Food allergy: everything you need to know about food allergies

Reactions triggered by food can occur suddenly, within 2 hours of ingestion, or delayed, up to 48 hours later. This fact sheet focuses only on immediate reactions caused by a food allergy. To learn more about gluten intolerance, food poisoning or food sensitivities, please consult our factsheets dedicated to these subjects.

A food allergy is an abnormal defense reaction of the body following the ingestion of a food.

Often, the symptoms are mild: tingling on the lips, itching or a rash. But for some people, the allergy can be very serious and even fatal. In this case, the food or foods in question must be banned. In France, 50 to 80 people die each year from a food allergy.

Food allergies usually appear before the age of 4. At this age, the digestive and immune systems are not yet mature, making them more susceptible to allergies.

There is no cure. The only solution is to ban the consumption of allergenic foods.

Note: Although it is quite rare, some people react strongly to the ingestion of various food additives. The reaction can be a true allergy if the additive, even if it does not contain protein, has been contaminated by another food containing protein. For example, soy lecithin, which is not allergenic, can be contaminated with soy protein. But most often, it is a food intolerance with symptoms similar to those of an allergy. Additives such as sulfites, tartrazine and salicylates can cause an anaphylactic reaction or asthma attack. One in 100 people with asthma is sensitive to sulfites2.

Symptoms of food allergy

Signs of allergy usually appear within minutes of eating the food (and up to 2 hours later).

They vary in nature and intensity from person to person. They may include any of the following symptoms, alone or in combination

  • Skin symptoms: itching, rash, redness, swelling of lips, face and limbs.
  • Respiratory symptoms: wheezing, swollen throat, difficulty breathing, choking.
  • Digestive symptoms: abdominal cramps, diarrhea, colic, nausea and vomiting. (If these are the only symptoms detected, the cause is rarely a food allergy).
  • Cardiovascular symptoms: pallor, weak pulse, dizziness, loss of consciousness.


  • To be considered an anaphylactic reaction, the symptoms must be very pronounced. Usually more than one system is affected (skin, respiratory, digestive, cardiovascular).
  • For anaphylactic shock to occur, there must be a drop in blood pressure. This can lead to loss of consciousness, arrhythmia and even death.


The physician usually begins by asking about the patient’s personal and family history. He or she will ask questions about the onset of symptoms, the content of meals and snacks, etc. Finally, the doctor completes the diagnosis by performing one or more of the following tests, depending on the case

  • Skin tests. A drop of a series of solutions, each containing a small amount of the allergen, is applied to different areas of the skin. Then, using a needle, the skin is gently pricked where the extract is located.
  • Blood tests. The UNICAP laboratory test measures the amount of antibodies (IgE or immunoglobulin E) to a particular food in a blood sample.
  • Challenge test. This test requires the ingestion of a progressive amount of a food. It is only done in a hospital, with an allergist.

The main allergenic foods

The most allergenic foods are not the same from one country to another. They vary according to the type of food. For example, in Japan, allergy to rice predominates, while in Scandinavian countries, allergy to fish predominates. In Canada, the following foods account for approximately 90% of severe food allergies4 :

  • peanuts (peanuts);
  • nuts (almonds, Brazil nuts, cashews, hazelnuts or filberts, macadamia nuts, pecans, pine nuts, pistachios, walnuts)
  • cow’s milk;
  • eggs;
  • fish;
  • seafood (especially crab, lobster and shrimp)
  • Soybeans;
  • wheat (and related cereal varieties: kamut, spelt, triticale)
  • sesame seeds.

Allergy to cow’s milk is the most common allergy in infants before the introduction of solid foods. It occurs in approximately 2.5% of newborns1.

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What is an allergic reaction?

When the immune system is functioning properly, it detects a virus, for example, and produces antibodies (immunoglobulins or Ig) to fight it. In the case of a person with a food allergy, the immune system reacts in an inappropriate way: it attacks a food, believing it to be an aggressor to be eliminated. This attack causes damage, and the effects on the body are multiple: itching, redness of the skin, production of mucus, etc.

These reactions result from the release of several pro-inflammatory substances: histamine, prostaglandins and leukotrienes. It should be noted that the immune system does not react to all components of a food, but only to one or a few substances. It is always a protein; it is impossible to be allergic to a sugar or a fat.

In theory, the symptoms of allergy appear at the time of the 2nd contact with the food. At the first contact with the allergenic food, the body, more specifically the immune system, becomes “sensitized”. The next time it comes into contact with the food, it is ready to react. The allergy therefore develops in 2 stages.


These are allergies to substances that are chemically similar. For example, a person who is allergic to cow’s milk may also be allergic to goat’s milk because of the similarity of their proteins.

