When your gut bacteria run short on dietary fiber, they don’t just go hungry — they may start feeding on you, chewing up the protective lining of your intestines, new research shows.
Scientists at the Ludwig Institute for Cancer Research’s Princeton branch used isotope labeling to trace what gut microbes eat. When fiber was limited, the bacteria broke down proteins from the gut’s mucus layer — the slick barrier that protects the intestinal wall. That produces harmful phenol metabolites (such as p-cresol sulfate and phenol sulfate), which have been linked to worse outcomes in cancer patients and to systemic toxicity in people with kidney disease. The findings were published in PNAS and Nature Metabolism in 2026.
Fiber tips the balance
Reassuringly, the study also found a fix baked into diet: both fiber and indigestible plant proteins shift the microbes back toward making healthful byproducts instead of harmful ones. “Both the fiber and indigestible proteins from plants shift the balance of phenol metabolites from the harmful kind…to the healthful variety,” the researchers reported.
Why it matters
The work — done in mice, rats and human cell samples — adds mechanistic weight to a familiar message: feeding your gut microbes fiber isn’t just about digestion, it may protect the gut barrier itself and reduce the production of toxins tied to disease. It’s preclinical research, but it points to why fiber-poor diets could carry hidden costs.