Association Between Composite Dietary Antioxidant Index and Depressive Symptoms in Breast Cancer Patients: The Role of Oxidative Stress Biomarkers in a Cross-Sectional Study

Background: Depressive symptoms (DepS) are prevalent among breast cancer patients and are associated with poor treatment outcomes and prognosis. Oxidative stress has been implicated in the pathophysiology of depression, and dietary antioxidants may be associated with oxidative balance. This study aimed to evaluate the association between the composite dietary antioxidant index (CDAI) and DepS, and to explore the statistical relationships between CDAI, oxidative stress biomarkers, and DepS. Methods: In this cross-sectional study, 302 breast cancer patients were enrolled, of whom 113 provided blood samples for the assessment of plasma oxidative stress biomarkers. Dietary intake was assessed using 3-day 24 h dietary recalls. The CDAI was calculated based on the intake of six dietary antioxidants, including vitamins A, C, and E, zinc, selenium, and manganese. DepS were defined as a score of ≥8 on the Hospital Anxiety and Depression Scale—Depression subscale (HADS–D). Results: A total of 102 patients (33.77%) exhibited DepS, and these patients had significantly lower CDAI scores and reduced plasma levels of superoxide dismutase (SOD) and glutathione (GSH), compared with those without DepS (all p < 0.05). CDAI and dietary zinc intake were non-linearly associated with DepS (p for non-linearity <0.05). Vitamin C intake was inversely associated with DepS (OR = 0.989; 95% CI: 0.984, 0.993; p < 0.001). CDAI scores were positively associated with SOD and GSH levels (both p < 0.001), while SOD and GSH were inversely associated with DepS (SOD: β = −0.610; GSH: β = −0.900; both p < 0.001). In exploratory mediation analysis, GSH and SOD statistically accounted for part of the association between CDAI and DepS (ACME = −0.192 and −0.228, respectively; all p < 0.01). Conclusions: Higher dietary antioxidant intake, as reflected by CDAI, is associated with lower DepS in breast cancer patients, and oxidative stress biomarkers may be statistically involved in this association.