Food Allergy and Food Intolerance

People often have an unpleasant reaction to something they ate and think they have a food allergy. But they may be having something else: a reaction called food intolerance.

What's the difference?

A food allergy is caused by your immune system reacting to the food when it doesn't need to.

With a food intolerance, your immune system isn't responsible. Most of the time it's a problem with digesting the food.

For example, being allergic to milk is different from not being able to digest it properly because of lactose intolerance.

Some people come from families where allergies are common -- not necessarily food allergies, but perhaps hay fever, asthma, or hives. When both of your parents have allergies, you're more likely to have food allergies than if only one parent has allergies.

If you think you have a food allergy, see a doctor to confirm what's triggering it and get help managing and treating it. Sometimes allergic reactions to food can be severe, even life-threatening.

How Food Allergies Work

Food allergies involve two parts of your immune system. One is immunoglobulin E (IgE), a type of protein called an antibody that moves through the blood. The other is mast cells, which you have in all body tissues but especially in places like your nose, throat, lungs, skin, and digestive tract.

The first time you eat a food you're allergic to, certain cells make a lot of IgE for the part of the food that triggers your allergy, called an allergen. The IgE gets released and attaches to the surface of mast cells. You won't have a reaction yet, but now you're set up for one.

The next time you eat that food, the allergen interacts with that IgE and triggers the mast cells to release chemicals such as histamine. Depending on the tissue they're in, these chemicals will cause various symptoms. And since some food allergens aren't broken down by the heat of cooking or by stomach acids or enzymes that digest food, they can cross into your bloodstream. From there, they can travel and cause allergic reactions throughout your body.

The digestion process affects the timing and the location. You may feel itching in your mouth. Then you may have symptoms such as vomiting, diarrhea, or belly pain. Food allergens in your blood can cause a drop in blood pressure. As they reach your skin, they can trigger hives or eczema. In the lungs, they may cause wheezing. All of this takes place within a few minutes to an hour.

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Which Food Allergies Are Most Common?

In adults, they include:

  • Peanuts
  • Tree nuts, such as walnuts
  • Shellfish, including shrimp, crayfish, lobster, and crab

For children, the food allergens that most often cause problems are:

Adults usually don't lose their allergies, but children do sometimes. Kids are more likely to outgrow allergies to milk, eggs, and soy than to peanuts, fish, and shrimp.

The foods that you'll react to are often those that you eat regularly. In Japan, for example, you'll find rice allergy. In Scandinavia, codfish allergy is common.

Cross Reactivity and Oral Allergy Syndrome

When you have a life-threatening allergic reaction to a certain food, your doctor will probably recommend that you avoid similar foods, too. For example, if you react to shrimp, you're probably allergic to other shellfish like crab, lobster, and crayfish. This is called cross-reactivity.

Another example of cross-reactivity is oral allergy syndrome. It happens in people who are highly sensitive to ragweed. During ragweed season, when they try to eat melons, especially cantaloupe, their mouths may itch. Similarly, people who have severe birch pollen allergy may also react to apple peels.

Exercise-Induced Food Allergy

At least one type of food allergy needs more than simply eating the allergen to cause a reaction. If you have exercise-induced food allergy, you won't have a reaction unless you then do something physically active. As your body temperature goes up, you'll begin to itch, get lightheaded, and could have hives or even anaphylaxis.

Fortunately, the cure is simple: Don't eat that food for a couple of hours before you work out.

Is It Really a Food Allergy?

A differential diagnosis is the process of telling the difference between a food allergy, a food intolerance, and other illnesses. When you go to the doctor's office and say, "I think I have a food allergy," they have to consider a list of other things that could cause similar symptoms and be confused with a food allergy. These include:

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Foods can get contaminated with bacteria and toxins. Tainted meat sometimes mimics a food allergy when it's really a type of food poisoning.

Histamine can reach high levels in cheese, some wines, and in certain kinds of fish, especially tuna and mackerel, if it hasn't been refrigerated properly. When you eat foods with a lot of histamine, you could have a reaction that looks like an allergic reaction. It's called histamine toxicity.

Sulfites are made naturally during the fermentation of wine, and they're added to other foods to enhance crispness or prevent mold growth. High concentrations of sulfites can pose problems for people with severe asthma. They give off a gas called sulfur dioxide, which the person breathes in while they're eating the food. This irritates their lungs and can trigger an asthma attack. That's why the FDA banned sulfites as spray-on preservatives for fresh fruits and vegetables. But sulfites are still used in some foods.

