In this study, we comprehensively evaluated the relationship of urinary Cr, Mn, As, Cd, and Pb contents with eGFR levels and explored potential metal interactions among participants using the BKMR model. The BKMR study results as follows: the mixture of five metals had a positive combined effect on eGFR levels, and Pb (PIP = 0.966) contributed the most to the eGFR levels. Furthermore, Pb and As were positively correlated with eGFR levels in women. Cr, Mn, As, Cd, and Pb showed no significant correlation with eGFR when analyzed as a mixture of contaminants rather than as a single contaminant.
Pb is a nephrotoxic metal that can be harmful at even extremely low doses. Pb nephrotoxicity presents with proximal renal tubular nephropathy, glomerulosclerosis, interstitial fibrosis, and associated functional deficits. Moreover, Pb also can enter the mitochondria in renal proximal tubular cells, thereby impairing oxidative metabolism in the kidney . The effect of Pb on the kidney has also been reported in many studies. Hui-Ju Tsai et al found that high concentrations of blood Pb were associated with the occurrence of proteinuria and the reduction of eGFR . A perspective study showed that low concentrations of blood Pb can cause a decrease in eGFR, increasing the risk of CKD . Blood Pb that increased the risk of decreased proteinuria and eGFR was also found in the study by a joint analysis . Nevertheless, the results of the association between urinary Pb and eGFR are inconsistent. A decrease in eGFR with increasing urinary Pb was found in a cross-sectional study in Taiwan . Xiao Chen et al  found a positive association between urinary lead and renal effect biomarkers such as urinary microalbuminuria, urinary N-acetyl-β-D-glucosaminidase and urinary total protein, but no significant correlation with eGFR. In the BKMR model, we found a significant positive correlation between urinary Pb and eGFR. A study in the United States found that urinary Pb discharge increased with increasing eGFR . Rufeng Jin et al  also found that the urinary excretion rate of metals increased with increasing levels of eGFR, in which eGFR is more susceptible to Pb. Some studies believe that this is a poor hyperfiltration, which leads to subsequent adverse renal reactions. In an animal experiment, rats showed a positive correlation between GFR and blood lead within one month of lead exposure, thereafter decreased significantly [21, 28]. Some studies also regard this as a reverse causality prediction [26, 27, 29]. Most metals are excreted by renal excretion and should be reduced after impaired renal function. Thus, eGFR decreases and filtration of metals decreases, resulting in a decrease in urine and increased metal levels in the blood.
Mn and Cr are essential trace element, but little attention has been paid to their renal effects. A cross-sectional study of Chinese people over 90 years found the dose-response relationship between manganese and eGFR consistent with the trace element dose effect curve, the U-type curve . A cross-sectional study in Korea found that low blood Mn concentrations (1.28 µg/dL in the participants with renal dysfunction) can increase the risk and prevalence of renal dysfunction . Jingli Yang et al.  also found plasma Mn (median concentration, 9.34 µg/L) and urinary Mn (median concentration, 0.106 µg/g, creatinin) were positive correlation with eGFR. In this study, urinary Mn was positively associated with eGFR in women, and the urinary Mn levels was 0.78 µg/g, creatinine. Some studies of renal function in occupational Cr exposure found negative or equivocal results . In our study, we found that significantly positively associated between urinary Cr and eGFR in women. A few studies have found a negative correlation between urinary chromium and eGFR [24, 10], but others have not found a significant correlation between urinary chromium and eGFR . Due to the limitations of cross-sectional study design, it is not possible to elucidate the causal relationship between chromium and kidneys, which requires more research to confirm our findings. At present, whether chromium is a trace element necessary for humans is still controversial, but the effect of chromium-induced oxidative stress on the body should be worth noting.
In the BKMR model, we found that As had a significant positive correlation with eGFR, and Cd had no significant association with eGFR. Cd and As are known nephrotoxic heavy metals . Significant associations between these heavy metals and eGFR has been rarely observed, and more have demonstrated the association between heavy metals and albuminuria, α1-microglobulin and other indicators [8, 24, 38, 39]. A few studies demonstrated that exposure to Cd or As exerted toxicity through an oxidative stress process, that generation of ROS, reduced levels of glutathione, decrease in superoxide dismutase, and induction of DNA adduct, which would aggravate the development of kidney disease [40–41].
Interestingly, we found that eGFR-associated metals were mostly different between men and women. First, urinary metal concentrations were higher in women than in men. This is due to the lower iron storage in women, which leads to the up regulation of the intestinal divalent metal transporter (DMT1), increasing the absorption of other metals . On the other hand, we found that women's eGFR was more susceptible to the effects of metals, especially Pb. Studies have shown that metals affect metabolic disorders in the body or induce the upregulation of inflammatory biomarkers, leading to an increased risk of diseases such as cardiovascular disease, diabetes and hypertension, especially in postmenopausal women. For example, Estradiol (E2) disorder may be a risk factor for metabolic diseases in postmenopausal women, while E2 is susceptible to Pb interference . The mean age of the women in this study was 55.87 years, so the majority were postmenopausal women, and this population was more susceptible to metal effects. Furthermore, serum creatinine is limited by age and sex, so urinary metal excretion may also be influenced by gender . Although the current study cannot clearly explain the sex-specific effects on the relationship between urinary metals and eGFR, we should focus more on sex differences-related health effects, especially the role of hormones in sex differences.
Our study has several advantages. First, BKMR analysis was used to analyze the single and combined effects of metals and to assess the metal interactions that may occur at the eGFR level. Second, we performed different stratified analyses, which helped us realize that different metal exposure environments may affect the relationship between metal and eGFR levels. However, our study’s limitations cannot be ignored. First, the present study was a cross-sectional study, and the determination of the causal relationship between metal exposure and eGFR was not possible. Second, some measurements may be affected by certain factors. For instance, creatinine may be affected by drugs and intestinal bacteria , which may exhibit extreme variation. Thus, we were unable to rule out the possibility of false-positive or false-negative results. Finally, considering that our results were obtained only from the excretion of these metals in the urine, we cannot rule out the possibility of false-positive results. Therefore, the correlations we found require further investigation.