The relative fat mass/albumin ratio as a nutritional–inflammatory marker is associated with the risk of gallstones: a population-based study

Gallstones, a major health burden globally, are intricately associated with obesity, nutrition and inflammation. Relative fat mass (RFM) represents a newly developed parameter for assessing obesity. Albumin was widely used in clinical practice for its role in anti-inflammation and assessing nutritional levels. The relationship of these two factors with gallstones has been separately analyzed. However, to date, no study has jointly assessed their combined association with gallstones. Data from NHANES spanning 2017–2020 were selected. The relative fat mass/albumin ratio (RAR) was constructed and its association with gallstone risk was confirmed via multivariate regression. Restricted cubic spline analysis was used to evaluate the non-linear relationship. Subgroup and interaction analyses were performed to strengthen the robustness of the study. The receiver operating characteristic (ROC) curve assessed the predictive ability of RAR. Mediation analysis was applied to explore potential mediating variables. The study’s findings were derived from an analysis of 2794 participants. RAR was identified as an independent risk factor for gallstone risk, showing a positive linear correlation. Individuals in the high RAR category showed an odds ratio (OR) of 3.69 (2.01–6.76) relative to those in the low RAR group. For every increase of 10 units in RAR, the risk of gallstones rose by approximately one-third (34%). All subgroups in the study exhibited a consistent trend with the overall population. Compared to RFM and albumin alone, the composite RAR index showed significantly enhanced predictive performance (0.653 vs. 0.634, P = 0.011; 0.653 vs. 0.578, P < 0.001). Age served as a partial mediator (7.33%) in the link between RAR and gallstone risk. The newly formulated RAR index was introduced for the first time to evaluate their combined association with gallstone risk. The prediction performance of RAR was superior compared to RFM or albumin alone. Findings implied that clinicians should focus more on RAR levels rather than isolated evaluation of RFM and albumin for high-risk individuals (such as those with obesity and low albumin). Notably, RAR should be used with caution in clinical applications. Given the straightforward nature and accessibility of its constitutive elements, RAR was conducive to clinical application.