Monosodium glutamate (MSG) is naturally in foods including tomatoes, cheese, and mushrooms. It's added to others to boost flavor. When eaten in large amounts, it can cause flushing, warmth, headache, pressure in your face, chest pain, or feelings of detachment.

Yellow dye number 5 can cause hives, although that's rare.

Lactose intolerance, the most common food intolerance, affects at least 1 out of 10 people. Lactase is an enzyme in the lining of the gut. It breaks down lactose, a type of sugar in milk and other dairy products. If you don't have enough lactase, you can't digest lactose. Instead, bacteria eat the lactose, which creates gas, and you can get bloating, stomach pain, and diarrhea. Your doctor can measure your body's response to lactose by testing blood samples.

Gluten intolerance is related to celiac disease. It's caused by an abnormal immune response to gluten, a protein found in wheat and some other grains.

Several other diseases share symptoms with food allergies, including ulcers and digestive system cancers. These can lead to vomiting, diarrhea, or cramping pain that gets worse when you eat.

Some people may have a food intolerance with a psychological trigger. An unpleasant event, often during childhood, that's tied to eating a particular food can bring on a rush of unpleasant sensations when you eat that food later, even as an adult.

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Diagnosing Food Allergies

First, the doctor asks detailed questions like:

  • Did the reaction come on quickly, within an hour of eating the food?
  • Did anyone else get sick?
  • How much did you eat before the reaction started?
  • How was the food prepared?
  • Did you eat anything else at the same time?
  • Did you take an antihistamine or do something else? Did it help?
  • Does this always happen when you eat that food?

These help the doctor understand what's going on and could point to another explanation. For example, if you ate fish contaminated with histamine, everyone who ate that same fish would have gotten sick, too. Some people will have a violent allergic reaction only to raw or undercooked fish because heat destroys the allergens they're sensitive to. Or other foods in the meal can delay digestion so the allergic reaction starts later.

Your doctor may ask you to keep a food diary, a record of each meal and any reaction you have. This gives more detail for you both to look for patterns. You may find that the severity of your reaction is related to the amount of food that you ate.

The next step might be an elimination diet, which you do with the help of your doctor. You start by not eating a suspect food, like eggs. If your symptoms go away, that strongly suggests an allergy. Then you try eating that food again to see if the symptoms come back, which confirms the diagnosis. But you can't do an elimination diet if your reactions are severe (because you wouldn't want to trigger it) or you don't have them often.

Tests for Food Allergies

If your doctor thinks a specific food allergy is likely, you may get tests to measure your allergic response.

One of these is a scratch puncture test. The doctor or technician puts a drop of a solution made with the food on your forearm or back. Then they'll prick your skin with a needle through the drop and watch for swelling or redness.

Skin tests are quick, simple, and relatively safe. But experts don't recommend making a diagnosis based on a skin test alone. Your skin test may show an allergy to a food without you having allergic reactions when eating that food. So your doctor will diagnose a food allergy only when you have a positive skin test and a history of reactions to the same food.

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If you're extremely allergic and have severe reactions, skin testing could be dangerous. It also can't be done if you have severe eczema. Instead, your doctor can use blood tests such as RAST and ELISA that measure the amount of food-specific IgE. These tests may cost more, and results take longer. Again, a positive result doesn't necessarily mean you have a food allergy.

A food challenge, or feeding test, is another way to confirm or rule out an allergy. It's done with your doctor there. You eat small servings of food every 15-30 minutes that have increasing amounts of the suspected allergen in them until you either have a reaction or eat a meal-sized portion.

In a "double-blind" test, neither you or your doctor know if what you're eating has the allergen in it. This type of testing is actually most common when the doctor believes that your reaction is not from a specific food. The test can provide evidence to look elsewhere to find the real cause of the reaction.

Of course, people with severe reactions can't do food challenges, and it's hard to test more than one food allergy at the same time. It's also expensive because it takes a lot of time.

Unproven Ways to Diagnose Food Allergies

Some techniques can't effectively identify food allergies. These include:

Cytotoxicity testing. A food allergen is added to your blood sample. A technician then checks the sample under the microscope to see if white cells in the blood "die."

Sublingual or subcutaneous provocative challenge. It's similar to a skin test, but the sample of food allergen goes under your tongue or gets injected under your skin.

