Associations of Dietary Antioxidant and Phytochemical Indices with Cognitive Function: Mediating Roles of Basal Metabolic Rate and Systemic Inflammation

Dietary antioxidants and phytochemicals are believed to support cognitive health, but evidence on composite dietary indices remains limited. This cross-sectional study of 1845 community-dwelling older adults in China investigated the associations of the composite dietary antioxidant index (CDAI) and dietary phytochemical index (DPI) with cognitive function and mild cognitive impairment (MCI). Cognitive function was assessed using the Montreal Cognitive Assessment (MoCA; Beijing version). MCI was diagnosed through a two-stage procedure: MoCA-based preliminary screening (with education-stratified cutoffs: 13/14 for illiterate, 19/20 for 1–6 years, 24/25 for ≥7 years) followed by neurologist confirmation. CDAI was calculated as the sum of the standardized intakes of six antioxidants (selenium, zinc, carotenoids, vitamin A, vitamin C, vitamin E); DPI was defined as the percentage of the total energy intake from phytochemical-rich foods (fruits, vegetables excluding potatoes, legumes including soy products, nuts, seeds, and whole grains). Multivariable linear regression, logistic regression, and receiver operating characteristic (ROC) curve analyses were performed. The basal metabolic rate (BMR) and systemic immune-inflammation index (SII; platelets × neutrophils/lymphocytes) were tested as potential statistical mediators. Each one-unit increase in CDAI was associated with a 0.068-point higher MoCA score (95% CI: 0.012–0.123), and each one-unit increase in DPI was associated with a 0.029-point higher MoCA score (95% CI: 0.008–0.050). BMR and SII partially mediated the association between CDAI and MoCA score, but temporal ordering remains unclear due to the cross-sectional design. When both CDAI and DPI were in the highest quartile, participants had a 46.3% lower risk of MCI compared with those with both indices in the lowest quartile (OR = 0.537, 95% CI: 0.308–0.935). A predictive model incorporating CDAI, inflammatory markers, and red blood cell parameters showed moderate discriminatory ability in this study sample (apparent AUC = 0.731, bootstrap-corrected AUC = 0.728). These findings suggest that a higher combined dietary antioxidant and phytochemical intake may be jointly associated with better cognitive function, although the cross-sectional design precludes causal inference.