Dietary index for gut microbiota (DI-GM) and irritable bowel syndrome: a case–control study

This study investigated whether the Dietary Index for Gut Microbiota (DI-GM), a measure of diet quality for gut health, is associated with IBS risk, and symptom severity, inflammation. A case-control study included 350 adults (175 IBS patients per Rome IV and 175 matched controls). Dietary intake was assessed using a validated food frequency questionnaire to calculate DI-GM scores. Irritable Bowel Syndrome severity and quality of life were measured via the Irritable Bowel Syndrome Symptom Severity Scale (IBS-SSS), Irritable Bowel Syndrome Extended Symptom Severity Scale (IBS-EISSS), and Irritable Bowel Syndrome Quality of Life questionnaire (IBS-QOL). Serum inflammatory markers-including C-reactive protein (CRP), lipopolysaccharide (LPS), zonulin, tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), and brain-derived neurotrophic factor (BDNF)-were quantified using ELISA. IBS patients had significantly lower DI-GM scores than controls (7.69 ± 3.12 vs. 12.15 ± 2.60; p < 0.001). Participants in the highest DI-GM tertile showed significantly lower zonulin (31.33 ± 0.58 vs. 56.01 ± 22.49 ng/mL; p < 0.001), LPS (1.03 ± 0.30 vs. 1.84 ± 0.47 EU/mL; p < 0.001), and CRP (3.10 ± 0.03 vs. 5.53 ± 1.41 mg/L; p < 0.001). Higher DI-GM scores were associated with lower psychological scores and IBS symptom severity (all p < 0.001). Importantly, IBS symptom severity showed strong positive correlations with inflammatory markers, suggesting that symptom severity may drive dietary modifications and inflammatory responses. Logistic regression indicated that individuals in the highest DI-GM tertile had substantially lower odds of IBS compared to the lowest tertile. Higher DI-GM scores were associated with lower IBS odds and favorable profiles. However, strong correlations between symptom severity and inflammation suggest reverse causality-symptoms may drive dietary changes rather than diet determining disease. Due to the cross-sectional design, causal inference is not permitted, and prospective studies are required.