Probiotic shifted bile acids but not symptoms

Bile acid malabsorption is a common but under-recognised cause of chronic diarrhoea, arising when bile acids that should be reabsorbed in the small intestine instead reach the colon. In some patients, abnormalities of the gut microbiota are thought to contribute, because gut bacteria are responsible for converting primary bile acids into secondary forms. That raises an obvious question: could altering the microbiota with a probiotic change the bile acid profile itself? Effects of probiotics on faecal secretory bile acids, and particularly on the primary bile acid chenodeoxycholic acid, have been unclear.

The investigators ran a randomised, double-blind, placebo-controlled trial of the De Simone formulation 8-strain probiotic in 24 patients previously diagnosed with bile acid malabsorption. Participants took either the probiotic, at 900 billion bacteria, or maltose placebo, three times daily for three weeks. At baseline and after the intervention they measured symptoms, serum 7αC4, faecal primary bile acids, intestinal permeability using a 13C-mannitol-lactulose test with 0.1 and 1 g of the sugars respectively over 24 hours, faecal short-chain fatty acids, the microbiome and the metabolome. Data from 22 patients were included in the analysis.

The primary biochemical signal was clear: probiotic supplementation was associated with a significant fall in the percentage of faecal primary bile acids — chenodeoxycholic acid and cholic acid — with a median change from baseline of −5.7% (IQR −10.3 to 0.9) compared with +9.8% (IQR 0.2 to 20.6) on placebo. The faecal microbiome differed significantly in the probiotic group after the intervention, driven by probiotic-specific species. The nulls matter just as much: there were no significant differences in symptoms, intestinal permeability or short-chain fatty acids. A numerical but non-significant difference appeared in 0–2 hour 13C-mannitol excretion, −1.9 mg for probiotic versus 1.0 mg for placebo.

This trial fills a specific gap — prior data on how probiotics affect faecal secretory bile acids were unclear — and shows that a high-dose multi-strain preparation can measurably shift bile acid composition over just three weeks. But it also illustrates the gap between a biomarker moving and a patient feeling better: the bile acid and microbiome changes occurred without accompanying symptom improvement. Whether three weeks is simply too short for clinical benefit to emerge, or whether the percentage of primary bile acids is not the driver of symptoms in these patients, cannot be resolved here. The permeability signal was suggestive but did not reach significance.

This is a reason to think multi-strain probiotics can genuinely alter bile acid chemistry and gut bacterial composition, and simultaneously a reminder that changing a laboratory measurement is not the same as feeling better. In this trial, symptoms did not improve. The dose used was very high and the population had a specific prior diagnosis, so the result does not generalise to ordinary digestive complaints or to supermarket probiotic products. Anyone with chronic diarrhoea needs a diagnosis first, since bile acid malabsorption is identified with specific testing rather than guessed at.