We never stopped to consider how gosh dang weird that all is until reader Scott Ross wrote in from Derwood, Md., just north of Rockville, to ask who was most susceptible to food allergies. Wherever we looked, we found data that defied easy explanation.
For starters, food allergies are on the march. Among children, they doubled from 2000 to 2018, according to our analysis of the Centers for Disease Control and Prevention’s National Health Interview Survey, and they probably have risen further in the years since. We don’t have comparable data for adults, though other research tends to reinforce the trend.
Boys and girls are equally likely to have food allergies, but as both age past puberty, food allergies among women grow much more common, especially in middle age. This gender gap has been observed around the world, and it seems to be widest in allergic reactions that cause hives or migraines, according to a 2017 analysis in the World Allergy Organization Journal. Women are more likely to be allergic to fruits and berries, while men are more likely to have peanut allergies.
This is all self-reported (or parent-reported) data: We don’t know how many of these people who say they have allergies actually do. We’re more than 3 percentage points more likely to report an allergy (9.4 percent in 2021) than to report receiving a professional diagnosis (6.2 percent). That means about a third of us who say we have allergies have not actually had them confirmed by a doctor.
On the hunt for deeper data, we found a series of analyses published by Northwestern University’s Center for Food Allergy and Asthma Research. Based on a survey of almost 80,000 people in more than 50,000 U.S. households in 2015 and 2016, this data goes into far more detail than the feds could — and raises even more questions about the weird world of food allergies.
For one thing, most food allergies are diagnosed in younger folks and tend to fall off in adulthood. An extreme example is cow’s milk: Toddlers are much more likely to be allergic to dairy products from cows than their older neighbors.
Shellfish is at the other end of the spectrum: Adults are allergic to crabs, shrimp and mussels at about twice the rate of children. Shellfish is also the most common allergy among adults, followed by milk, peanuts, and tree nuts such as almonds and walnuts.
Other notable findings: Allergies are more common in low-income families than in high-income ones. Black and Hispanic Americans are more likely to have food allergies than their White colleagues, with the one exception being wheat allergy, which is most prevalent in White people. (What we’re calling a wheat allergy looks similar to other food allergies, and is not to be confused with celiac disease, an autoimmune condition, or other gluten intolerance.)
We called Christopher Warren, a Northwestern professor who helped design the survey in his capacity as director of population health research at the allergy center, mainly just to make sure we’d measured everything correctly. But Warren instead opened our eyes to a grand, unified theory of why everybody is suddenly allergic to everything.
First, he patiently explained that what we commonly call a food allergy is an immune response, particularly one from an antibody called immunoglobulin E, which causes your body to release the biochemical histamine to eject an invader. (Histamine causes itching and swelling and, if too much of it builds up, serious gastrointestinal reactions and even anaphylaxis.)
“The nature of the reaction,” Warren said, “has to do with your antibodies identifying something … and being like, ‘We got to get this out of here! Get ’em sneezing! Get ’em inflamed! Get ’em vomiting! Get ’em diarrhea!’”
So one reason women might have more allergies is that they have more aggressive immune systems, especially during the childbearing years. As Warren points out, that hypervigilance might make sense from an evolutionary perspective: Humans and other animals are more likely to pass on their genes if they aggressively avoid infections at the very moment when those pathogenic invaders might infect their offspring or limit their ability to gestate or care for them.
But why does the histamine howitzer sometimes get trained on harmless sesame seeds or soybeans? Regardless of gender, Warren says, it begins in the skin.
We’re oversimplifying a bit here, but a baby’s immune system follows a simple rule of thumb: If a protein comes in through the mouth, it’s probably a helpful nutrient. But if it comes in through the skin, it’s probably a perilous parasite. Anything that inflames the skin, or makes it easier for foreign substances to enter, can make a haywire immune response more likely.
That’s why a baby with eczema, an inflammatory skin condition, is five times more likely to develop a food allergy than her friends without the condition. A remarkable study out of Australia shows that the more severe the eczema, the more likely a baby is to develop a food allergy, with about half the kids with the earliest and most severe eczema cases developing such allergies.
