Assessing the Impact of Composite Dietary Antioxidant Index on Gastric Cancer Risk: A Case–Control Study in Southeast China

<b>Objective</b>: To examine the association between the composite dietary antioxidant index (CDAI) and gastric cancer (GC) risk among adults in Southeast China, and to provide evidence for region-specific nutritional interventions. <b>Methods</b>: In this case-control study (July 2023-November 2024), 336 newly diagnosed GC patients were recruited from a hospital in Southeast China, and 336 sex-matched healthy controls were selected from local communities. Dietary data from a validated food frequency questionnaire were used to calculate CDAI scores. <b>Results</b>: A total of 672 participants (56.5% male) were included. The mean CDAI value was 0.47 ± 4.23 in cases versus -0.04 ± 4.61 in controls (<i>p</i> = 0.134), but CDAI quartile distribution differed significantly (<i>p</i> = 0.009). In multivariable analysis of individual CDAI components, vitamin C intake demonstrated a significant inverse association with GC risk, with the strongest protective effect observed in the highest quartile (OR = 0.48, 95% CI: 0.30-0.77, <i>p</i> = 0.002). Selenium intake also showed significant protective effects in the second (OR = 0.52, 95% CI: 0.32-0.83, <i>p</i> = 0.006) and third quartiles (OR = 0.50, 95% CI: 0.30-0.82, <i>p</i> = 0.006). Compared with the lowest quartile, adjusted odds ratios (95% CI) for GC in the second, third, and fourth CDAI quartiles were 0.56 (0.36-0.87), 0.59 (0.38-0.90), and 0.60 (0.39-0.92), respectively. The inverse association was stronger in participants aged >55 years, unmarried, and nonsmokers. Restricted cubic spline analysis revealed a significant nonlinear dose-response relationship. <b>Conclusions</b>: Higher dietary antioxidant intake is associated with lower GC risk. Personalized dietary strategies to enhance antioxidant intake may be particularly beneficial in high-risk populations.