According to recent research led by Ludwig Princeton scientists Jenna AbuSalim and Joshua Rabinowitz, plant-based diets support human health by encouraging diverse gut bacteria, yet the precise mechanisms of how gut microbes process plant components are still being uncovered. Two distinct studies published in the Proceedings of the National Academy of Sciences and Nature Metabolism reveal that plant fiber and overlooked plant proteins actively reshape microbial metabolism while challenging long-held assumptions about where circulating metabolites originate.
How Plant Fiber and Prifs Shift Gut Metabolites
Plant-based diets support gut health, immunity, and cardiovascular function partly by feeding intestinal bacteria that break down dietary fiber. According to research published in the Proceedings of the National Academy of Sciences, investigators examined how plant-based foods influence phenol metabolites, which are compounds created when gut microbes digest the amino acids tyrosine and phenylalanine. These resulting metabolites carry drastically different health impacts, according to the findings.
Bacteria process phenylalanine to produce phenylpropionate and hippuric acid, compounds associated with healthy body weight and strong intestinal health. Conversely, tyrosine processing yields p-cresol sulfate and phenol sulfate, which are linked to systemic toxicity in kidney disease patients and poorer outcomes in cancer research. The study showed that both plant fiber and indigestible plant proteins—dubbed “proteins imitating fiber,” or Prif—shift the balance from harmful tyrosine derivatives to healthful phenylalanine metabolites.
“Our studies showed that both the fiber and indigestible proteins from plants — which we call ‘proteins imitating fiber,’ or Prif — shift the balance of phenol metabolites from the harmful kind made from tyrosine to the healthful variety derived from phenylalanine,” said Jenna AbuSalim, according to the PNAS study.
Did you know? Indigestible plant proteins typically escape early human digestion and reach the colon, where gut microbes utilize them much like dietary fiber to alter host metabolism.
Tracing the Source of Good and Bad Phenols in the Gut
To pinpoint the exact origins of these metabolic compounds, researchers utilized stable non-radioactive isotopes to label proteins and track their digestion inside the mouse gut. According to the study data, harmful phenols were produced primarily when bacteria consumed host proteins, including those found in the protective mucus lining of the intestines. When dietary fiber intake ran low, intestinal bacteria turned to feeding on the gut lining itself, as noted in additional coverage by ScitechDaily.
Fiber actively reduced this bacterial breakdown of the gut mucus lining, which successfully lowered harmful phenol production. Meanwhile, Prif increased the volume of dietary protein reaching the microbes, fueling the creation of beneficial phenols. “We think Prifs represent an emerging class of dietary nutrients that shape the composition of the gut microbiome and could have a far-reaching influence on metabolic health,” said AbuSalim. “Food packaging may eventually list Prif right below fiber,” added Joshua Rabinowitz.
Challenging Assumptions About Mammalian Versus Microbial Metabolism
While the PNAS paper focused on phenol pathways, a second study published in June in Nature Metabolism investigated both phenol and indole metabolites, which derive from the amino acid tryptophan. Indoles connect to conditions like inflammatory bowel disease, neurodegenerative disorders, and cancer progression. Scientists historically assumed that phenols and indoles were manufactured exclusively by gut bacteria.
Using isotope tracing across mice, rats, and human cells, AbuSalim, Rabinowitz, and their team tested that core assumption and discovered that mammalian metabolism independently produces many essential indole and phenol metabolites, including indole-3-lactate and indole-3-acetate. In mice and human patient samples—including data from cancer patients undergoing antibiotic treatment—circulating levels of these specific metabolites remained high even after antibiotics disrupted the microbiome. Conversely, metabolites generated strictly by microbes, such as indole-3-propionate and p-cresol sulfate, declined sharply post-antibiotic treatment.
“There’s growing interest across medical disciplines in manipulating the human microbiome or using its metabolic products themselves for therapy,” said Rabinowitz. “Diet holds great promise for controlling the microbiome and its outputs. But to devise effective therapeutic interventions, we need to understand what aspects of the diet control which microbial outputs.”
Future Implications for Dietary and Microbiome Therapies
These dual findings provide a precise framework for understanding how diet interfaces with human biology and microbial outputs. By identifying which specific plant nutrients govern individual metabolic products, researchers can better design targeted dietary interventions, probiotics, and medical therapies aimed at raising or lowering specific metabolites.
“A clearer picture of how different foods interact with the microbiome to modulate the production of bacterial metabolites will help sharpen the guidance nutritionists and doctors can give to people for disease prevention and therapy,” Rabinowitz noted. The research efforts received funding support from the Ludwig Institute for Cancer Research, the National Institutes of Health, the National Institute of Diabetes and Digestive and Kidney Diseases, the Princeton Alliance for Collaborative Research and Innovation, and Princeton University.
Frequently Asked Questions
What are Prifs in plant-based diets?
Prifs, or “proteins imitating fiber,” are indigestible plant proteins that escape early digestion, reach the colon, and act similarly to dietary fiber by shaping gut microbiome composition and encouraging beneficial metabolic outputs, according to researcher Jenna AbuSalim.
Do gut bacteria produce all indole and phenol metabolites?
No. While scientists long assumed microbes generated all of these compounds, isotope tracing research published in Nature Metabolism demonstrates that mammalian metabolism independently produces substantial amounts of several vital indole and phenol metabolites.
How does dietary fiber protect the gut lining?
According to isotope-tracing studies in mice, fiber prevents intestinal bacteria from turning on the host by reducing the bacterial breakdown of the protective mucus lining of the gut.
What are your thoughts on these findings? Do you expect food labels to begin tracking indigestible plant proteins alongside fiber in the near future? Share your perspective in the comments below, or subscribe to our newsletter for ongoing updates on microbiome research and nutritional science.
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