Pulses, where this newly named plant protein fraction is found.
Harborsparrow
Illustrative. Not the specific foods analysed in this work.
Fiber Isn't the Only Plant Nutrient Feeding Your Gut Bacteria, and the Other One Just Got a Name
Every gut health conversation I have had in the last decade eventually collapses into one word: fiber. Eat more fiber. Get thirty plants a week. Feed the bacteria. I believe it, I do it, and I have written about it here more than once. A new paper out of Ludwig Cancer Research and Princeton suggests we have been giving fiber credit for a job that two nutrients were doing together.
- Researchers at Ludwig Cancer Research and Princeton found that the indigestible protein in plant foods does its own separate job in the gut. They named the class Prif, short for Proteins Imitating Fiber.
- By tracing labeled proteins through the mouse gut, they showed your bacteria make two families of phenol metabolites: harmful ones built mostly from your own gut lining mucus, and beneficial ones built almost entirely from indigestible dietary protein.
- Fiber and Prif pull different levers. Fiber stops bacteria from eating your mucus lining. Prif gives them dietary protein to work with instead.
- It is mouse data from one lab, published in PNAS and announced July 28, 2026. No food amounts, no human numbers, no product exists.
Phenol sulfate, cresol sulfate
The unwelcome ones. Made mostly from bacteria consuming the mucus layer that lines your own gut.
Phenylpropionate, hippuric acid
The beneficial ones. Almost exclusively from indigestible dietary protein, the part your small intestine cannot break down.
Traced with isotope labelled proteins in mice, so the source of each molecule is measured rather than inferred. Human relevance is inferred, not tested.
How they figured it out
The team used isotope labeled proteins, essentially tagged versions of the real thing, so they could follow individual molecules through the mouse gut and see exactly what the bacteria were building things out of. That is the clever part. Most microbiome work tells you which bacteria are present and which metabolites appear. This tells you where the raw material came from.
Two families of metabolites, two very different sources
Gut bacteria produce a group of compounds called phenol metabolites, and they sort into good and bad. The unwelcome ones, phenol sulfate and cresol sulfate, turned out to be made mostly from bacteria consuming the mucus layer that lines your own gut. Your bacteria were eating you, and then producing something you would rather not have circulating.
The beneficial ones, phenylpropionate and hippuric acid, came almost exclusively from indigestible dietary protein. Not from fiber. Not from your mucus. From the part of plant protein your small intestine cannot break down, which travels onward and reaches the bacteria intact.
So the two nutrients work through separate mechanisms that happen to point the same direction. Fiber suppresses the mucus eating, which lowers the harmful phenols. Prif increases how much dietary protein actually reaches the bacteria, which raises the beneficial ones. Neither one substitutes for the other. That is the whole finding, and it is a tidier explanation than "plants are good for you."
Why one does not substitute for the other.
Direction only. Nobody has quantified how much Prif is in any food, so this figure carries no amounts and no target.
Honest caveat
This is mouse data. Human relevance is inferred, not tested. It comes from a single research group, and while a companion paper from the same lab supports the broader picture of phenol metabolism, that is not independent replication of this specific claim. Most importantly for anyone tempted to act on it: nobody has quantified how much Prif is in any particular food, or how much you would need. There is no intake target here, no food ranking, and no supplement. If you see one marketed in the next few months, it is running ahead of the evidence.
What this means for you
The practical takeaway is smaller than the headline and still worth having. If beneficial phenol metabolites come from the protein matrix of whole plant foods, then an isolated fiber powder is only pulling one of the two levers by design. It was never carrying protein. Beans, lentils, whole grains and vegetables arrive with both, which is a mechanistic reason to favor the food over the scoop, on top of all the reasons we already had.
an isolated fiber powder is only pulling one of the two levers by design
I am not throwing out my fiber supplement over a mouse study, and you should not either. But I have quietly moved "actually cook the lentils" a little higher up my list. And selfishly, I like knowing the word Prif before the wellness industry gets hold of it, because it will, and it will oversell it.
Primary sources
- AbuSalim, J.E. et al. (2026), Digestion resistant proteins support the healthy metabolite profiles associated with plant based diets, Proceedings of the National Academy of Sciences.
- A new light on the microbiome and its metabolites, Ludwig Cancer Research, July 28, 2026.
Common questions
What exactly is Prif?
Prif stands for Proteins Imitating Fiber, and the researchers coined the term in this paper, so you have not missed anything by never hearing it before. It refers to the portion of plant protein your small intestine cannot fully digest, which therefore travels onward and reaches the bacteria in your large intestine. It behaves like fiber in the sense that it arrives intact and becomes bacterial food, but chemically it is protein, not carbohydrate, and it produces different metabolites.
Should I stop taking my fiber supplement?
No. Fiber did real work in this study, just different work. It suppressed the bacterial breakdown of your gut mucus lining, which is what cuts production of the harmful phenol metabolites. That is a genuine benefit and an isolated fiber supplement can deliver it. The finding is that fiber alone appears to pull one of two levers, and the second lever comes from the protein in whole plant foods. It is an argument for adding whole plants, not for subtracting fiber.
Which foods give me the most Prif?
Nobody knows yet, and I would be inventing numbers if I gave you a list with amounts. The researchers have not quantified how much Prif specific foods contain or how much you would need to eat to shift your metabolite profile. What the study establishes is that the source is the indigestible protein in plant foods generally, which points at legumes, whole grains and vegetables rather than at any single hero ingredient. Anyone selling you a Prif supplement right now is ahead of the science by at least a year.
Stay Curious.
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