Some people who know they are allergic to a particular food prefer not to eat other foods in the same family for fear of triggering a severe reaction. However, it is best to consult a physician before making this decision, as excluding foods can create deficiencies. Cross-allergies can be identified by skin testing.

Here is an overview of the main cross allergies .

If allergic to:Possible reaction with:Risk assessment:
A legume (peanut is one of them)Another legume5%
PeanutA nut35%
A nutAnother nut37% to 50%
A fishAnother fish50%
A cerealAnother cereal20%
SeafoodAnother seafood75%
Cow’s milkBeef5% to 10%
Cow’s milkGoat’s milk92%
Latex (gloves, for example)Kiwi, banana, avocado35%
Kiwi, banana, avocadoLatex (gloves, for example)11%

Sometimes people who are allergic to pollen are also allergic to fresh fruits, vegetables or nuts. This is called oral allergy syndrome. For example, a person who is allergic to birch pollen might have itchy lips, tongue, palate and throat when eating a raw apple or carrot. Sometimes swelling of the lips, tongue and uvula, and a tightness in the throat may occur.

The symptoms of this syndrome are usually mild and the risk of anaphylaxis is low. This reaction occurs only with raw products since cooking destroys the allergen by changing the structure of the protein. Oral allergy syndrome is a form of cross-reactive allergy.


Allergies that tend to fade or disappear over time: cow’s milk, egg and soy allergies.
Allergies that tend to persist throughout life: peanut, tree nut, fish, seafood and sesame allergies.

Anaphylactic reaction and shock

It is estimated that 1% to 2% of the Canadian population is at risk of anaphylactic reaction6 , a sudden severe allergic reaction. Approximately 1 in 3 anaphylactic reactions are caused by a food allergy3. If not treated promptly, the anaphylactic reaction can progress to anaphylactic shock, which is a drop in blood pressure, loss of consciousness and possibly death, within minutes (see symptoms below). The word anaphylaxis comes from the Greek words ana = opposite and phulaxis = protection, to mean that this response of the body goes against what we want.

Allergies to peanuts, tree nuts, fish and seafood are the most common ones involved in anaphylactic reactions.

Vapours and odours: can they cause an anaphylactic reaction?

As a general rule, as long as the allergenic food is not ingested, it is very unlikely that a severe allergic reaction will occur.

However, a person who is allergic to fish may experience mild respiratory symptoms after breathing in the fumes from cooking fish, for example. When fish is heated, its proteins become very volatile. Therefore, if you have a fish allergy, you should not bake fish fillets with other foods at the same time to avoid contamination. Inhalation of food particles can cause an allergic reaction, but only mildly

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However, most of the time, smelling a food you are allergic to in the kitchen will simply create a reaction of disdain, without a true allergic reaction.

De plus en plus fréquente?

Today, 5% to 6% of children have at least one food allergy3. Some allergies diminish or disappear with age. It is estimated that nearly 4% of adults live with this type of allergy3.

According to a report by the Centers for Disease Control and Prevention, the U.S. government agency responsible for prevention, the prevalence of food allergy increased by 18% in children under 18 years of age between 1997 and 2007.20 The number of severe reactions also increased. However, as the authors of 2 studies published in 201021,22 point out, the prevalence statistics on food allergies vary greatly from one study to another. And although there appears to be an upward trend, it cannot be stated with certainty.

Overall, allergic diseases (some cases of eczema, allergic rhinitis, asthma and hives) are more common today than they were 20 years ago. The predisposition to allergies, called atopy in medical jargon, is becoming more and more widespread in the West. To what do we attribute the progression of these atopic diseases?

People and risk factors for food allergy

People at risk for food allergies

  • Children who suffer from eczema, asthma, hives or hay fever.
  • Those who have one or both parents who also have one of these forms of allergy. Only 5% to 15% of people with food allergies have no family predisposition.
  • Obese children, possibly. A U.S. study involving 4,200 children found that obese children were at greater risk of developing milk allergy.8 The causal link between obesity and food allergy is not clear. A causal link between obesity and food allergies has not been proven. It is possible that the chronic state of inflammation present in obese individuals contributes to the development of allergies12. There may also be a link between asthma and overweight.

People at risk of an anaphylactic reaction

  • People who have had a previous anaphylactic reaction.
  • People who, in addition to having food allergy(ies), also have asthma, especially if the disease is poorly controlled.
  • Adolescents are considered to be more at risk. They tend not to inform others of their food allergies and do not carry their epinephrine auto-injector with them at all times.

