Design: Systematic review and metabolomics meta-analysis of 28 studies, with exploratory transcriptomic analysis in two independent cohorts
Population: 5,056 patients with inflammatory bowel disease, 1,721 healthy controls and 314 non-IBD patients
Primary outcome: Circulating bile acid profiles across eight prespecified comparisons for diagnosis and disease activity
Effect: Secondary bile acids consistently lower in IBD than controls, in ulcerative colitis than Crohn's disease, and in active than quiescent disease; deoxycholic acid and its glyco- and tauro-conjugates most frequently reduced; glycocholic acid raised in some comparisons

Bile acid metabolism sits at the junction of the microbiome and the intestinal epithelium, and individual studies have reported disturbed circulating bile acids in inflammatory bowel disease without much agreement on the pattern. This meta-analysis pooled 28 metabolomics studies covering 5,056 patients with IBD, 1,721 healthy controls and 314 non-IBD patients, in which 131 distinct bile acids had been reported, and tested eight prespecified clinical comparisons.

One pattern held across comparisons: secondary bile acids — the ones produced by gut bacteria from primary bile acids — were lower in IBD than in controls, lower in ulcerative colitis than in Crohn's disease, and lower in active disease than in remission. Deoxycholic acid and its glycine and taurine conjugates were the most consistently reduced, while glycocholic acid, a primary conjugate, rose in some comparisons. Supporting transcriptomic work found differential expression of bile acid transport and metabolism genes in blood, including SLC51A and ABCB4.

The reading is mechanistic. Secondary bile acids are a direct readout of bacterial metabolic activity, so their consistent fall is evidence that the microbial deficit in IBD is functional rather than merely compositional. Whether measuring them adds anything to faecal calprotectin and endoscopy is untested, and that is the question any clinical use would have to answer.

Continue using faecal calprotectin and endoscopy for activity assessment; nothing here replaces them
No head-to-head comparison against existing markers was done
Note that the direction distinguishes ulcerative colitis from Crohn's disease, which few blood markers do
Metabolomics platforms differ between studies, which limits how comparable absolute values are
Read this as evidence about microbial function, not as a test to request

Why it matters

It shows the microbiome deficit in IBD as a loss of a specific metabolic function, which is a more tractable target than dysbiosis in general.

The statistics, in plain English

This is a meta-analysis of direction and consistency rather than of pooled effect sizes: the claim is that the same bile acids move the same way across studies, not that a particular concentration means anything. With 131 bile acids reported across 28 studies, the scope for chance findings is large, which is why the authors' emphasis on the consistently replicated ones matters more than any single molecule. No diagnostic accuracy figures are given, so nothing can be said about how well this would perform as a test.