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BACKGROUND: High glycemic index (GI) diets have been linked to elevated risk of cardiometabolic diseases. One possible underlying mechanism comes from high GI diet's potential to promote lipid peroxidation. OBJECTIVES: We aim to evaluate whether and to what extent dietary carbohydrate quality and quantity are associated with systemic levels of lipid peroxidation in females. METHODS: In this cross-sectional analysis of 2163 middle-aged women, a subset of the Shanghai Women's Health Study, we measured lipid peroxidation biomarkers F(2)-isoprostanes (F(2)-IsoPs) and its metabolite, 2,3-dinor-5,6-dihydro-15-F(2t)-IsoP (F(2)-IsoP-M), in urine. The quality of carbohydrate was defined by dietary GI, assessed using a validated FFQ via in-person interviews. A multivariable linear regression model with restricted cubic spline functions was used to evaluate the association of measured biomarkers with carbohydrate intake and dietary GI. RESULTS: After adjustment for potential confounding factors such as cigarette smoking, BMI, and comorbidities, among others, we found that F(2)-IsoP-M concentrations were positively associated with both carbohydrate intake and dietary GI. Carbohydrate intake and dietary GI were weakly correlated (r = 0.12). When further mutually adjusted for the 2 factors, the positive association with F(2)-IsoP-M remained statistically significant for GI (P = 0.004) but not for carbohydrate intake (P = 0.50). Compared with those in the 10th percentile of dietary GI, fold increases (95% CI) in F(2)-IsoP-M concentrations for those in the 30th, 50th, 70th, and 90th percentiles were 1.03 (1.00, 1.07), 1.06 (1.01, 1.10), 1.09 (1.03, 1.14), and 1.13 (1.05, 1.21), respectively. Moreover, there appeared a threshold regarding the association between dietary GI and F(2)-IsoP-M concentrations, with the dose-effect slope of GI being 2.3 times greater when GI was ≥75 relative to GI