Remark. An unusual case shows that food allergy can be transmitted by organ transplantation19. A 42-year-old woman developed a peanut allergy (with anaphylactic reaction) after a liver transplant. The organ donor was allergic to this food.

Risk factors

It is difficult to know why a food allergy develops. Some risk factors are currently being studied.

See our Allergies fact sheet to learn more about the factors that could explain the increase in the number of people with allergies to foods or other types of allergens (pollen, latex, etc.)

Medical treatments for food allergy

Parents who suspect that their child has a food allergy should inform their doctor as soon as possible. An evaluation by an allergist may be suggested by the physician, who can confirm an allergy using the tests described above.

There is no treatment that can eliminate a food allergy. The only option is to strictly ban the food (or ingredient) that causes the allergy from the diet. For tips on how to do this, read the Prevention section.

Some people with allergies carry an epinephrine auto-injector with them at all times so that they can react quickly if an anaphylactic reaction is triggered. This decision to carry the auto-injector is made with the physician.

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Treatment of anaphylactic reaction

If breathing problems and severe swelling are signs that an anaphylactic reaction is underway, the only medication that can be used is epinephrine, also called adrenaline. It provides rapid relief of symptoms.

Epinephrine has been synthetically manufactured since 1900. It is available as an injectable solution, with or without a prescription (in which case it is not covered by insurance). It is injected into the thigh muscle on the outer side.

In North America, there are two brands of epinephrine auto-injectors on the market: Epipen®, which contains a single dose, and Twinject®, which contains two doses. The second dose of Twinject® is injected differently (see video demonstration in Sites of Interest). In this case, the second dose can be administered if symptoms worsen or do not improve. Wait 5 to 15 minutes before injecting the second dose. Usually only one dose is needed. Both brands of auto-injectors are available in 0.15 mg and 0.3 mg doses. The doctor will choose the dose based on the person’s weight. Epinephrine works quickly, but its effect is short-lived, which is why a second dose is sometimes necessary.

Note that adrenaline is a hormone that the body naturally releases in situations of stress or fear, and during physical activity. It dilates the airways, increases the heartbeat and tightens the small blood vessels that irrigate the skin and the digestive system. This hormone mobilizes all the resources of the body, to react quickly in an emergency situation.

What is important to know

It is very important to inform those around you of your condition (at school, at work, at leisure, etc.), which creates a safety net. You should also always carry your epinephrine auto-injector with you. You can get a case that you attach to your belt. Also wear your MedicAlert® bracelet, if applicable. During an anaphylactic reaction, the person may not be able to ask for help.

Some important information to pass on to those around you

  • Symptoms of an anaphylactic reaction (read the Symptoms section).
  • How to administer epinephrine using Epipen® and Twinject® auto-injectors (see video demonstrations in the Sites of Interest section).

How to react?

  • When you detect an anaphylactic reaction, act quickly.
  • Administer the dose of adrenaline contained in the autoinjector.
  • Call emergency medical services (9-1-1).
  • If the symptoms do not improve after the first injection of epinephrine and you have not yet obtained medical help, you can inject a second dose of epinephrine (the second dose of Twinject®, if needed). 5 to 15 minutes after the first injection.
  • Doctors monitor the patient for at least 4 hours to ensure that the reaction is definitely over 6 . It has happened that seizures are exacerbated after a few hours, without further exposure to the allergen.

After using an auto-injector, you should take the empty auto-injection device to a pharmacist and get a new one. In addition, epinephrine is sensitive to light and air. Watch the expiration date of the product.

In an emergency situation, doctors advise using epinephrine first. Other medications, such as antihistamines or bronchodilators (commonly used by people with asthma), have no effect on a severe reaction that has already occurred. Check with your doctor.

Important. It is important to note that beta-blocker medications, such as those used to treat high blood pressure, decrease the effectiveness of adrenaline. These medications are not recommended for people who need an epinephrine auto-injector.

Nutritional advice

In order to manage the dietary changes brought on by a food allergy and to find allergen-free substitutes and recipes, the advice of a nutritionist can be very useful. She will also be able to assess whether dietary supplements are necessary. Associations dedicated to food allergies can also be helpful. See the section Sites of Interest.

Note. Some forms of allergy (allergic rhinitis, insect venom allergy) can be reduced in intensity with desensitization treatment. Carrying out this type of treatment in the case of a severe food allergy is very dangerous, as it involves exposing the allergic person to increasing doses of the allergen. In fact, ingesting even a small dose of the allergenic food can trigger a potentially fatal anaphylactic shock. The only safe way to verify that a food allergy has subsided or disappeared is to perform a medical skin reaction test or to measure the immunoglobulins (antibodies) specific to the food in the blood.