Inverse Association Between Composite Dietary Antioxidant Index and Prevalence of Pelvic Inflammatory Disease Among Women: A Cross-Sectional Study of NHANES 2013–2018

Background: Pelvic inflammatory disease (PID) is a prevalent chronic inflammatory condition among women. The Composite Dietary Antioxidant Index (CDAI), a measure of dietary antioxidant capacity, has been associated with various inflammatory diseases, but evidence concerning its association with PID remains limited. Methods: The final analytic sample included 4539 women. CDAI was calculated from six dietary antioxidant components: vitamin A, vitamin C, vitamin E, carotenoids, zinc, and selenium. Survey-weighted multivariable logistic regression models were used to evaluate the association between CDAI and self-reported history of treated PID, incorporating the sampling weights, strata, and primary sampling units of NHANES. Restricted cubic spline (RCS) analysis was used to assess both linear and non-linear associations. Subgroup analyses and a machine learning model based on random forest, combined with SHapley Additive exPlanations (SHAP) value ranking, were conducted to evaluate the relative importance of individual components of CDAI. Results: In the fully adjusted spline model including smoking status, CDAI was inversely associated with the odds of self-reported history of treated PID, with no statistical evidence of nonlinearity. Compared with the lowest quartile (Q1), the odds ratios (ORs) for self-reported history of treated PID across higher quartiles of CDAI were as follows: Q2 (OR = 0.682, 95% CI: 0.485–0.959, p = 0.036), Q3 (OR = 0.524, 95% CI: 0.334–0.819, p = 0.009), and Q4 (OR = 0.666, 95% CI: 0.380–1.167, p = 0.167). Among the components of CDAI, vitamin E intake showed an independent inverse association with the odds of self-reported history of treated PID. The SHAP value interpretation indicated that vitamin A, vitamin C, and carotenoids were the three components in CDAI with the highest predictive contribution. Furthermore, subgroup analysis demonstrated a significant interaction effect of age on the association between CDAI and PID. Conclusions: This cross-sectional study suggests an inverse association between CDAI and self-reported history of treated PID, particularly in spline analyses; however, the quartile-based fully adjusted results were non-monotonic and attenuated after adjustment for smoking status. These findings provide hypothesis-generating evidence for future longitudinal and mechanistic studies on antioxidant-related dietary patterns and PID-related reproductive health.