Lets break down the differences between an actual food allergy and an intolerance.
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You finish a meal and, suddenly, your stomach is twisting into knots, you feel a burning in your throat, and that rumbling in your intestines is sending you straight to the bathroom. These may be signs you're allergic to whatever you just ate. But, more likely, you're actually just intolerant of it.

A new study in JAMA found that even though one in five adults think they have a food allergy (just about 20 percent), only one in 10 actually does. Most people probably just have a food intolerance instead, say the study authors.

"It's super common for people to confuse having an allergy with an intolerance," says Danielle Capalino, R.D., a New York–based nutritionist specializing in food intolerances and author of the newly released Healthy Gut, Flat Stomach: Drinks.

It's sometimes deliberate—it can be easier to explain in a restaurant that you have an allergy, and that wording increases the chances they'll take your restrictions seriously, explains Capalino. However, many people may innocently associate the negative effects of food "not agreeing with their system" as being an allergy, even if it doesn't meet the technical criteria, she adds.

A true food allergy causes an immune system reaction, while an intolerance does not. "A food allergy and intolerance can have many of the same symptoms. One reason for an intolerance is that your body doesn't have the enzymes to break that food down, like with lactose. This would cause digestive symptoms like gas, bloating, diarrhea," says Capalino. "If you're allergic to dairy, though, you don't lack the enzyme; instead, your body would actually mount an immune response which could also involve gastrointestinal symptoms, but may also cause a rash, breathing issues, or, in severe cases, anaphylaxis." 

More on food allergies: 

The Key Difference Between a Food Allergy and Intolerance

Food allergies can be life-threatening while food intolerances are not—that's why it's so important to correctly diagnose your food issues, and not simply dismiss it as an intolerance. (Speaking of: Is It Possible to Give Yourself a Food Intolerance?)

On the flip side, falsely claiming you have an allergy makes it riskier for people who actually do: "If you go to a restaurant and say you're allergic to wheat and need special gluten-free pasta, but then you manage to try the flour-filled chocolate cake for dessert, the server sees this and over time doesn't take allergies as seriously," says Capalino. (Related: What It's Really Like to Be On an Elimination Diet)

Interestingly though, the same JAMA study found nearly half of afflicted adults developed at least one of their food allergies after adolescence—so it is possible to be suddenly allergic to a food you've eaten hundreds of times. (Why? Science isn't quite sure yet.)

How to Know If You Have a Food Allergy

For starters, allergic reactions tend to happen very quickly and can involve manifestations on the skin (like a rash), diarrhea, vomiting, breathing problems, or anaphylaxis, says Capalino. It's true that intolerances may cause GI distress as well, but if it's an allergy, it'll come on fast and furious, she adds.

The most common dietary allergens are fish, peanuts, shellfish, tree nuts, soy, cow's milk, eggs, and wheat—which can be confusing since these last four are often intolerance offenders, too, says Capalino. And if your suspect isn't one of these eight, there's also a weird phenomenon called oral allergy syndrome where you're allergic to something like pollen but have a reaction to foods with the same enzyme, such as apples, peaches, parsley, or carrots (learn more about that from the American Academy of Allergy, Asthma & Immunology).

Mostly, though, if you think you have an allergy but have never been tested, Capalino recommends heading to an allergist. He or she can confirm whether your adverse reaction is your body telling you to steer clear of the offending food for a happy gut or to actually save your life.