Fecal Transplants Show Early Promise for Peanut Allergy Tolerance

Peanut allergy affects roughly 2% of children and a growing number of adults in the United States, and for most patients, strict avoidance remains the only reliable way to prevent a reaction. But a new study published in Science Translational Medicine is exploring a different approach entirely, one that starts not with the immune system directly, but with the gut microbiome.

A Trial Built on Earlier Mouse Findings

Rima Rachid, M.D., director of the Food Allergy Program and the Allergen Immunotherapy Program at Boston Children’s Hospital, and Talal Chatila, M.D., director of translational immunology at the hospital, led the new trial along with colleagues.

“People with food allergies want freedom from constant anxiety and extreme vigilance and to not have be on guard for potentially dangerous exposures every time they leave the house,” said Rachid. “Patients don’t want therapies that have to be taken indefinitely and instead are looking for a permanent disease modification or cure.”

The work builds on earlier research from Chatila and Rachid’s team, which found that transferring stool samples from infants with food allergies into allergy-prone mice caused the mice to go into anaphylaxis, while stool from healthy infants protected mice from allergic reactions. That study identified several “good” bacteria strains in healthy infants’ guts that appeared to guard against allergic reactions.

Testing “Poop Pills” in Young Adults

For the new trial, the researchers used frozen, encapsulated fecal matter, informally described as “poop pills,” to transfer these protective bacteria strains from healthy donors into 10 young adults with severe peanut allergies who reacted to trace amounts of peanut. Each participant took a one-time dose of 36 pills over a few hours. When tested four months later, three participants could tolerate eating multiple peanuts without reacting.

In a second phase of the trial, five additional participants took antibiotics before their fecal transplant, to see whether clearing out their existing gut bacteria would help the donor bacteria take hold more effectively. Three of those five participants improved, tolerating more than four peanuts before reacting, a result suggesting the antibiotics did help the transplant succeed.

Bile Salts May Be the Missing Link

To understand why the treatment worked for some participants and not others, the team analyzed blood samples and found that responders had higher levels of bile salts than nonresponders. Bile salts are made in the liver, stored in the gallbladder, and released into the small intestine to help digest fat. The investigators found that the beneficial bacteria introduced through the fecal transplants appeared to be more efficient at metabolizing bile salts than the bacteria already present in participants’ guts. In further experiments, the team found that this bile salt metabolism was linked to increased levels of immune regulatory cells that help protect against allergic reactions.

An Early but Notable Signal

Rachid described the findings as significant for the field. “This landmark study was the first to demonstrate that a microbiome-based therapy may improve food allergy in people while also revealing how gut bacteria, their metabolites and the immune system work together to influence treatment response.”

Rachid noted that larger studies of fecal and microbiota transplantation are now needed to confirm the results, identify which patients are most likely to benefit, and potentially develop targeted probiotic therapies based on the beneficial bacteria identified.

This was a small, early-phase trial designed primarily to evaluate safety, so the improvements in peanut tolerance, while encouraging, will need to be replicated in larger, controlled studies. Still, the findings offer an early look at how the gut microbiome might one day be harnessed to treat food allergy, rather than simply avoided around.

“Food allergy reflects a failure of oral tolerance, the process by which the gut immune system learns to accept food, and what this study shows is that the right bacteria can help restore that process, working through bile acid metabolites to promote the immune cells that enforce tolerance,” said Chatila. “Knowing how the bacteria restore tolerance to food in allergic individuals allows us to optimize the therapy for more effective outcomes.”