Is it a food intolerance, allergy, or something else?

Learn how to tell the difference, and what to do if you're reacting to wheat, milk, or other foods.

Walk down the aisles of your local supermarket, and you'll see something you likely wouldn't have encountered a decade ago—shelves devoted entirely to gluten-free cereals, breads, muffins, and other foods. Restaurants have also jumped onto the bandwagon, revising their menus to include dishes without gluten, a protein found in wheat.

The gluten-free diet was designed for people with celiac disease, who can't tolerate any foods containing gluten because their immune system reacts to it and damages the small intestine in response. Celiac disease is a very real, very uncomfortable, and potentially very serious condition. If left untreated, it can lead to anemia, osteoporosis, and intestinal cancers.

About 1% of Americans, or three million people, have true celiac disease. Another 6%, or 18 million people, are sensitive to gluten. Eating gluten-containing foods doesn't damage their intestines, but it can still produce gastrointestinal discomfort, along with symptoms like headaches and fatigue. People in a third group are allergic to wheat. When they're exposed, they get more traditional allergy symptoms, which can range from tingling around the mouth to hives, throat swelling, and difficulty breathing.

"It's confusing that people can have all these different reactions to the same food," says Dr. Ciaran Kelly, professor of medicine at Harvard Medical School and medical director of the Celiac Center at Beth Israel Deaconess Medical Center. "It's important to make the distinction between food allergies and intolerance, because there is a lot of confusion and there are differences in treatments."

A number of foods—including wheat, milk, eggs, and seafood—are notorious for triggering both food allergies and intolerances. If you have symptoms when you eat certain foods, it's important to distinguish what kind of reaction you're having and which foods are triggering it.

Food intolerance

When you're intolerant to a particular food, it's usually because your body lacks an enzyme needed to break down a component in that food (such as lactose, the sugar in milk). Or, your body might be sensitive to a particular chemical or additive in the food. The process leading to food intolerance often starts early in life, but symptoms can be too subtle to notice at first. "People may become more aware of intolerances as they get older," Dr. Kelly says.

Examples of food intolerance:

Lactose intolerance. Your body can't break down the sugar lactose because your gut contains reduced levels of the intestinal enzyme lactase. Lactose is found in dairy foods such as milk or ice cream. When you eat these foods, you can develop uncomfortable gastrointestinal symptoms like gas and diarrhea.

Gluten sensitivity. You have many of the same symptoms as someone with celiac disease after eating wheat or other foods containing gluten (stomach pains, bloating, fatigue), but your immune system doesn't produce the blood test abnormalities seen in people with celiac disease, and there is no evidence of damage in the intestines.

Sensitivity to food additives. You get symptoms like flushed skin and wheezing from eating additives such as sulfites (found in wine, dried fruits, and canned goods), or headaches, palpitations, or numbness after eating foods flavored with monosodium glutamate (MSG).

• Symptoms of food intolerance:
  You may be able to eat small amounts of the food without having any reaction to it. Your symptoms will come on gradually after you've eaten a particular food.
  Often, those symptoms will involve your digestive system—such as nausea, gas, or diarrhea. Your reaction will be uncomfortable, but it's usually not life-threatening.

• How to deal with food intolerance:
  Keeping a food diary can help you identify the source of the problem. Every day, write down the foods you eat and any symptoms that occur. Once you pinpoint one or a few foods that coincide with your symptoms, you can try cutting them all out of your diet. This is called an elimination diet. Then add one food back in every couple of days. When your symptoms return, you've found the offending food. Ask your doctor or a dietitian for help identifying your trigger food and eliminating it from your diet.

Food allergy

A true food allergy involves your immune system. Your body recognizes a normally innocuous food, such as peanuts or milk, as a potentially harmful foreign invader. It goes into defensive mode, producing high levels of an antibody called immunoglobulin E (IgE). Often food allergies start when you're young, but it's not impossible for them to appear for the first time later in life, Dr. Kelly says.

Examples of foods that commonly cause allergic reactions include eggs, fish and shellfish, milk, peanuts, soy, tree nuts (hazelnuts, walnuts, almonds), and wheat.

• Symptoms of a food allergy:
  You could have a reaction from eating just a tiny amount of the food, or simply from being around the food.

You can experience allergic symptoms such as hives, swelling, and itchiness, as well as gastrointestinal symptoms such as abdominal pain, vomiting, and diarrhea.

If your allergy is severe, you might have an anaphylactic reaction, which can begin with a rash, swelling of the tongue and throat, trouble breathing, dizziness, or fainting. It can be life-threatening.

• How to treat a food allergy:
  See an allergist who has experience treating food allergies. The doctor can do a skin test, placing a solution containing an extract of the food just beneath the skin of your forearm or back. Or you may get a blood test to look for IgE antibodies to the food. If you have an allergy, you'll need to avoid the food. Your doctor might also recommend that you carry around an epinephrine injector (EpiPen) to treat anaphylaxis if your allergy is severe.

Don't shortchange your diet

Avoid foods that bother you, but don't do a full-scale purge of your diet without good cause (for example, celiac disease or true food allergies). Because of the abundance of gluten-free foods available, many Americans have begun to think that all wheat and other grain products are bad for them. "There's a way of thinking that gluten is an unhealthy food," Dr. Kelly says. "Somehow if a food is gluten-free, it's considered healthier, and there's little basis for that."

Cutting out foods like wheat, barley, and rye can rob your diet of nutrients such as fiber, calcium, and B vitamins. Going gluten-free could have a similar effect on your purse. One Canadian study found that gluten-free foods cost 242% more than comparable regular foods. Work with a doctor or dietitian to create a diet that's safe for your system, while still healthy and well rounded.