| Summary: | Abstract Background Dietary antioxidants have been postulated to prevent age‐related cognitive decline. However, the optimal antioxidant level for cognitive benefits remains unknown. Method MEDLINE, Embase, and PubMed were searched from inception to 5th September 2021 for cohort studies reporting the associations between the levels of dietary antioxidant, including flavonoids, vitamin E, carotenoids, and vitamin C and cognitive impairment, and dementia. The quality of all studies was assessed by the NHLBI Study Quality Assessment Tool. Random‐effects model was used to calculate the pooled relative risks (RRs) and 95% confidence intervals (CIs). Dose‐response meta‐analysis using the Greenland and Orsini method with random‐effects models were performed. Result A total of 14 cohort studies were included in the meta‐analysis. Comparing highest to lowest dietary intake levels, flavonoids was associated with lower odds of cognitive impairment and dementia (RR Q4 vs. Q1 = 0.84, 95% CI = 0.72–0.98), while no significant association was observed for vitamin E, carotenoids and vitamin C (RR Q4 vs. Q1 = 0.86, 95% CI = 0.58–1.27, RR Q4 vs. Q1 = 0.84, 95% CI = 0.64–1.10, RR Q4 vs. Q1 = 0.87, 95% CI = 0.68–1.11, respectively). In dose‐response analysis, a J‐shaped relation was observed for dietary vitamin C intake with protective associations detected for less than 85 mg/day intake (P<0.05 for non‐linearity). Meanwhile a significant linear protective relation was observed for dietary carotenoids (range, 0‐20mg/day, RR = 0.98, [0.95‐1.00] for each 1mg/day increment) (P<0.05 for linearity). Conclusion The meta‐analysis results suggest potential beneficial roles of long‐term intake of dietary flavonoids, carotenoids and vitamin C in the primary prevention of cognitive impairment and dementia.
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