Blocking a Gut Bacterial Enzyme Reversed Fatty Liver Disease in Mice

If you have been told your liver enzymes are up, or you are managing fatty liver disease and wondering what your gut has to do with it, this study is worth knowing about. Researchers at the University of California, San Diego gave mice with diet-induced fatty liver disease a drug that never leaves the intestine, yet it reshaped bile acid chemistry enough to shrink liver fat, calm liver inflammation, and in some cases blunt scarring. The drug works by shutting down a single bacterial enzyme in the gut, not by acting on the liver directly.

Key takeaways

• A gut-restricted drug called GR-7 blocked a bacterial enzyme called bile salt hydrolase (BSH) in mice with diet-induced metabolic dysfunction-associated steatohepatitis (MASH), the advanced, inflammatory form of fatty liver disease.
• Started early, GR-7 suppressed liver scarring (fibrosis) even at a low dose; started late, a high dose cut liver fat, liver weight, and blood markers of liver injury and gut leakiness.
• The drug worked by changing which bile acids gut bacteria produce, shifting the liver's own bile acid factory onto a different, protective pathway, and by curbing appetite.
• This is a mouse study only. Late-stage liver scarring improved at the gene-expression level but not yet on tissue biopsy, and no human trial has been run.

Bile Salt Hydrolase Inhibitor GR-7 Improves MASH in Mice

Researchers tested GR-7, a non-absorbable inhibitor of bile salt hydrolase, a bacterial enzyme (BSH, a protein gut bacteria use to chemically modify bile acids), in mice with diet-induced MASH (metabolic dysfunction-associated steatohepatitis, the inflamed and scarring stage of fatty liver disease). When GR-7 was given early in disease development, it suppressed liver fibrosis (scar tissue buildup) even at the lowest dose tested. When given later, after the disease was already established, a high dose reduced liver weight, hepatic triglycerides and cholesterol, and blood levels of alanine aminotransferase (ALT, a standard liver-injury blood marker) and lipopolysaccharide (LPS, a bacterial toxin that leaks into the blood when the gut barrier weakens), with differences reaching strong statistical significance (P values from 0.05 down to below 0.0001, meaning the changes were very unlikely to be due to chance).

Fatty Liver Disease Affects Millions Who May Not Know It

MASH sits at the far end of a disease spectrum that starts with simple fatty liver and can progress to cirrhosis, liver cancer, or the need for a transplant. The researchers note that fatty liver disease broadly (MASLD) affects up to 38% of the adult population globally, and the condition is closely tied to obesity, type 2 diabetes, and high cholesterol, meaning many readers managing those conditions are also, knowingly or not, managing some degree of liver fat buildup. Until recently there was no dedicated drug for it. Resmetirom, approved in 2024, was the first FDA-approved MASH treatment, and the injectable diabetes and weight-loss drug semaglutide has also shown benefit, but both act directly on the liver or on metabolism broadly. A drug that works entirely from inside the gut, without entering the bloodstream, would open an entirely different route to treating the disease, one with a different side-effect profile because it never has to reach the liver, brain, or any other organ to work.

Mice Studied Across Diet, Dose, and Timing

The team used two mouse models: Foz/Foz mice, which are genetically prone to overeating and reliably develop MASH on a high-fat, high-fructose "Western diet," and ordinary wild-type mice, to confirm the drug's effects were not specific to one genetic background. Foz/Foz mice were fed the Western diet for 24 weeks and split into six groups, including a normal-chow control (7 mice), Western diet alone (12 mice), and four treatment groups receiving low or high doses of GR-7 started either early (after 3 weeks on the diet) or late (after 12 weeks, once disease was established), with group sizes of 6 to 12 mice. A separate short-term experiment in wild-type mice, and a third experiment giving GR-7 by direct gavage instead of mixing it into food, ruled out the possibility that the drug's effects were just a side effect of mice eating less because their food tasted or smelled different.

