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IMPORTANCE: Healthier diets are generally believed to benefit cognitive health; however, the evidence remains inconsistent, and a systematic examination of multiple healthy dietary patterns within the same context is lacking.OBJECTIVE: To evaluate the associations of 6 healthy patterns with subjective cognitive decline (SCD) and objective cognitive function.DESIGN, SETTING, AND PARTICIPANTS: This was a prospective cohort study based on the Nurses' Health Study (NHS, 1986-2014), NHSII (1991-2017), and the Health Professionals Follow-Up Study (HPFS, 1986-2012). Included were adults with available data on diet and cognitive function. Data analyses were performed from September 2024 to November 2025.EXPOSURES: Six dietary pattern scores, including the Alternate Healthy Eating Index 2010 (AHEI-2010), the Dietary Approaches to Stop Hypertension (DASH) diet score, the Healthful Plant-Based Diet Index (hPDI), the Planetary Health Diet Index (PHDI), and the reversed empirical dietary indices for hyperinsulinemia (rEDIH) and inflammatory pattern (rEDIP).MAIN OUTCOMES AND MEASURES: SCD was assessed using 7 questions on perceived cognitive changes. Cognitive function was objectively measured via telephone in the NHS.RESULTS: A total of 159 347 individuals (mean [SD] age, 44.3 [9.3] years; 131 560 female [82.6%]) were included in this analysis. Among the study participants, higher adherence to all 6 dietary patterns was associated with a lower SCD risk. The DASH diet showed the strongest magnitude (risk ratio [RR] comparing 90th vs 10th percentile of adherence: 0.59; 95% CI, 0.57-0.62), followed by the hPDI (RR, 0.76; 95% CI, 0.65-0.85), rEDIH (RR, 0.76; 95% CI, 0.73-0.80), PHDI (RR, 0.80; 95% CI, 0.75-0.86), AHEI-2010 (RR, 0.84; 95% CI, 0.80-0.89), and rEDIP (RR, 0.89; 95% CI, 0.85-0.93). Higher DASH diet score at ages 45 to 54 years showed the strongest association with SCD. Higher adherence to the DASH diet also showed the strongest association with a higher objectively measured global cognition (mean z score difference comparing 90th vs 10th percentile: 0.05; 95% CI, 0.02-0.09). Key food groups associated with better cognitive function included higher vegetable and fish intake and lower red and processed meats intake.CONCLUSIONS AND RELEVANCE: Results reveal that healthy diets, exemplified by the DASH diet for blood pressure control and diets with lower hyperinsulinemia and inflammation potentials, were associated with a lower SCD risk and better cognitive function. These findings underscore the importance of a healthy diet for maintaining long-term cognitive health.