What causes some people’s gut microbes to produce high alcohol levels?

(Press-News.org) Researchers at University of California San Diego, Mass General Brigham, and their colleagues have identified specific gut bacteria and metabolic pathways that drive alcohol production in patients with auto-brewery syndrome (ABS). The rare and often misunderstood condition causes people to experience intoxication without drinking alcohol. The study was published in Nature Microbiology on January 8, 2026.

ABS occurs when gut microbes break down carbohydrates and convert them to ethanol (the alcohol found in intoxicating beverages), which then enters the bloodstream. While the metabolism of carbohydrates can produce small amounts of alcohol in everyone, levels can be high enough to cause intoxication in people with ABS. The condition is extremely rare but likely underdiagnosed due to a lack of awareness, diagnostic challenges, and stigma.

Medical consequences of ABS can include liver damage, cognitive impairment, digestive issues and withdrawal symptoms, among others. Many patients experience years of misdiagnosis and social, medical, and legal consequences before receiving a diagnosis. Confirmation of ABS is also difficult to obtain, as the gold-standard diagnosis requires monitored blood alcohol testing under supervised conditions.

In the largest study of the condition to date, the researchers evaluated 22 patients with ABS, 21 unaffected household partners, and 22 healthy control participants, comparing gut microbial composition and function across groups.

They found that stool samples taken from patients during an ABS flare produced significantly more ethanol than samples from household partners and healthy controls.

Previously, scientists had very little information about which specific gut microbes (yeasts or bacteria) caused ABS. The stool analysis pointed to several bacterial species as key drivers in some patients. 

“We found that gut bacteria, including Escherichia coli and Klebsiella pneumoniae, ferment sugars into ethanol in the intestine in patients with ABS,” said Bernd Schnabl, MD, professor of medicine at UC San Diego School of Medicine. “These microbes use several ethanol-producing pathways and can drive blood-alcohol levels high enough to cause legal intoxication.”

While some patients had these organisms, the authors note that identifying the exact microbes responsible for ABS in individual patients is difficult. However, during ABS flares, some patients also had much higher levels of enzymes involved in fermentation pathways compared with controls. The authors note that rather than focusing on specific bacteria species, treatments that target microbial enzymes involved in ethanol production may be a more effective strategy.

The results also suggest that a stool-based test could serve as an alternative to monitored blood alcohol testing to diagnose ABS, leading to more rapid and accessible screening.

The researchers also followed one patient who experienced symptom relief after undergoing a fecal microbiota transplantation when other treatments had failed. Their patterns of relapse and remission corresponded with shifts in specific bacterial strains and metabolic pathway activity, providing further biological evidence for the condition. After a second fecal transplant, the patient was symptom-free for more than 16 months.

“Auto-brewery syndrome is a misunderstood condition with few tests and treatments. Our study demonstrates the potential for fecal transplantation,” said co-senior author Elizabeth Hohmann, MD, of the Infectious Disease Division in the Mass General Brigham Department of Medicine. “More broadly, by determining the specific bacteria and microbial pathways responsible, our findings may lead the way toward easier diagnosis, better treatments, and an improved quality of life for individuals living with this rare condition.”

Schnabl, Hohmann and their colleagues are currently conducting a phase 1 clinical trial evaluating fecal microbiota transplantation in eight patients with ABS. They believe that studies like this could also have lessons for treating other conditions, such as metabolic-dysfunction associated liver disease.

Additional co-authors on the study include: Cynthia L. Hsu, Shikha Shukla, Linton Freund, Annie C. Chou, Yongqiang Yang, Ryan Bruellman, Chitra Mandyam, Brigid S. Boland, Fernanda Raya Tonetti, Noemí Cabré, Susan Mayo, Cristina Llorente and Hyun Gyu Lim, UC San Diego; Bernhard O. Palsson, UC San Diego, Joint BioEnergy Institute and Technical University of Denmark; Valeria Magallan, Mass General Brigham; Barbara J. Cordell, Auto-Brewery Syndrome Information and Research, Inc.; Sonja Lang , University of Cologne and Peter Stärkel at St. Luc University Hospital, Catholic University of Louvain.

Funding: The study was funded in part by National Institutes of Health (grants K99 AA031328, R01 AA024726, R01 AA020703, U01 AA026939, P50AA011999, and T32 DK007202) and the American Association for the Study of Liver Diseases Foundation (grant #CTORA23-208366).

Disclosures: Schnabl consults for Ambys Medicines, Boehringer Ingelheim, Ferring Research Institute, Gelesis, HOST Therabiomics, Intercept Pharmaceuticals, Mabwell Therapeutics, Patara Pharmaceuticals, Surrozen and Takeda. UC San Diego has received research support from Axial Biotherapeutics, BiomX, CymaBay Therapeutics, Intercept, NGM Biopharmaceuticals, Prodigy Biotech and Synlogic Operating Company. Schnabl is founder of Nterica Bio. Hohmann has received research support from Seres Therapeutics, MicrobiomeX/Tend.

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