hispanic white individuals within the studied population.These findings emphasize the importance of paying special attention to the potential liver damage caused by urinary heavy metal exposure in these populations. Previous research has highlighted that Mexican Americans have the highest prevalence of NAFLD33, and this finding aligns with the results of Spaur M. et al., who also reported a significant positive association between blood manganese levels and NAFLD and liver fibrosis in Mexican Americans34. Interestingly, we discovered a negative correlation between urinary Cd levels and advanced liver fibrosis specifically among individuals of Other Hispanic ethnicity. This unexpected finding highlights the need for further investigation into the sensitivity of different populations to the potential liver damage caused by urinary heavy metals. Previous research has established that individuals with a high BMI are more susceptible to developing NAFLD and advanced liver fibrosis35, 36. This notion is supported by the findings of Spaur M. et al., who observed BMI-specific differences, where positive associations were observed between blood manganese (Mn) levels and liver steatosis among participants with a BMI of 30 or higher34. In our study, we consistently observed a significant impact of urinary heavy metal levels on the development of advanced hepatic fibrosis across various BMI groups. Nevertheless, our subgroup analysis revealed that among individuals with BMI greater than 30, there was no significant association observed between most urinary heavy metals and the presence of NAFLD or advanced liver fibrosis. However, we did find a positive correlation between urinary levels of Tu and NAFLD, as well as urinary Sb and advanced liver fibrosis, within this higher BMI population. It is important to note that the findings in this subgroup analysis mirror the rationale behind the age-based subgroup analysis, as the sample sizes and compositions of these two groups differ substantially. These divergent results highlight the potential influence of factors such as BMI and suggest the need for further investigations with larger sample sizes to fully understand the associations between urinary heavy metals and NAFLD or advanced liver fibrosis in specific subpopulations.
The pathogenesis of NAFLD and advanced liver fibrosis is widely acknowledged to involve metabolic disorders such as hypertension and diabetes mellitus37–39. Consistent with this understanding, our study revealed a positive correlation between urinary levels of Sb and Sn with advanced liver fibrosis specifically among participants with T2DM. Additionally, we observed a higher prevalence of urinary Pb levels among individuals with hypertension who also had advanced liver fibrosis. Reja D et al. similarly reported an independent association between increased serum lead levels and an elevated risk of advanced liver fibrosis28. However, when examining the association between urinary heavy metals and NAFLD, we only identified a significant effect of Ur among individuals with diabetes or hypertension. These findings underscore the intricate interplay between urinary heavy metals, metabolic disorders, and the development of advanced liver fibrosis, emphasizing the need for further investigation in this field.
This study is subject to several limitations that warrant consideration. Firstly, the cross-sectional design of the NHANES dataset used in this study prevents the establishment of causality and the reliance on self-reported alcohol consumption may introduce recall bias. Secondly, the diagnosis of NAFLD and advanced hepatic fibrosis relied on the use of FLI and FIB-4 scores after excluding other causes of chronic liver disease. This approach may have inherent limitations and could potentially lead to misdiagnosis of NAFLD. While liver biopsy is considered the gold standard for NAFLD diagnosis, its impracticality in large population-based studies necessitated the use of alternative diagnostic methods. Moreover, the relatively smaller sample sizes of Non-Hispanic and Mexican American participants limited our ability to assess differences between subgroups, and some of the findings may be unstable. Lastly, despite adjusting for various potential confounding factors using NHANES data, there remains the possibility of unmeasured confounders that may influence our findings. Therefore, further studies are necessary to elucidate the causal relationship between urinary heavy metal concentrations and NAFLD, as well as to validate our current findings.