From overly clean dishes to skyrocketing rates of cesarean-section births, scientists have proposed dozens of explanations for the sharp rise in food allergies in recent years.
Now, several new studies suggest another factor that could play a role in food allergies: dietary fiber.
This notion is based on the idea that bacteria in the gut have the enzymes needed to digest dietary fiber, and when these bacteria break down fiber, they produce substances that help to prevent an allergic response to foods, said Charles Mackay, an immunologist at Monash University in Melbourne, Australia.
So far, the research related to this idea has been done mainly in mice, and dietary factors are unlikely to be the sole explanation for why allergy rates have skyrocketed, researchers say. But if the results were to be replicated in human studies, they would suggest that promoting the growth of good gut bacteria could be one way to protect against, and possibly even reverse, certain allergies, researchers say. [5 Surprising Things About Your Microbiome]
Up to 15 million Americans have food allergies — a number that increased by 50 percent between 1997 and 2011, according to Food Allergy Research & Education, a nonprofit organization that advocates for people with food allergies. About 90 percent of people with food allergies are allergic to one of eight types of foods: peanuts, tree nuts, wheat, soy, eggs, milk, shellfish and fish.
It's not clear why these foods are so allergenic, but it may be that large parts of these foods remain undigested when they reach the gut. From there, the undigested compounds may go from the gut into the bloodstream, where they can be recognized by antibodies, or immune cells designed to recognize foreign invaders, said Cathryn Nagler, a food allergy researcher at the University of Chicago.
"Somehow, they're getting into the bloodstream intact, and we're wondering if that may be a unifying feature of food allergens," Nagler said.
Mackay and Nagler believe the modern Western diet may play a role in this process.
"Bacteria are required to digest many indigestible fibers that were, at one time, a large part of the diet," Nagler told Live Science.
But the modern diet — laden in sugar, fat and refined carbs — seems to promote the growth of different types of bacteria in the gut than the human ancestral diet did, she added.
That may not be a good thing.
It turns out that fiber promotes the growth of a class of bacteria called Clostridia, which break down fiber and are some of the biggest producers of byproducts called short-chain fatty acids. (These types of Clostridia differ from the kind that causes deadly C. difficile infections.)
In a 2011 study in the journal Nature, researchers found that these short-chain fatty acids normally prevent gut cells from becoming too permeable, and letting food particles, bacteria or other problematic compounds move into the blood.
"A leaky gut is bad because all these undesirable things go from the gut into the bloodstream, and they screw up the immune system," Mackay told Live Science.
In an August 2014 study in the journal Proceedings of the National Academy of Sciences, Nagler and her colleagues found that implanting Clostridiainto the guts of mice sensitized to the peanut allergen could prevent peanut allergies.
Other environmental factors may also act in conjunction with a low-fiber diet to promote allergies. Antibiotics, which are widely used in agriculture and for treating ear infections in babies and toddlers, kill the bacteria in the gut. So the combination of antibiotics and low-fiber diets may be a "double whammy," that predisposes people to allergic responses, Nagler said. [8 Strange Signs You're Having an Allergic Reaction]
The new findings also suggest a way to prevent, or possibly even reverse some allergies. For instance, allergy treatments could use probiotics that recolonize the gut with healthy forms of Clostridia, Nagler said. In fact, in a small study published in January in the Journal of Allergy and Clinical Immunology, children with peanut allergies who received probiotics were able to eat the nut without having an allergic reaction, and their tolerance to peanuts persisted even after the treatment.
Many factors may contribute to the rise in food allergies, said Dr. Robert Wood, director of pediatric allergy and immunology at the Johns Hopkins Children's Center in Baltimore. Epidemiological studies have found that having pets, going to day care, having a sibling, being born vaginally and even washing dishes by hand can affect the risk of allergies.
The fiber-allergy link isn't the whole story but "is on the list of 15 or 20 theories that do make some sense and do have a little evidence to support it," Wood told Live Science.
Still, it's important not to make recommendations from this preliminary data, experts say. For years, doctors told parents of children at a high risk of developing allergies to wait until the children were 3 years old before giving them peanuts or other allergy-inducing foods, Wood said.
"We really thought we knew what we were doing, and it turns out it was 100 percent wrong," Wood said.
In fact, a recent study found that serving kids peanuts frequently from a young age sharply reduced their odds of developing a peanut allergy.