Why Do Some People Suddenly Develop Food Allergies Later in Life?

It can be confusing and daunting to suddenly not be able to eat a food you've eaten your whole life.
food in jars

In 2013, my digestive life went to crap, nearly literally. I started to have strange episodes where my stomach would feel like it was crushing in on itself, leaving me bent over in the bathroom for hours. Then, I would randomly break out in hives on my arms, or around my mouth. Other times my throat felt extremely tight, never enough to cause breathing problems, but enough to freak me out. I sought out a doctor to tell me what was going on.


She put me on an elimination diet, when you stop eating the top eight food allergens, and made an appointment for me to get tested for a food allergy. I had never been allergic to anything before (though I am lactose intolerant), so I was skeptical that a food was the culprit.

But a few weeks later, she gave me the news: Congratulations, I was allergic to soy. (I’ve since confirmed this again and again through accidental and painful exposures to it.)

I ate soy throughout my whole childhood. My mother is Chinese and did the bulk of the cooking: soy sauce, tofu, edamame, tempeh—we regularly indulged in the soy smorgasbord. How could I suddenly have become allergic?

My doctor’s answer: “I don’t know.”

Food allergies are a mysterious affliction, and perhaps the one thing we know for sure about them is that they are increasing—as much as 20 percent in the past decade. A study that came out in the first week of 2019 in T he Journal of the American Medical Association (JAMA) reported that in a survey of 40,443 US adults, an estimated 10.8 percent of them had a food allergy, and nearly half of those people had at least one food allergy that had cropped up in adulthood. A quarter of them, like me, had never had a food allergy as a child.

“That was pretty surprising to me,” says first author Ruchi Gupta, a professor of pediatrics and medicine at Lurie Children's Hospital in Northwestern Medicine. “You can extrapolate that to say that at least half of adults carry their food allergy into adulthood. And then there's this additional [group] that are developing newer food allergies as adults.”


Gupta confirms it’s a confusing situation for patients. We wonder to ourselves, what happened to our bodies? What has changed?

Cathryn Nagler, a scientist and professor at the University of Chicago, says that the speed at which food allergies have been increasing rules out certain causes. “Our hypothesis is the increase in prevalence can't be genetic,” she tells me. “Genetics don't change that quickly. It must be due to the environment.”

She tells me that she, and others, think the rise in allergies is explained by environmental factors changing our microbiome, the bacteria that live in our bodies.

The two biggest offenders? “Misuse of antibiotics and the loss of dietary fiber from our diets,” Nagler says. “Even if you personally shun antibiotics, they're fairly stable as drugs, and they're present in more foods and in our water supply than you'd like to know. The loss of dietary fiber from the diet has been observed pretty consistently. Americans are famously enamored with fast food, or processed food that all of which are low in fiber.”

Certain bacteria feed off of fiber, and by lowering the amount of it in our diets, we could also be altering our bacteria populations—starving some bugs and encouraging the growth of others. Similarly, antibiotics might be changing the types and amounts of bacteria that make up our microbiome by killing some and favoring others.

In another recent study, out in Nature last week, Nagler and her colleagues took one step towards uncovering how exactly the microbiome could be linked to food allergies. They found that when gut bacteria from healthy human babies were put into germ-free mice, those mice were protected from an allergic reaction to cow’s milk. But if the mice got bacteria from babies who were allergic to milk, the mice had an allergic reaction to milk, and not a mild one. The mice experienced anaphylaxis when they were given cow’s milk for the first time.


“We can show very, very clearly that the mice that got the microbiota from the healthy infants were completely protected from an allergic response,” Nagler says.

The researchers looked at which microbes were present in both the healthy and allergic mice, and found that one particular bacteria species, called Anaerostipes caccae, seemed to be helping protect the mice from the allergic reaction when it was present.

When they compared differences in the mice’s gene expression, or which genes were more or less active, they saw differences in the intestinal epithelium, which is the thin lining of the small and large intestine. The next steps are to uncover exactly what Anaerostipes caccae is doing to change this lining, and how exactly that change alters the immune response.

The new findings line up with her and her collaborators’ other work: In 2014, Nagler and others found that a class of bacteria called Clostridia could help protect against nut allergies. (Intriguing, Anaerostipes caccae is part of the Clostridia class.) This class of bacteria produces a compound called butyrate, which is an important nutrient for a healthy microbiome, she tells me.

In 2015, Nagler and her collaborator Roberto Berni Canani at the University Federico II of Naples in Italy found there were big differences in the gut bacteria of babies with and without cow’s milk allergies. Children who became tolerant to cow’s milk through dietary management had higher levels of butyrate in their feces, emphasizing that butyrate might play an important role in allergy protection.


In June 2016, Nagler co-founded ClostraBio, a company dedicated to creating a pill that could reintroduce a man-made version of butyrate to the gut. Given their new promising results with Anaerostipes caccae, she says they’re now also interested in developing that bacteria separately as a live therapeutic.

