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Data from twins suggests that gut bacteria are important in food allergies

A Stanford-led study of twins with and without food allergies has uncovered differences in the fecal bacteria of allergic and non-allergic individuals.

A comparison of twins with and without food allergies has uncovered distinct differences in the fecal bacteria of allergic and non-allergic individuals.

The study, a collaboration between Stanford Medicine and the University of Chicago, included 13 twin pairs in which one person has food allergies and the other does not, as well as five pairs in which both people have at least one food allergy.

The participants without food allergies had different, more diverse microbes in their feces than the participants who are allergic. This suggests that gut bacteria may play a role in food allergies.

"Is this a cause-and-effect relationship? We do not know. We want to understand what bacteria are associated with food allergies and what bacteria are linked with protection against food allergies," said Stanford immunologist Kari Nadeau, MD, PhD, the study's co-senior author and director of the Sean N. Parker Center for Allergy and Asthma Research at Stanford University. The research appeared recently in the Journal of Clinical Investigation.

Clues from twins

Studies of twins in which only one has a disease have resolved other medical mysteries, Nadeau noted, including identifying genetic risk factors for colon cancer. Being able to compare two people who have similar or identical genes -- and who live in extremely similar environments until adulthood -- adds controls that are not usually possible in long-term studies of people.

About 10 years ago, Nadeau began collecting information on twin pairs in which one person had a food allergy and the other did not. Her team later expanded to studying pairs of twins in which both people have food allergies.

The researchers verified that all of the pairs grew up in the same household as their twin, eating the same foods. Four of the pairs in the study were fraternal twins and the rest were identical. When their data was collected, they ranged in age from 6 months to 70 years old.

A colleague of Nadeau's, Cathryn Nagler, PhD, at the University of Chicago, tested the twins' fecal samples to characterize what bacteria were present. Nagler also measured fecal levels of bacterial metabolites, the molecules the bacteria made during their life processes.

Different fecal bacteria profiles

The fecal bacteria profiles in the allergic twins were similar to those seen in prior studies Nagler conducted of food allergies in mice. The scientists also found that certain classes of bacteria and bacterial species were more prevalent in the non-allergic twins than in the twins with allergies.

The study found that bacterial differences identified in the twins persisted throughout life, even though the adult participants lived apart from their siblings after reaching adulthood.

The data also showed key differences in the microbial metabolites present in feces, with some metabolites more abundant in non-allergic twins and others more prevalent in allergic twins. This is the first ever study to compare microbial metabolites in people with and without food allergies. The differences discovered raise many new questions about what the metabolites do.

One possibility is that certain metabolites may be needed to keep the intestine healthy, Nadeau said. Other research has shown that certain bacterial metabolites form a protective microfilm inside the intestine, a physical barrier that helps maintain the health of the intestinal wall. This barrier may prevent the body from incorrectly seeing food proteins as dangerous and triggering an allergic response.

"The microfilm protects against injury in the gut and establishes the right pH so you don't have ulcerative breakdown of the intestinal lining," said Nadeau.

The research suggests that probiotic supplements, which provide healthy bacteria, might help prevent or even treat food allergies, said Nadeau. More research is needed to understand the specifics, such as which probiotics to give, and what specific roles are played by different metabolites made by the bacteria.

"This is a small study. We need bigger studies on how food allergies and the microbiome are linked," Nadeau said. "But I think the data really does speak to how important the gut is early in life."

Photo by Juliane Libermann

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