Serum albumin mediates the associations between heavy metals and two novel systemic inflammation indexes among U.S. adults

The effects of heavy metal exposure on immunological function have sparked widespread concern, but unequivocal evidence on the association between mixed metal exposure and novel systemic inflammatory indexes remains scarce. This study aimed to analyze the associations of heavy metals with two novel systemic inflammation indexes and the mediated effects of serum albumin. Nineteen metals were detected among 4082 U.S. adults based on the NHANES. A linear regression, restricted cubic splines (RCS) regression, weighted quantile sum (WQS), Quantile-based Gcomputation (qgcomp), and Bayesian kernel machine regression (BKMR) were conducted to evaluate the associations of single metal and mixed metals with systemic immune-inflammation index (SII) and systemic inflammation response index (SIRI) levels, respectively. A series of subgroup analyses were used to identify potentially vulnerable populations. Furthermore, we conducted mediation analyses to investigate the mediated effects of serum albumin on the associations of metals with SII and SIRI. In the single-exposure model, exposure to various metals such as urinary Co, As, and serum Zn, Cu was associated with SII and SIRI (PFDR<0.05). Simultaneously, the above metals were linear positively correlated with SII and SIRI. Mixed-exposure analyses consistently showed that overall mixed urinary metal levels were positively pertinent for SII and SIRI levels, and the metal Co played a significant role in the urinary metal mixtures. Subgroup analyses showed that exposure to urinary Cd in men and elderly people increased SII and SIRI levels. The results of mediation analyses suggested the association of urinary metal mixture with SII and SIRI was mediated by albumin, and the proportion of mediation was 14.45% and 9.49%, respectively. Our findings suggested that metal exposure is strongly associated with the levels of system inflammation indexes and that serum albumin is, in part, a mediator of this association.