Evaluating the role of anxiety on the association between irritable bowel syndrome and brain volumes: a mediation analysis in the UK Biobank cohort

There is a strong link between irritable bowel syndrome and brain volumes, yet, to date, research examining the mediators of this association has been little. Based on the phenotypic data of 15 248 participants from the UK Biobank, a two-stage mediation analysis was performed to assess the association among brain volumes, anxiety, and irritable bowel syndrome. In the first stage, we identified the candidate mediating role of anxiety for irritable bowel syndrome associated with brain volumes using regression models. Then, we quantified the magnitude of the mediation effects by evaluating the average causal-mediated effect and proportion of mediation through performing mediation analyses in the R package in the second stage. In the first stage, we identified the partly mediating role of anxiety in the association between irritable bowel syndrome and the volume of thalamus (P(left) = 1.16 × 10(−4), P(right) = 2.41 × 10(−4)), and grey matter (P(left) = 3.22 × 10(−2), P(right) = 1.18 × 10(−2)) in the VIIIa cerebellum. In the second stage, we observed that the proportion of the total effect of irritable bowel syndrome on volume of thalamus mediated by anxiety was 14.3% for the left region (β(Average causal-mediated effect) = −0.008, P(Average causal-mediated effect) = 0.004) and 14.6% for the right region (β(Average causal-mediated effect) = −0.007, P(Average causal-mediated effect) = 0.006). Anxiety mediated 30.8% for the left region (β(Average causal-mediated effect) = −0.013, P(Average causal-mediated effect) = 0.002) and 21.6% for the right region (β(Average causal-mediated effect) = −0.010, P(Average causal-mediated effect) x= 0.018) of the total effect of irritable bowel syndrome on the volume of grey matter in the VIIIa cerebellum. Our study revealed the indirect mediating role of anxiety in the association between irritable bowel syndrome and brain volumes, promoting our understanding of the functional mechanisms of irritable bowel syndrome and its related psychosocial factors.