Millions of Americans might be mistaken about their self-professed food allergy, suggests a new survey. It found that while nearly 20 percent of people said they had a food allergy, only half as many people reported the sort of symptoms you’d expect from eating something you’re allergic to.

Researchers surveyed more than 40,000 adults via the phone and internet between October 2015 to September 2016. The volunteers were asked if they had any food allergies and about what symptoms they typically had. They were also asked if they had ever been formally tested and diagnosed with a food allergy by a doctor.

All told, 19 percent of the nationally representative group reported having a food allergy. But only 10.8 percent said they had symptoms consistent with an allergic reaction to food, such as hives, swelling of the lips or throat, and chest pain. The main culprits behind these allergies were shellfish, milk, and tree nuts. Those who didn’t have a convincing food allergy instead reported symptoms like stomach cramps, a stuffy nose, or nausea.

The findings, published Friday in JAMA Network Open, roughly match up to estimates from other studies, including those that confirmed a person’s food allergy with testing or medical records. In terms of the U.S. population, the study estimates, there are about 26 million adult Americans with a food allergy—and there are likely nearly as many Americans who wrongly say they have one. But that doesn’t mean huge swaths of people are pretending to have food allergies; it’s just that we could be a little confused about the terminology.

True allergies, as they’re known, happen when the immune system overreacts very quickly and in a specific way to a foreign substance harmless to us, whether it’s food or a piece of clothing. The antibodies usually responsible for an allergic reaction are called immunoglobulin E, or IgE. When doctors test for allergies, it’s IgE antibodies they’re looking for. But people can react badly to food for other reasons outside of this process.

Lactose intolerance is probably the best known example of this, and it happens because many adults are less able to break down lactose, the sugar commonly found in dairy products, into simpler sugars. Another genetic condition, celiac disease, makes people unable to digest gluten. Some people also seem to have delayed immune reactions to food without IgE in the picture, though we’re less sure about how commonly this happens and how to accurately diagnose it. Many doctors, for instance, criticize tests that promise to find these so-called food sensitivities with ease.

It’s likely then, the researchers say, that people might be mixing up a food intolerance or sensitivity with a food allergy.

What’s also concerning is that many people with likely food allergies in their survey have seemingly never talked to a doctor about it. Only half of the group said they had an official diagnosis from a physician. And while many of us develop food allergies early on in childhood, just about half reported finding out about their allergy as adults.

Source: Millions of Americans Are Wrong About Having a Food Allergy, Study Suggests