Association of Dietary Fiber Intake with Depression Severity in US Adults: A Cross-Sectional Study of NHANES 2021-2023

Background: Emerging evidence implicates the gut-brain axis as a pathway through which dietary fiber may influence depressive symptoms, yet the shape of this associa-tion, its specificity to individual symptom domains remain underexplored. This study examined the association between dietary fiber intake and depression severity among US adults using data collected during a pandemic period of markedly elevated depres-sion prevalence. Methods: Cross-sectional data from 4043 adults (≥18 years) in the National Health and Nutrition Examination Survey (NHANES) 2021-2023 cycle were analyzed. Die-tary fiber intake was derived from two 24-hour dietary recalls. Depression severity was classified into five ordered categories using the Patient Health Questionnaire-9 (PHQ-9). Survey-weighted ordinal logistic regression models with progressive covari-ate adjustment were fitted. Dose-response patterns were evaluated via quartile analy-sis and restricted cubic splines. Secondary analyses examined fiber source specificity and associations with individual PHQ-9 items. Results: In the fully adjusted model, higher fiber intake was inversely associated with depression severity (OR 0.89 per 1 SD increase, 95% CI 0.80-0.97, p = 0.012). Partici-pants in the highest intake quartile had 40% lower odds of greater severity compared with the lowest quartile (ptrend < 0.001). The dose response curve showed the steepest decline at lower intakes, plateauing near 25 g/day. Among fiber sources, only grain fi-ber was significantly associated with depression severity. Symptom-level analyses re-vealed selective inverse associations with anhedonia and sleep problems, although these findings should be interpreted as exploratory given the lack of correction for multiple comparisons. Conclusions: Dietary fiber intake is independently and inversely associated with de-pression severity in a dose-dependent manner. Observed syndrome-specific associa-tions with anhedonia and sleep disturbance warrant further investigation through prospective and interventional studies to clarify causality.