British Journal of Health Psychology (2018), 23, 171–185 © 2017 The British Psychological Society www.wileyonlinelibrary.com The longitudinal effects of emotion regulation on physical and psychological health: A latent growth analysis exploring the role of cognitive fusion in inflammatory bowel disease In^ es A. Trindade* , Cl  audia Ferreira and Jos e Pinto-Gouveia CINEICC – Cognitive and Behavioural Centre for Research and Intervention, Faculty of Psychology and Education Sciences, University of Coimbra, Portugal Objective. This study thus aims to test differences between patients with inflammatory bowel disease (IBD) regarding IBD symptomatology, cognitive fusion, and psychological and physical health, as well as to explore whether the maladaptive emotion regulation process of cognitive fusion longitudinally impacts on the baseline and evolution of these outcomes over a period of 18 months. Design and methods. Participants include 116 IBD patients with a mean age of 36.76 (SD = 11.39) of both genders (69.83% females) that completed the self-report measures of interest in three different times, equally spaced 9 months apart, over a period of 18 months. Latent growth curve models were conducted using structural equation mod- elling to estimate the growth trajectory of the variables in study. Results. Inflammatory bowel disease symptomatology and cognitive fusion’s levels were negatively associated with psychological health and physical health’s baseline levels. Furthermore, IBD symptomatology did not influence the growth of psychological health, while cognitive fusion did (b = .30, p = .007). The same result was found for physical health (b = .26, p = .024). These findings indicate that individuals with higher levels of cognitive fusion present lower levels of psychological health and physical health that tend to further decrease over the time through the effects of this maladaptive emotion regulation process. Conclusions. This study implies that it is of crucial importance to include psychotherapeutic interventions in the health care of patients with IBD. If successful, these interventions could represent decreases in the cost of IBD treatment and in the use of drugs with adverse side effects, in addition to improving patients’ mental health and quality of life. Further implications for clinical and research work are discussed. Statement of contribution What is already known on this subject?  Research has demonstrated the impact of emotion regulation on both physical and mental health.  Nevertheless, the longitudinal effects of the specific emotion regulation process of cognitive fusion on physical and psychological health have never been explored. *Correspondence should be addressed to In^ es A. Trindade, Faculty of Psychology and Education Sciences, University of Coimbra, Rua do Col  egio Novo, 3000-115 Coimbra, Portugal (email: ines.almeidatrindade@gmail.com). DOI:10.1111/bjhp.12280 171