Background: Long COVID (LC) is associated with persistent cognitive symptoms, frequently described as brain fog, and with inflammatory, immunometabolic, and energetic mechanisms potentially involved in cognitive functioning. Vitamins participate in biological processes relevant to brain function, but the relationship between habitual dietary vitamin intake and cognitive performance in LC remains poorly studied. Objective: To analyze the relationship between dietary vitamin intake and cognitive performance in adults with LC, considering both objective performance and self-reported cognition. Methods: A cross-sectional study was conducted in 304 adults with LC. Vitamin intake from foods and beverages was estimated using the EVIDENT seven-day dietary record, previously compared with a validated food-frequency questionnaire. Objective cognitive performance was assessed using normative scores from the Montreal Cognitive Assessment (MoCA) and the Memory Index Score (MIS), as well as MoCA domains and items. Self-reported cognition was assessed using the 30-item Memory Failures of Everyday Questionnaire (MFE-30) and the cognitive subscale of the Modified Fatigue Impact Scale (MFIS). Primary global associations were examined using multiple linear regression with HC3 robust standard errors, whereas bounded MoCA domains were analyzed using proportional-odds ordinal regression and binary item-level outcomes using logistic regression. Benjamini-Hochberg correction was applied within prespecified families of contrasts. Results: No associations between vitamin intake and global normative cognitive performance remained significant after false discovery rate (FDR) correction. Median MoCA and MIS scores were 24.00 (IQR, 21.00-26.00) and 11.00 (IQR, 9.00-13.00), respectively. Normative MoCA and MIS scores showed medians of -0.32 (IQR, -0.82 to 0.19) and -0.32 (IQR, -0.76 to 0.14), respectively. Most participants (95.1%) met the dietary reference value for vitamin B3. In secondary ordinal analyses, higher vitamin B3 intake was associated with better MoCA language performance in the fully adjusted model (OR per 1 SD = 2.18, 95% CI 1.45-3.27, p < 0.001, q = 0.0116). No associations remained significant after FDR correction for everyday cognitive complaints, cognitive fatigue, or MoCA Memory and MoCA Multiple scores. Conclusions: In adults with LC, the primary analyses did not identify FDR-significant associations between habitual dietary vitamin intake and global normative cognitive performance. The secondary vitamin B3-language association requires independent confirmation and does not establish a cognitive benefit of higher intake or supplementation. These findings require confirmation in longitudinal studies incorporating nutritional biomarkers, information on supplementation, and broader neuropsychological assessment.
