Nonlinear association between urinary nitrate and gallstone risk: metabolic regulatory factors revealed by the NHANES study

Gallstone disease imposes a substantial US healthcare burden. Rising environmental nitrate exposure from agriculture may influence gallstone pathogenesis through inflammatory pathways. We investigated urinary nitrate (log-transformed)–gallstone associations. Using 2017–2020 NHANES data (n = 1643), we employed weighted multivariable logistic regression to assess the linear association as our primary analysis, adjusting for sociodemographic, lifestyle, and metabolic covariates. In exploratory analyses, we employed weighted restricted cubic splines and threshold models to evaluate potential nonlinear relationships. Subgroup analyses were conducted to generate hypotheses regarding effect modification. In fully adjusted models, urinary nitrate (log-transformed) was not significantly associated with gallstone risk. However, exploratory analysis revealed a nonlinear relationship with a threshold effect at lnNO₃ = 10.61. Levels below this threshold were associated with a significantly lower gallstone risk (OR = 0.49, 95% CI 0.29–0.84, P = 0.025), while levels above it suggested a non-significant increase (OR = 1.68, 95% CI 0.49–5.72, P = 0.271). In exploratory subgroup analyses, lower urinary nitrate was associated with reduced gallstone odds among women, individuals without diabetes, and vigorous exercisers. Formal interaction tests showed no significant effect modification (all P-interaction > 0.05). The primary analysis found no significant linear association. Exploratory analyses suggest a potential nonlinear, threshold-dependent relationship, with patterns that may vary by metabolic factors. These hypothesis-generating findings warrant validation in longitudinal studies.