Illness perceptions and perceived functional health literacy in inflammatory bowel disease: a multicentre longitudinal study

Perceived Functional Health Literacy (HL) and Illness Perceptions (IPs) are key yet understudied determinants of self-care in Inflammatory Bowel Disease (IBD). Although cross-sectional studies suggest a relationship between these constructs, their temporal directionality and within-person dynamics remain unclear, as previous research has failed to distinguish stable between-person differences from true intra-individual change processes. To examine the longitudinal and reciprocal relationship between perceived functional HL and IPs in people living with IBD, separating within-person dynamics from stable between-person differences. A multicenter prospective longitudinal study included 455 people living with IBD from nine Italian specialized centers, with data collected at baseline, 6, and 12 months. Perceived functional HL (self-reported difficulty in understanding written health information) was assessed using the Single Item Literacy Screener (SILS) and IPs via the Brief Illness Perception Questionnaire (BIPQ). Temporal associations were analyzed using a Bayesian Random-Intercept Cross-Lagged Panel Model (RI-CLPM). Worsening IPs were temporally associated withsubsequent increases in perceived HL difficulties (posterior M = 0.18, 95% Credible Interval (CrI) [0.07, 0.28]); the reverse effect was not credible (posterior M = 0.25, 95% CrI [− 0.04, 0.53]). Older age, male gender, and Crohn’s disease were stable predictors of both constructs. SILS showed strong persistence (β = 0.63); BIPQ showed greater variability with mean reversion (β = −0.27). The relationship between functional HL and IPs is asymmetric and within-person driven: deteriorating illness perceptions were temporally associated with subsequent increases in perceived HL difficulties, but not vice versa, highlighting the need for integrated psychological support in IBD care.