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Background: The neuroprotective benefits of combined folic acid and docosahexaenoic acid (DHA) on cognitive function in mild cognitive impairment (MCI) patients are suggested but unconfirmed. Objective: To explore the effects of 6-month folic acid + DHA on cognitive function in patients with MCI. Methods: Our randomized controlled trial (trial number ChiCTR-IOR-16008351) was conducted in Tianjin, China. We divided 160 MCI patients aged >  60 years into four regimen groups randomly: folic acid (0.8 mg/day) + DHA (800 mg/day), folic acid (0.8 mg/day), DHA (800 mg/day), and placebo, for 6 months. Cognitive function and blood amyloid-β peptide (Aβ) biomarker levels were measured at baseline and 6 months. Cognitive function was also measured at 12 months. Results: A total of 138 patients completed this trial. Folic acid improved the full-scale intelligence quotient (FSIQ), arithmetic, and picture complement scores; DHA improved the FSIQ, information, arithmetic, and digit span scores; folic acid + DHA improved the arithmetic (difference 1.67, 95% CI 1.02 to 2.31) and digital span (1.33, 0.24 to 2.43) scores compared to placebo. At 12 months, all scores declined in the intervention groups. Folic acid and folic acid + DHA increased blood folate (folic acid + DHA: 7.70, 3.81 to 11.59) and S-adenosylmethionine (23.93, 1.86 to 46.00) levels and reduced homocysteine levels (–6.51, –10.57 to –2.45) compared to placebo. DHA lower the Aβ40 levels (–40.57, –79.79 to –1.35) compared to placebo (p <  0.05), and folic acid + DHA reduced the Aβ42 (–95.59, –150.76 to –40.43) and Aβ40 levels (–45.75, –84.67 to –6.84) more than DHA (p <  0.05). Conclusion: Folic acid and DHA improve cognitive function and reduce blood Aβ production in MCI patients. Combination therapy may be more beneficial in reducing blood Aβ-related biomarkers.

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