Fiber does more than keep you regular. New research from Princeton’s Ludwig Institute for Cancer Research suggests that when your gut bacteria run short of fiber, they may turn to a different food source: the protective mucus lining of your intestines. The finding puts gut health and fiber in a new light, though most of the evidence so far comes from animals and lab work.
Gut Health and Fiber: What the Research Says
According to a summary released by the Ludwig Institute, two papers from the lab of Joshua Rabinowitz, director of the institute’s Princeton Branch, shed light on how gut bacteria and the body produce certain metabolites called phenols and indoles.
- PNAS (2026; 123(32)): “Digestion-resistant proteins support the healthy metabolite profiles associated with plant-based diets.”
- Nature Metabolism (2026; 8(7):1528): “Host metabolism can produce many indoles and phenols independently of the microbiome.”
Lead author on both was Jenna AbuSalim. The studies used isotope tracing in mice, rats and human cells, and one included samples from patients receiving antibiotics.
Why Fiber Protects the Gut Lining
The PNAS work found that fiber and certain indigestible plant proteins shift the balance of phenol compounds in the body. In the summary’s description, potentially harmful phenols come from bacteria breaking down the body’s own proteins, including proteins in gut mucus. Beneficial phenols came almost entirely from indigestible proteins in food.
In plain terms, fiber appears to suppress bacterial breakdown of the mucus lining, and indigestible plant proteins give bacteria an alternative food supply. The researchers call these “proteins imitating fiber,” or Prif.
What Are Prifs?
Prif is a new term for dietary proteins that resist digestion and travel to the large intestine, where bacteria can use them. Lead author AbuSalim said the team thinks Prifs represent “an emerging class of dietary nutrients that shape the composition of the gut microbiome.” Rabinowitz added that “Diet holds great promise for controlling the microbiome and its outputs,” and suggested that “Food packaging may eventually list Prif right below fiber.”
That last remark is a forecast, not a current labeling rule. Today, no standard Prif label exists, and the clinical meaning of these proteins in people has not been established.
The Second Surprise: Your Body Makes Some of These Compounds
The Nature Metabolism paper found that mammalian cells can produce many phenol and indole metabolites without help from gut bacteria. Many stayed at substantial blood levels in antibiotic-treated mice and in patients on antibiotics. Two compounds, indole-3-propionate and p-cresol sulfate, did decline after antibiotics, which the summary says are produced only by microbes.
This matters because scientists often assume a metabolite in blood reflects the microbiome. These results show that assumption can be wrong for many compounds, which could change how researchers design microbiome-based treatments.
Important Caveats
The summary itself cautions that much of the evidence comes from animals and laboratory studies, and that more research is needed to learn how these mechanisms influence long-term health in people. No sample sizes were given in the public summary. These findings should not be read as proof that low fiber damages anyone’s gut lining.
How to Eat More Fiber Starting Today
Whatever the final science shows, fiber is already linked to better digestion and heart health. Simple ways to add it include:
- Swap refined grains for whole grains. Oats, brown rice and whole-wheat bread are easy starts.
- Add beans or lentils. One extra serving a few times a week makes a difference.
- Eat the skin. Apples, pears and potatoes keep their fiber when you leave the skin on.
- Increase slowly. A rapid jump can cause gas and bloating, so add fiber gradually and drink water.
- Check with your doctor. If you have a digestive condition, ask what level of fiber suits you.
The Bottom Line
New lab research suggests fiber may help keep gut bacteria from feeding on the intestinal lining, and that diet could one day be used to steer the microbiome. The work is early. For more on the gut, read our report on how a 24-hour fast affected gut repair in mice and our look at new brain-health research. The studies are available at PNAS and Nature Metabolism.
This article is for general information and is not medical advice.
