Onion Sensitivity: What It Is, And How It Could Already Be Affecting You

They're in almost every recipe you eat, and doctors report it's not an uncommon sensitivity for people with gastrointestinal distress.
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Onions are found in most cuisines across the world. They’re cheap, they have a good shelf life and they grow year-round. About 6.75 billion pounds of onion are produced each year just in the United States, according to Colorado State University’s food source database, and global production reaches as high as 105 billion pounds per year. You probably have onions in your pantry, and they could even be in the food you are eating right now.

But did you know onions may make a large portion of the population feel sick?

Yes, the onion and its relatives (chives, shallots and even garlic) can cause a wide range of symptoms including gastrointestinal distress, migraines and, though rare, anaphylactic shock. And yet onions are found everywhere. U.S. consumption has grown 70% since 1982, according to the National Onion Association.

But for some people, onions can be part of a recipe for disaster. For patients with irritable bowel syndrome, or IBS, onions are “one thing most of our patients can’t tolerate,” said Dr. Jane Muir, associate professor and head of translational nutrition science at Monash University. Onions and garlic are high in fructan, a carbohydrate found in many fruits and vegetables that many people cannot digest well and can cause bloating or diarrhea. The Cleveland Clinic estimates that 10% to 15% U.S. adults have IBS, so that’s a lot of folks who may have issues with onion and garlic.

Some people may even have an allergic response to onion, but that is far rarer. In a patient registry maintained by the organization Food Allergy Research and Education, “139 of 11,411 individuals (or 1.2%) self-report as having an onion allergy,” said Dr. Bruce Roberts, chief research strategy and innovation officer at FARE. While you have to take the numbers with a grain of salt since they are self-reported, a 2020 Spanish study of garlic and onions had similar results, with 1.1% of people reporting an onion and garlic allergy.

While it may be a rare allergy, it is more common as a food intolerance. “Onion and garlic are common [food intolerances] that I definitely hear” from patients, said Dr. Kara Wada, assistant clinical professor in allergy and immunology at Ohio State University. The other two common categories of food intolerances that she regularly sees are lactose intolerance and fructose intolerance.

Dr. Sai Nimmagadda, attending physician and clinical assistant professor of allergy and immunology at Northwestern University, estimated that 1%-5% of his patients have reported some kind of onion sensitivity. When asked, Wada said that figure was comparable to her practice. But the actual number of people with an onion and garlic food intolerance in the U.S. or elsewhere is not currently known due to challenges in testing for it, and is likely to be higher.

However, cases of food intolerance, along with food allergies, are rising according to all the doctors interviewed in this article. Given the significant amount of onions and garlic consumed, it’s not something to sniff at. The Spanish study mentioned above concluded, “Allergic hypersensitivity to garlic and onions should not be underestimated and, given their high consumption, should be included in the diagnostic food allergy battery.”

So what’s the difference between a food allergy and food intolerance? The latter is “a condition when the body is unable to digest or absorb certain nutrients,” explained Dr. Ruslan Medzhitov, professor of immunobiology at Yale University and chief scientific officer at the Food Allergy Science Initiative. The offending nutrient then builds up in the gut and draws in water, which causes bloating and diarrhea.

Then there are allergies. These involve the immune system, which attacks the noxious food in the gut. There are two types, Roberts noted: IgE mediated response and non-IgE mediated response. The former is what most people think of as an allergy; when someone ingests, inhales or comes into contact with an allergen, the entire body reacts, involving multiple organs like your skin and your cardiovascular and respiratory systems. It’s pretty immediate and can be lethal.

Non-IgE mediated responses tend not to be quite as potent and are less likely to be life-threatening. The reaction tends to be delayed by a few hours and can cause nausea, vomiting and/or diarrhea.

While food reactions can cause headaches, Roberts noted, “The question concerning migraines is challenging because patients can experience headaches due to sinus congestion in the case of IgE mediated food allergy or anxiety but not a true migraine. The literature suggests food intolerance is more likely to trigger a migraine.”

See that marinara sauce? It's probably has onions in it.
Cris Cantón via Getty Images
See that marinara sauce? It's probably has onions in it.

As to why food intolerance could cause migraines, Roberts noted that the gut-brain connection is strong. There are sensory receptors in our guts that will send up signals to the brain that “there’s something amiss,” he explained.

“And so that’s when you have things like nausea, headaches and other things,” Roberts said.

Getting To The Root Cause

While it stinks to have a true onion allergy, researchers and medical professionals know a lot more about IgE mediated allergic reactions since they are something that can be tested for and possibly treated. Non-IgE mediated allergic response and food intolerance are a lot harder to uncover, Roberts pointed out. There’s “no reliable testing [for food intolerances] worth spending your time, energy or money on,” Wada added.

So it’s a challenge to know exactly how many people have an intolerance to onion or garlic without an easy way to evaluate for it.

On top of it all, Nimmagadda said the challenge is that “there’s a lot that we don’t understand about foods.” Food is so complex, filled with all sorts of proteins, carbohydrates, lipids and more; humans are all different, including the microbiome in our stomach. For instance, he noted, “I’ve had some patients that cannot have Chilean wine, but they’re fine with Italian wine.” And of course, there’s such a spectrum of reactions on top of it all. Wada noted that someone may have issues with fructan in the onions and garlic, or something else like the sulfites in it.

The symptoms and root causes of food intolerances can also differ from person to person. For instance, people with food intolerances may be able to tolerate a certain quantity of onions, or cooked onions. But when the person eats over that threshold or eats raw onions, that’s when all bets are off. Nimmagadda noted that cooking onions can degrade some of the proteins that cause people trouble. However, he said cooking food can sometimes make the reaction even stronger, such as with peanuts. (Raw peanuts are less potent than cooked peanuts.)

It’s important to note that people with a food allergy cannot tolerate any amount of the allergen.

You may not know what is actually causing your upset stomach or your migraine. It could be the sauce that you put in your favorite dish or maybe the ingredient not even listed on the restaurant menu.

So how do you figure it out? All the doctors recommended seeking medical professionals if you suspect a certain food or foods are bothering you. Nimmagadda recommends keeping a food diary so that you may be able to correlate your symptoms with the foods you eat and even the quantity of food. Working with a doctor and a reliable food allergy may help to figure out which food is problematic for your system. Perhaps it’s a single restaurant that’s causing all your digestive problems.

For people medically diagnosed with IBS, there’s the FODMAP diet, developed by Muir and her colleagues at Monash University. FODMAPs are a variety of carbohydrates that are naturally found in food that can trigger symptoms; onions and garlic are one of the offending foods.

The basic idea is that for two to six weeks you swap out high-FODMAP foods with low ones and then gradually introduce them back into your diet one at a time. Muir said, “It’s a learning diet or process where people have to learn which of their FODMAPs [are causing problems] because everyone is different.” But she noted that this diet is intended only for people medically diagnosed with IBS and collaborating with a dietitian.

Hold The Onions

For many people, avoiding the offending food might be the best practice, but that’s not always possible when we eat food that we haven’t cooked ourselves. Nimmagadda said you can talk to the chef about what’s in the food to get in front of any unpleasant surprises. Or maybe find restaurants that you’ve had a favorable history with. Definitely let people who cook for you know about it.

If you have been feeling less than stellar after eating, it may be time to investigate whether you have an intolerance, or worse, an allergy to onions or other food. Nimmagadda summed it up best: “Educate yourself; know your body, know your symptoms, and then seek out treatment. Get evaluated. You can find solutions. You don’t have to live with this.”

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