Fecal Transplant Trial Let Some Peanut Allergy Patients Eat 2.5 Peanuts

If you or your kid carries an EpiPen because of a peanut allergy, you already know the fear behind an unlabeled snack. A new study offers an early, unusual proof of concept: capsules filled with gut bacteria from healthy, non-allergic donors were given to 15 adults with severe peanut allergy, and in the people who responded, the amount of peanut protein they could safely handle jumped many times over. It is the first published human trial of fecal microbiota transplant (FMT, the transfer of gut bacteria from a healthy donor into another person, usually as freeze-dried capsules) aimed specifically at treating a food allergy.

Key takeaways:

• Researchers gave 15 adults with severe peanut allergy capsules of donor gut bacteria; 6 of them (40%) went on to tolerate far more peanut protein.
• Before treatment, participants reacted to under 100 milligrams of peanut protein; by 4 months, 5 of the 6 responders tolerated 600 milligrams or more, roughly 2.5 peanuts.
• The other 9 participants saw no meaningful change, and responders skewed heavily male (5 of 6), for reasons the researchers can't yet explain.
• This is a small, open-label trial with no placebo comparison, so it's an early signal, not a ready treatment.

What the Peanut Allergy Fecal Transplant Trial Found

Researchers led by Cathryn Nagler at the University of Chicago, working with Edwin Kim at UNC Chapel Hill and Talal Chatila at Harvard Medical School, gave 15 adults with severe, confirmed peanut allergy capsules containing gut bacteria collected from healthy donors without food allergies. Most participants swallowed 36 odorless, tasteless capsules in a single sitting; five of the 15 also received a four-day course of antibiotics before the transplant, meant to clear space for the new bacteria to establish. Six of the 15 participants, 40%, showed a significant jump in how much peanut protein they could tolerate without reacting, measured through supervised oral food challenges (a monitored test where a patient eats gradually larger amounts of an allergen under medical supervision). "Up until now, it's been in mouse models. Now we have people. And that's exciting," Nagler told Nature News.

Why This Matters for People Living With a Severe Peanut Allergy

Peanut allergy has no approved cure. The only current options are strict avoidance and, for some patients, oral immunotherapy (a supervised regimen of eating slowly escalating peanut doses to build tolerance), which requires daily dosing indefinitely and doesn't work for everyone. A treatment that works by resetting the gut's bacterial community, rather than by repeated exposure to the allergen itself, would be a genuinely different approach. Allergist Cecilia Berin of the Icahn School of Medicine at Mount Sinai, who wasn't involved in the study, told Science News that the fact the pills helped a subset of participants without any exposure to a specific allergen suggests the approach could eventually help patients who live with multiple food allergies at once, not just peanut.

Inside the 15-Person Open-Label Trial Design

This was a small, open-label trial, meaning everyone knew they were getting the real treatment since there was no placebo group for comparison, which is standard for an early-phase safety and feasibility study. All 15 participants had a documented severe peanut allergy and, at baseline, reacted to under 100 milligrams of peanut protein, less than half a peanut kernel. After receiving the donor-bacteria capsules, participants were followed for 4 months and periodically retested. A companion analysis pointed to a possible biological explanation: the six responders showed expansion of RORgt+ regulatory T cells (Treg, a subset of immune cells shaped by gut bacteria that help the body tolerate rather than attack a substance), a mechanism also flagged in outside coverage of the findings, and linked it to bile acids (chemical byproducts made when gut bacteria break down substances secreted by the liver) that donor bacteria produce. In a related mouse experiment, bacteria taken from human responders protected mice from developing an egg allergy, hinting the mechanism may not be peanut-specific.

What This Means If You or Your Child Has a Peanut Allergy

For now, this changes nothing about how peanut allergy should be managed. Nobody should attempt a do-it-yourself fecal transplant, and there is no approved or commercially available version of this treatment. What it does offer is a concrete, human proof of concept: that shifting gut bacteria can change how the immune system responds to a food allergen, not just in mice but in people. If you or a family member manages a peanut allergy, this is a study worth mentioning to an allergist as a marker of where the field is heading, particularly if you've discussed oral immunotherapy and want to know what else is in the pipeline. It is not evidence that any current supplement, probiotic, or over-the-counter product can replicate this effect.

The Open Questions This Trial Left Unanswered

The biggest caveat is that more than half the group, 9 of 15 participants, saw no meaningful change, and the trial can't yet explain why. The responder group also skewed heavily male, 5 of the 6 people who improved, which the researchers can't yet explain either. Because the trial was open-label with no placebo group, it can't fully rule out that expectation or the food-challenge process itself shaped some results, though the size of the tolerance jumps in responders, from under 100 milligrams up to 600 milligrams or more, makes a pure placebo effect less likely. The researchers floated giving all the bacteria in a single day, rather than spreading out dosing, and inconsistent antibiotic pretreatment, as possible explanations for non-response, but none of that is confirmed.

What Comes Next for This Peanut Allergy Fecal Transplant Research

The team's next step is to try to improve on this first attempt, using higher concentrations of the beneficial bacteria and eventually working toward isolating specific bacterial strains rather than delivering a full fecal transplant, according to Science News. Allergist Stephen Tilles at the University of Washington, who wasn't involved in the study, told STAT News that "we, as the food allergy community, have been waiting for these results," calling them "very exciting" while cautioning the approach isn't "ready for prime time" yet.

Common Questions

Is this fecal transplant treatment for peanut allergy available now? No. This is an early-phase research trial, not an approved treatment, and there is no commercial or clinical version available outside of research settings.

How much more peanut could people tolerate after the treatment? Before treatment, participants reacted to under 100 milligrams of peanut protein; the 6 responders reached between 300 and 600-plus milligrams by 4 months, with 5 of them tolerating roughly 2.5 peanuts worth of protein.

Why did some people not respond to the treatment? The researchers don't fully know. They've floated the single-day dosing schedule, inconsistent antibiotic pretreatment, and an unexplained skew toward male responders as possible factors, but none of these has been confirmed.

Could this approach work for other food allergies, not just peanuts? It's possible but unproven in humans. A companion mouse experiment found that bacteria from human responders also protected mice from developing an egg allergy, hinting the mechanism isn't peanut-specific, though that hasn't been tested in people with other allergies.

References

1. Edward Chen (2026). First poo transplant to treat food allergy in people has ‘exciting’ results. Nature. https://doi.org/10.1038/d41586-026-02440-5
2. Fecal transplant pills helped peanut allergy sufferers in a small trial. Science News. https://www.sciencenews.org/article/fecal-transplant-pills-peanut-allergy-clinical-trial-poop-bacteria
3. Could poop pills be the next treatment for food allergies?. STAT. https://www.statnews.com/2026/08/05/fecal-microbiome-transplant-for-peanut-food-allergy/