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Background The decline in pulmonary function among older adults is a widespread issue, lacking specific dietary guidelines for optimising respiratory health.Methods Data from the Guangzhou Biobank Cohort Study (GBCS) and National Health and Nutrition Examination Survey (NHANES) were analysed to assess the association of the Composite Dietary Antioxidant Index (CDAI) with key pulmonary function indicators in individuals aged 50 and older.Results The study included 8640 participants from GBCS and 4029 from NHANES, with a mean age of 62.5 years. After adjusting for confounders, higher CDAI scores were significantly linked to increased forced expiratory volume in 1 s (FEV 1) and FEV1 percentage predicted (FEV1pp) in GBCS (β=0.004 L, 95% CI 0.001 to 0.008; β=0.193%, 95% CI 0.045% to 0.342%). Similar trends were observed in NHANES for FEV1pp (adjusted β=0.353%, 95% CI 0.059% to 0.646%). Subgroup analyses showed that women exhibited consistent positive associations between CDAI and both FEV1pp and forced vital capacity (FVC) percentage predicted (FVCpp) in both cohorts. In GBCS, stronger associations were found for participants over 65 years.Conclusions Higher CDAI scores correlate with improved pulmonary function, particularly in women and older adults, underscoring the need for dietary recommendations that emphasise antioxidant-rich foods to enhance respiratory health and support healthy ageing.