Effects of a Combined Vitamin D3 and Docosahexaenoic Acid Intervention on Cognitive Function and Blood Pyroptotic Biomarkers in Older Adults with Mild Cognitive Impairment: A 12-Month Randomized, Double-Blind, Placebo-Controlled Trial

Background: This study aimed to assess the effects of a combined vitamin D3 (VD3) and docosahexaenoic acid (DHA) intervention on cognitive function and blood pyroptotic biomarkers in older adults with mild cognitive impairment (MCI). Methods: In total, 360 participants with MCI were recruited, and randomly assigned into four groups: VD3 (800 IU/day), DHA (1 g/day), VD3+DHA (VD3 800 IU/day+DHA 1 g/day), and placebo. Cognitive function was evaluated by the Wechsler Adult Intelligence Scale-Revised in China (WAIS-RC), and blood biomarkers were measured by ELISA and Western blot at baseline, 6 months and 12 months. Linear mixed-effects models (LMMs) were employed to analyze longitudinal interaction effects on cognitive function and blood biomarkers. Structural equation modeling (SEM) was conducted to evaluate the parallel mediation effects of blood biomarkers on cognitive improvements. Results: A total of 355 participants completed the trial. Over the 12-month intervention, all intervention groups demonstrated significant improvements in cognitive function and reductions in five blood biomarkers (IL-1β, IL-18, NLRP3, Gasdermin D (GSDMD), cleaved caspase-1) compared to the placebo group (p < 0.05). Compared to each single intervention, the combined VD3 and DHA intervention yielded superior cognitive improvements in scores for full-scale IQ (FIQ), performance IQ (PIQ), verbal IQ (VIQ) and several subtests (information, comprehension, digit span, vocabulary, picture completion, block design, and picture arrangement), and significantly reduced five blood pyroptotic biomarkers (all p < 0.05). Path analysis revealed that longitudinal GSDMD reduction mediated the improvements in FIQ. Conclusions: A 12-month intervention with VD3, DHA, or their combination significantly improves cognitive function and mitigates pyroptosis, and the combined intervention showed superior benefits, potentially mediated by downregulating the protein GSDMD.