That also helps to explain why shellfish is such a common adult-onset allergy. Allergies often travel in packs; if you’re allergic to peanuts, you’re more likely to be allergic to tree nuts such as cashews and walnuts. Shellfish such as shrimp and crayfish are biologically similar to another even more common arthropod allergen: dust mites.
“If you put a shrimp and a dust mite like face to face, they look pretty darn similar,” Warren said. Their muscles are made of the same proteins, and once your immune system starts reacting to the mites, it might detect in shrimp the same, presumably dangerous, compound.
Dust mites (and their poop) may be unusually likely to trigger allergic reactions, and they bombard us constantly — far more frequently than our guts probably get treated to Maine lobster or jumbo prawns — giving our bodies plenty of chances to consider arthropods a threat. Research in adult-onset allergies remains scarce — they’re rare enough that you’d have to follow tens of thousands of healthy adults to get a big enough subgroup that would develop any given allergy — but it seems reasonable to assume that adult exposure to dust mites may trick your body into thinking shellfish are dangerous.
One reason Black children have higher rates of food allergies is that they’re about twice as likely to be reactive to dust mite and cockroach allergens as their White friends, according to another analysis by Warren, his boss Ruchi Gupta at the Center for Food Allergy and Asthma Research, and several collaborators. Presumably, inequalities in U.S. housing led those children to be exposed to more of those allergens, generating a sensitivity that triples your risk of developing a shellfish allergy.
So why have our immune systems suddenly gone haywire? One theory notes that we (mostly) eradicated hookworms by the 1980s in the United States. And ringworms. And tapeworms. All the classic parasites are mostly kaput. Without those actual threats, our immune system downshifts to tackle the biggest possible threat on the horizon. Which, these days, might be cashew butter or Camembert.
“It’s looking for stuff to do and it’s staying busy,” Warren said. “But it’s busy doing stupid stuff like reacting to walnuts and birch pollen.”
Some support for this theory comes from anecdotes offered by experts who infected themselves with hookworms to distract their overactive immune systems. While this method achieved some success in curbing stubborn allergies and other conditions, it seems unlikely we’ll see a massive experiment anytime soon that randomly infects healthy Americans with hookworms. Still, this so-called hygiene hypothesis helps explain why allergies may be on the march: Back when they were more widespread, hookworms and their friends may actually have reined in our immune system’s most aggressive tendencies.
But that’s not the only explanation for today’s allergy epidemic. Another is that we’re over-protecting our kids. That practice was tragically exacerbated by the American Academy of Pediatrics, which, alarmed by rising food allergies, advised parents in 2000 to avoid exposing kids to peanuts or peanut products until age 3. That’s way too late to avoid most food allergies through exposure.
“That led to this whole epidemic of people not feeding their kids the allergens early, which then probably contributed to the increase in allergies, because you’re not giving the immune system a chance to train when it’s best suited to learn,” Warren said.
Those guidelines are now reversed. The National Institute of Allergy and Infectious Diseases recommended in 2017 that children be exposed to peanuts as early as 4 months — particularly if they have eczema. The new advice was based on a landmark study out of the United Kingdom that found that early exposure led to a massive drop in the incidence of allergies.
“If you can get anywhere close to an 81 percent decrease that we saw in the original study, we’re talking about saving thousands and thousands of kids from peanut allergies,” Anthony S. Fauci, then the institute’s director, said at the time.
Greetings! The Department of Data craves queries. What do you wonder about: Which states have the most (and least) childhood vaccination exemptions? Where are pubs, taverns and other alcohol-serving establishments least likely to have the word “bar” in their names? Which communities have the highest share of vacant buildings? Just ask!
If your question inspires a column, we’ll send an official Department of Data button and ID card. This week’s button is en route to tiny Derwood — near bucolic, bass-laden Lake Needwood — where Scott Ross has some other food allergy questions we may tackle at a later date. We’ll also send one to Andrew Dulude in Colrain, Mass., who asked why children today are more prone to attention-deficit/hyperactivity disorder, peanut allergies and other ailments.