Food allergy has a biology problem: the treatments that exist stop working when you stop taking them. Oral immunotherapy raises tolerance, but the protection wanes without continued exposure, and patients live with the constant arithmetic of avoiding a single peanut. A team at Boston Children’s Hospital tested a fundamentally different idea: changing the immune system’s relationship with food by changing the bacteria in the gut. In the first trial of its kind in people, fecal microbiome transplants quieted peanut allergies in six of fifteen adults, and the researchers identified the mechanism.
The trial, published in Science Translational Medicine, enrolled fifteen young adults with severe peanut allergies who reacted to less than 100 milligrams of peanut protein (under half a peanut). Ten received a one-time dose of 36 frozen capsules of stool from healthy donors without food allergies. Five were pretreated with antibiotics for four days to clear space for the donor bacteria. Four months later, six participants, three in each group, could tolerate meaningfully more peanut: some reached 600 milligrams or more, roughly two and a half peanuts. No severe side effects were reported.
Why antibiotics mattered
The response pattern hinted at the biology. Three of five antibiotic-pretreated participants responded, versus three of ten who received the transplant alone, and those pretreated could eat more than four peanuts before reacting. Clearing the resident gut microbes appeared to give the donor bacteria a better chance of establishing themselves. The non-responders, the authors hypothesize, may not have let the new bacteria take root.
The team then took the experiment into mice, transplanting gut bacteria from the participants’ stool into allergy-prone animals. Bacteria from responders protected the mice from developing an allergy to egg protein; bacteria from highly sensitive participants caused egg allergy. Because the mouse experiment used a different allergen, the effect is likely not peanut-specific: the therapy may work across food allergies.
The mechanism: bile acids
The study’s deeper contribution is mechanistic. In both humans and mice, protection correlated with increased bile acid metabolites, the products of bacterial processing of bile acids in the gut. The key bacteria were Bacteroides species, and the causal test was decisive: deleting a bile salt hydrolase gene in a protective Bacteroides strain eliminated its ability to suppress food allergy in mice.
Bile acids have emerged over the past decade as a signaling axis between gut bacteria and the immune system. Bacterial bile acid metabolites help shape regulatory T cells, the cells that put the brakes on allergic inflammation. In responders, the transplant boosted ROR-gamma-t-positive regulatory T cells and reduced the T-helper-2 cells that drive allergic responses. The picture is coherent: donor bacteria, particularly Bacteroides, process bile acids into metabolites that expand regulatory T cells, restoring oral tolerance.
What it means
The trial is a phase 1 proof of concept: fifteen patients, open-label, no placebo control. The response rate is real but modest, the durability beyond four months unknown, and the practical formulation, frozen stool pills, is not how this would ultimately be delivered. The researchers are already testing a purified, concentrated microbial formulation that can be stored in a home refrigerator, and a study combining the approach with peanut oral immunotherapy.
The significance is the demonstration that a microbiome-based therapy can improve food allergy in people, and the identification of a plausible molecular pathway, bile acid metabolites and regulatory T cells, that can be measured and optimized. If the mechanism holds, the path forward is not necessarily transplants at all but targeted probiotics: defined bacterial strains engineered or selected for their bile acid processing, delivered as a pill. For the roughly 33 million Americans with severe food allergies, the promise is a treatment that changes the immune system rather than one that must be taken forever.
Sources
- Rachid, R. et al. “Fecal microbiome transplant in food allergy in humans and mice identifies a role for bile acid metabolites in oral tolerance.” Science Translational Medicine 18: eaee3263 (2026). DOI: 10.1126/scitranslmed.aee3263
- Nature news, “First poo transplant to treat food allergy in people has ‘exciting’ results,” August 5, 2026
- Medical Xpress / Boston Children’s Hospital, August 2026