What This Means If You Are Managing Liver or Gut Health

This study strengthens the case that the gut microbiome, not just diet and body weight, actively shapes liver disease risk through the bile acids bacteria produce, but it does not hand readers a new supplement or treatment to try. GR-7 is an experimental compound tested only in mice; it is not available as a drug or supplement, and there is no equivalent over-the-counter product that reliably inhibits bile salt hydrolase the way GR-7 does. What is worth discussing with a doctor, if you have fatty liver disease or risk factors for it (obesity, type 2 diabetes, high cholesterol), is where bile acid and gut-liver axis testing fits into your workup, since prior clinical research the authors cite has linked BSH activity to MASLD severity in people, not just mice. This is a mechanism to watch, not an intervention to adopt.

Late-Stage Liver Scarring Did Not Fully Reverse

The clearest limitation is timing-dependent: when GR-7 was started early, it protected against fibrosis from the start. When started late, after fibrosis had already developed, high-dose GR-7 improved liver fat and inflammation but did not produce a statistically significant reversal of scarring on tissue staining, only a downward trend alongside reduced activity of several fibrosis-related genes (including Col1a1, Col1a2, and Tgfbr1). The authors suggest longer treatment might be needed to see fibrosis actually reverse once it has taken hold. It is also a single study, in mice only, funded by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) with no industry funding or disclosed conflicts of interest reported.

Next Steps for a Gut-Restricted MASH Drug

The researchers built on earlier chemistry: GR-7 belongs to a class of non-absorbable, covalent inhibitors designed specifically to stay in the gut and act only on bacterial BSH, sparing the rest of the body. The logical next steps are longer dosing studies to see whether extended late-stage treatment can reverse established fibrosis, followed by the safety and dosing studies needed before any human trial could begin. No timeline for human testing has been announced.

Common Questions

Is GR-7 available as a treatment or supplement right now? No. GR-7 is an experimental compound used only in this mouse study; it has not been tested in humans and is not sold as a drug or supplement.

Does this mean probiotics or diet can fix fatty liver disease by changing gut bacteria? Not based on this study. GR-7 works by chemically blocking one specific bacterial enzyme, which is a different mechanism than taking a probiotic, and no comparable effect has been demonstrated from diet or supplements alone.

What is the difference between MASLD and MASH? MASLD (metabolic dysfunction-associated steatotic liver disease) is the umbrella term for fat buildup in the liver; MASH is the more advanced, inflamed and scarring stage within that spectrum that this study specifically targeted.

Why does a gut-acting drug affect the liver at all? Bile acids travel a loop between the liver and intestine; gut bacteria chemically modify them via bile salt hydrolase, and those modified bile acids signal back to the liver, so changing what bacteria do to bile acids in the gut can reprogram how the liver makes and processes bile acids in turn.

References

1. Wenchao Wei, Radka Graf, Yanhan Wang, et al. (2026). Targeting Microbial Bile Salt Hydrolase Reprograms Bile Acid Metabolism and Ameliorates Metabolic Dysfunction–Associated Steatohepatitis in Mice. Molecular Metabolism. https://doi.org/10.1016/j.molmet.2026.102430
2. Weixing Zhao, Huiying Wang, Minghua Zheng, Yan Ni (2025). Bile salt hydrolase: a key player in gut microbiota and its implications for metabolic dysfunction-associated steatotic liver disease. Microbiome Research Reports. https://doi.org/10.20517/mrr.2025.12
3. Definition & Facts of NAFLD & NASH – NIDDK. NIH. https://www.niddk.nih.gov/health-information/liver-disease/nafld-nash/definition-facts
4. Susan J. Keam (2024). Resmetirom: First Approval. Drugs. https://doi.org/10.1007/s40265-024-02045-0
5. Development of a Covalent Inhibitor of Gut Bacterial Bile Salt Hydrolases – MIT CMIT. microbiome.mit.edu. https://microbiome.mit.edu/microbiome_news/development-of-a-covalent-inhibitor-of-gut-bacterial-bile-salt-hydrolases/