Background:
Inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS) patients suspect that diet can exacerbate their symptoms. However, how diet quality relates to inflammation markers and symptom occurrence is unclear. We aimed to investigate the relationship of dietary indices with inflammatory markers and gastrointestinal symptoms in IBD and IBS patients.

Methods:
A cross-sectional study was performed using 239 IBD patients, 261 IBS patients and 195 healthy controls (HC). A validated 147-item food frequency questionnaire was used to calculate the inflammatory potential of the diet using the Adapted Dietary Inflammatory Index (ADII) and the Empirical Dietary Inflammatory Index (EDII), and the diet quality using the Dutch Healthy Diet index 2015 (DHD15-index). Faecal calprotectin was used as marker of intestinal inflammation and symptom domains were assessed using the Gastrointestinal Symptom Rating Scale (GSRS).

Results:
The ADII (0.0522.405 vs. 0.0552.465 vs. 0.0542.333, p>0.099) and EDII (0.2841.399 vs. 0.3121.292 vs. 0.0781.138, p=0.125) were not significantly different between IBD, IBS and HC. The DHD15-index was significantly lower in IBS (64.64616.041) compared to IBD (68.36616.546, p=0.036) and HC (69.61816.976, p=0.005). Faecal calprotectin levels correlated significantly with the EDII (r=0.279, p<0.001) and the DHD15-index (r=-0.238, p=0.001) in IBD, but not in IBS (p=0.272 and p=0.295) and HC (p=0.347 and p=0.316), nor with the ADII in either of the subgroups (p=0.084, p=0.854 and p=0.141). In IBS, GSRS domains abdominal pain and reflux syndrome correlated significantly with the DHD15-index (r=-0.184, p=0.003; r=-0.186, p=0.003), while domain diarrhoea syndrome correlated significantly with the ADII (r=0.162, p=0.009) and the EDII (r=0.144, p=0.020).

Conclusion:
A more pro-inflammatory diet and a lower overall diet quality correlated with more intestinal inflammation in IBD and higher symptom scores in IBS. Current findings support the need for further research into diet in these patients.