Food allergies can be linked to the type of food being fed to gut bacteria — at least in mice, says a study published June 21 in Cell Reports. Rodents that ate a diet with average calories, sugar, and fiber content from birth were shown to have more severe peanut allergies than those that received a high-fiber diet. Australian researchers found that gut bacteria release a specific fatty acid in response to fiber intake, which impacts allergic responses by causing changes to the immune system.

"We felt that the increased incidence of food allergies in the past 10 years had to relate back to our diet and our own microbiome rather than a lack of exposure to environmental microbes — the so-called 'Hygiene Hypothesis,'" says study co-author Laurence Macia, an immunologist at Monash University. "Most researchers in this field look at excess fat as the problem — we were one of the first looking specifically at fiber deficiency in the gut."

Bacteria in the digestive system break down dietary fiber into byproducts, mainly short-chain fatty acids. The new study showed that these fatty acids support the immune system by binding onto specific receptors on T regulatory cells, immune cells known to suppress the immune response. This uniting stimulates a cascade of events that regulate inflammation in the gut, which doesn't function properly during an allergic reaction to food.

For the study, mice that were bred to have an artificially-induced peanut allergy were fed a high-fiber diet to produce a healthy population of gut bacteria. The bacteria were then given to a group of "germ-free" mice that had no gut microbes of their own.

Despite not having consumed any fiber themselves, this second group of mice was protected against allergy, showing a less severe response when exposed to peanuts. In short, their microbiota was "reshaped" by having this transplant, says co-author Charles Mackay, adding that these mice clearly evolved mechanisms for responding to fiber and its byproducts. "It's almost an essential component of their nutritional health," he says.

"My theory is that the beneficial bacteria that predominate under consumption of fiber promotes the development of regulatory T cells, which ensures the bacteria have a healthy, anti-inflammatory system to thrive in," says Macia. "So it's a win-win for everybody."

The researchers are optimistic that their research applies to humans. "It's likely that compared to our ancestors, we're eating unbelievable amounts of fat and sugar, and just not enough fiber," says Mackay, "and these findings may be telling us that we need that high-fiber intake, not just to prevent food allergy, but possibly other inflammatory conditions as well."