At the moment, there aren’t many options for people with food allergies. The best advice is to avoid, avoid, avoid. I obsessively read food labels and bully waiters at restaurants, but it’s not a perfect system. My most recent exposure was when, on an airplane, a friendly stranger lent me a cough drop for a tickle in my throat. That cough drop had soy in it. It wasn’t pleasant, but I’m lucky that my allergy isn’t life-threatening. Last year, a 15-year-old girl died on a plane after eating a sandwich from Pret a Manger that had sesame in it.

People with fatal allergies need solutions now. Some are trying oral immunotherapy, which exposes you to tiny amounts of the food you’re allergic to in order to become desensitized to it. In a 2018 article in Science Magazine, Jennifer Couzin-Frankel reported that more than 3,000 people worldwide are now trying peanut immunotherapy and that the technique is being extended to other foods, like eggs, milk, and tree nuts. “In a field that for decades has had nothing to offer patients beyond avoidance, immunotherapy marks a seismic shift,” Couzin-Frankel wrote.


The treatment hasn’t gone perfectly, since it does require exposing someone to something they’re allergic to—often extremely allergic to. In 2017, a three-year-old died in Alabama during an oral food challenge, and there have been other bad outcomes while the procedure and dosages get figured out.

Personally, I would rather take a pill that simulates a healthy microbiome than eat small amounts of soy and risk feeling lousy. But Nagler tells me that she sees any treatment inspired by the microbiome as something that would be used alongside immunotherapy. She thinks that tolerance needs both desensitization and a bacteria-induced barrier protective response, they’re two interlocking pieces of a puzzle.

“If you desensitize without dealing with the underlying bacteria barrier protective response, you're still going to have problem,” Nagler says.

“The increase in prevalence of food allergy is likely to be a multifactorial issue,” says Ahmad Hamad an allergist and immunologist at the University of North Carolina Chapel Hill, who wasn’t involved in Nagler’s work. But he added that: “Microbiota seems indeed to play a role in inducing sensitization or tolerance to food antigens as Dr. Nagler showed beautifully in her research.”

He tells me about another recent randomized controlled trial that found that adding probiotics to peanut oral immunotherapy helped kids not respond to the allergen, an example of the two approaches working hand in hand.


When I tell people I’m now allergic to soy, they often ask if it’s because I ate too much soy. I’ve also had servers not take my allergy seriously, because there’s some people who avoid soy for other “health” reasons besides an allergy.

Gupta tells me that when her study in JAMA came out, many media outlets focused only one statistic from it: While 1 in 10 people had a food allergy, almost double believed they had a food allergy, but their symptoms didn’t match a true food allergy.

On The Daily Show, Trevor Noah commented on the research, saying, “Allergies: they affect the lives of many bitch-ass Americans. But according to a new study, the most common affliction is hypochondria.”

Thing is, it’s likely not hypochondria but rather another reaction that’s not an allergy—like an intolerance. Allergies are responses from the immune system, while intolerances are a predictable negative response to a certain food, but not because of an immune response. Gupta says she thinks intolerances are real. For example, lactose intolerance is very real, but is caused by the deficiency of an enzyme, not the immune system. Both she considers valid, and she included the figure about those without true food allergies not to mock them, but to show how many of us have negative side effects after eating.

Is it annoying that everyone suddenly has a grudge against gluten? Sure. But that doesn’t justify what feels like a growing antagonism against specific dietary needs.


When people ask me if I’m allergic to soy because I ate too much it, it weirdly feels like it’s my own fault, rather than a product of systemic diet and environmental issues. Gupta tells me that when she looks at food allergies around the world, there do tend to be more food allergies of what people eat more often, and each country has slightly different top allergens. For example, in Europe, hazelnuts are one of the most common allergies, but not in the United States. Nonetheless, there’s no evidence that eating too much of something ever leads to an allergy to it.

In fact, exposure to top allergens can also be protective (and is the whole premise behind oral immunotherapy). The LEAP study (Learning Early About Peanut allergy) found that in 640 children who were high-risk for a peanut allergy, those who ate peanuts at an early age were prevented from developing that allergy. The results suggest that “the previous recommendations of delaying introduction of allergenic food into infants’ diet (which have been changed recently) could have contributed to an increase in food allergies,” Hamad tells.

So at the moment, there’s no current evidence to support that overexposure leads to allergies, or that it’s anyone’s personal responsibility that they’re allergic to a food.

Nagler says that she would guess what happened to me was something outside of my control.

“It suggests environmental influence, right?” She says. “Your genetics haven't changed. The way that I would explain it, in theory, would be to say that something has happened to change the composition of your microbiome at a point in time that allowed your body to start to become sensitized to this. Some defect perhaps in barrier function, or depletion of healthy populations of bacteria.”

It’s unclear how long adult-onset allergies will last. Some kids grow out of their allergies when they grow up, but if I’m already grown, what will happen? “That's a great question that we're going to have to observe moving forward,” Gupta tells. “I think we don't have any large adult cohorts of adults that we're following over time, and that's really what you need to figure that out.”

For now, those of us with adult allergies can decide to try oral immunotherapy, or wait until potential microbiome drugs come to market. Until then, be kind to your food allergic friends, and I will be wistfully watching others eat miso soup.

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