Immune complex assay. This blood test looks for groups of certain antibodies bound to the food allergen. But these clusters normally form as part of food digestion, and everyone, if tested with a sensitive enough measurement, has them.

IgG subclass assay. This blood test looks specifically for certain kinds of IgG antibody, but they're part of a normal immune response.

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Treatment for Food Allergies

The main way to deal with food allergies is to avoid them. For highly allergic people, even tiny amounts of an allergen (as little as 1/44,000 of a peanut kernel) can trigger a reaction. Less-sensitive people may be able to have small amounts of a food that they're allergic to.

Once you've identified the food, you have to stop eating it. That may mean reading long, detailed ingredient lists because many allergy-triggering foods are in things you wouldn't expect to find them in. Peanuts, for example, may be included for protein, and eggs are in some salad dressings. At restaurants, you might have to ask about the ingredients that are in specific dishes or in the kitchen.

Even people who are very careful can make a mistake, so if you have severe food allergies, you must be prepared to treat an accidental exposure. If you've had anaphylactic reactions to a food, you should wear a medical alert bracelet or necklace. And you should carry two auto-injectors of epinephrine (Adrenaclick, Auvi-Q, EpiPen) and be ready to use them if you think a reaction is starting. Mild symptoms such as tingling in your mouth and throat or an upset stomach might not be an allergic reaction, but you should still give yourself an injection. It won't hurt, and it could save your life. Then call 911 or get a ride to the emergency room.

Parents and caregivers should protect children from their trigger foods and know what to do if the child eats one. Schools should have plans in place to address any related emergency.

Medications can help relieve food allergy symptoms that aren't part of an anaphylactic reaction:

But these won't prevent an allergic reaction if you take them before eating the food. No medication can. Putting a dilute solution of a food under your tongue about a half hour before you eat it as a way to "neutralize" your exposure doesn't work either.

Allergy pills and shots are being studied as way to desensitize people to food allergens. You regularly get small amounts of food extracts over a long period of time to help your body build up a kind of tolerance. But researchers haven't yet proven that allergy shots work for food allergies.

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Food Allergies in Infants and Children

Milk and soy allergies are particularly common in infants and young children, probably because their immune and digestive systems are still developing. These allergies can appear within days to months of birth. They may not show up as hives and asthma, but rather lead to colic and perhaps blood in poop or poor growth.

Typically, the doctor sees a very unhappy colicky child who may not sleep well at night and diagnoses a food allergy partly by changing their diet, like switching from cow's milk to soy formula. This type of allergy tends to disappear within a few years.

Doctors recommend only breastfeeding infants for the first 4-6 months, if possible, for many reasons, but there's no proof that it prevents food allergies later in life. While some pregnant women may hope limiting their diets while they're pregnant or breastfeeding may help their children avoid allergies, the experts disagree and don't suggest it. Soy formula isn't a good way to prevent allergies either.

Issues Mistakenly Linked to Food Allergies

Although some people think certain illnesses can be caused by food allergies, evidence doesn't back up such claims. Histamines in cheese or red wine, for example, can trigger migraines. But we can't say that food allergies actually cause migraines. Rheumatoid arthritis and osteoarthritis aren't made worse by foods. Food allergies don't cause "allergic tension fatigue syndrome," where people get tired, nervous, and may have problems concentrating or headaches.

Cerebral allergy is a term that describes when mast cells are supposedly releasing their chemicals in the brain -- and nowhere else in the body -- causing trouble concentrating and headaches. Most doctors don't recognize cerebral allergy as a disorder.

Even when their surroundings are very clean, some people have many general complaints like problems concentrating, fatigue, or depression. Environmental illness may be the result of small amounts of allergens or toxins, but not food allergies.

Researchers have found that hyperactivity in children may be related to food additives, but only occasionally and only when the child has had a lot of them. A food allergy won't directly affect a child's behavior, although their symptoms might make them cranky and difficult, and allergy medications can make them sleepy.

WebMD Medical Reference Reviewed by Brunilda Nazario, MD on October 29, 2018

Sources

SOURCES:

The National Institute of Allergy and Infectious Diseases: "Fact Sheet: Food Allergy and Intolerances."

UpToDate.

American Academy of Allergy Asthma & Immunology.

Advances in Dermatology and Allergology: "Role of immunoglobulin G antibodies in diagnosis of food allergy."

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