When you think of food allergies, you might picture someone eating something and then breaking out in hives or even gasping for breath.

But often the symptoms people report as allergy-related are due to food intolerances instead. Though intolerances can cause considerable discomfort, the reactions are never life-threatening.

“Research shows that as many as 20 percent of people claim to have food allergies, when the number is actually around 3 to 4 percent,” says Hugh Sampson, M.D., director of the Jaffe Food Allergy Institute at Mount Sinai School of Medicine in New York City.

That said, you can develop food allergies in adulthood, and people can become less tolerant of certain foods as they get older. Milk is a common example: Most adults who are lactose-intolerant had no trouble drinking milk when they were young.

Whether you suspect that you have a true allergy or think you might be just intolerant of certain foods, the stakes are high enough that it doesn’t make sense to guess. We interviewed experts to gather essential facts about food allergies and intolerances. Here’s what you need to know:

Different Foods, Different Symptoms

A handful of culprits trigger most allergies. In fact, just eight foods are responsible for 90 percent of food-allergic reactions in the U.S.: eggs, fish, milk, peanuts, shellfish, soy, tree nuts (almonds, cashews, pecans, pistachios, and walnuts), and wheat.

And though some of the symptoms of food intolerances and food allergies are similar, the differences between the two are critical.

More on Food Allergies

“A lot of the time when people say they are allergic to food, they mean that too much of it gives them a stomachache or they just don’t like it. But it’s very dangerous to confuse an allergy with an intolerance,” says Maria Acebal, former chief executive officer of the Food Allergy and Anaphylaxis Network (FAAN), a nonprofit organization dedicated to food-allergy education and awareness.

“People with intolerances can still eat the food without serious consequences,” she says. “But for someone with an allergy, touching, inhaling, or ingesting even a microscopic amount of the allergenic food can be deadly.”

Symptoms can vary widely. Food intolerances take place in the digestive system and can cause gastrointestinal complaints such as nausea, bloating, cramps, gas, and diarrhea.

Chemicals and food additives can cause a wide range of intolerance symptoms in a few susceptible people. For example, chocolate, cheese, and red wine can trigger migraines.

Food allergies, on the other hand, are an overreaction of the immune system.

Within minutes or sometimes hours after coming into contact with even a trace amount of a food they are allergic to, victims might experience symptoms in the gastrointestinal tract (nausea, vomiting, or diarrhea), skin (itching, swelling, or hives), or respiratory system (stuffy, runny, or itchy nose; sneezing; coughing; or wheezing). Or they might have a combination of any of those symptoms, among others.

'A Little Allergic'? No Such Thing.

Food allergies can be fatal.

In the most serious cases, a food allergen can trigger anaphylaxis, a severe reaction that happens very quickly and can cause swelling of the throat, difficulty breathing, and loss of consciousness. Without immediate treatment—an injection of epinephrine (adrenaline) and expert care—anaphylaxis can be fatal.

Even a small amount of a trigger food can cause a big reaction, and anything that has touched a trigger food can be dangerous.

When you’re allergic to something, you can’t get away with eating even a little bit of the forbidden food. People who have received an allergy diagnosis have to be scrupulous in avoiding that food—and that can be trickier than it sounds.

You might not expect to find wheat in soy sauce, for example, or soy in vegetable broth, but hidden ingredients are present in many foods. And the formulations of even familiar foods can change, so you have to make label-reading a habit. Food makers are legally required to state on labels whether a product contains any of the eight major food allergens.

Even if you avoid trigger foods such as nuts, you need to be aware
that kitchen utensils and factory equipment can harbor trace amounts of allergens. So read food labels, and wash your hands and kitchen surfaces with soap and water to effectively remove allergens; alcohol sanitizers won’t do the trick.

Remember that a mild attack doesn’t indicate that the next one won’t be severe. A review of food-allergy fatalities by the FAAN found that most of the victims had not suffered a severe allergic reaction before the one that caused their death. 

If you suspect you might have a food allergy, it’s critically important to confirm your suspicions with a doctor who specializes in allergies.

Identifying exactly what you are allergic to and getting nutritional counseling on how to avoid it could save your life.

What About Gluten?

Avoiding gluten, a protein found in wheat, barley, and rye, is essential for people with a condition called celiac disease.

Celiac disease is not a food allergy, per se. The two conditions are believed to involve different parts of the immune system.

A wheat allergy, for example, causes symptoms similar to other allergies, and it’s often outgrown by adulthood. In celiac disease, on the other hand, gluten triggers an autoimmune condition in which the body starts attacking normal tissue in the small intestine. The condition can be inherited, and it persists through life.

The only treatment is to avoid gluten completely.

But going gluten-free has become a trendy way to eat for a wide range of people, some of whom say they have nonceliac gluten sensitivity—a little-understood condition where eating gluten is linked to gastrointestinal symptoms. 

“Gluten intolerance is real, but it probably affects only a small subset of people,” says Charles Elson III, M.D., professor of medicine and microbiology at the University of Alabama at Birmingham who was chairman of a National Institutes of Health panel on celiac disease.

More research is needed, but there are downsides to avoiding gluten when it’s not necessary, so CR recommends consulting your doctor before deciding to cut gluten out of your diet.