It's not uncommon to hear someone say, "I can't eat that, I'm allergic to it." As many as one in five individuals have bad reactions to specific foods or classes of foods.1 And the number of adults with reported food allergies is increasing.1,2 

Feeling sick after eating a certain food doesn't mean you actually have a true food allergy.3 If the offending food doesn't set off a specific immune reaction in your body, then you probably have a food intolerance, rather than a real allergy.

Know the terms

Many different terms and phrases are used to refer to unpleasant food reactions, and it can be easy to mix them up. Knowing their definitions and differences will help you better communicate with your health-care professional and better understand your specific health needs.

In the United States, food allergy specialists (allergists) use the following terms.4

  • Adverse food reaction: Any unpleasant reaction that occurs after you eat a certain food or a substance in a food.
  • Food intolerance: An adverse food reaction that doesn't involve activation of your body's immune system.
  • Food allergy / food hypersensitivity: An adverse food reaction that involves an immediate immune response. According to this definition, food allergy and food hypersensitivity are the same thing.

True food allergies: Sudden and unpredictable

A true food allergy is an immune response that occurs quickly and repeatedly when you are exposed to a certain food.1 When you eat a food that you're allergic to, your body mistakenly marks it as foreign.

This triggers your immune system to release a flood of inflammatory chemicals to activate cells that cause allergy-related inflammation, including mast cells, eosinophils, and basophils.

This food hypersensitivity reaction is also known as immunoglobulin E (IgE)-mediated food allergy, because it is the food-specific IgE that binds to the food, triggering the cascade of allergic reactions.

Why does the body identify some foods as allergens? Largely because of a person’s genetic make-up, in addition to environmental factors that shape the immune response.

Certain proteins on a food drive the allergic reaction, although other factors can play a role.

Some believe that having a low level of stomach acid – which can happen if you use antacids or acid blockers – results in incomplete digestion of these proteins. This, combined with inflammation in the small intestine, can trick the immune system into thinking the protein is foreign, causing the immune system to launch a defense.

Food allergy symptoms occur quickly, usually in seconds or minutes after eating the food. In unusual cases, it can take several hours.1,3 Symptoms can affect the gastrointestinal tract, respiratory tract, and skin, and can include:3

  • Stomach pain
  • Nausea and vomiting
  • Diarrhea
  • Runny nose
  • Cough
  • Wheezing
  • Hives and itchy skin
  • Swelling of the tongue, lips, and throat

The severity of the symptom(s) has nothing to do with the amount of food you eat. Even a tiny amount of an offending food can set off an immune reaction. Also, a food that causes hives one time can cause a life-threatening symptom the next time.

Symptoms of a life-threatening food allergy reaction include:5

  • Swelling of the tongue, lips, and throat
  • Chest tightness
  • Severe wheezing
  • Inability to breathe
  • Low blood pressure leading to a weak and rapid pulse
  • Fainting or passing out

More than 170 foods are linked to IgE-mediated food allergies.3 The most common ones are cow’s milk, chicken eggs, fish, shellfish, tree nuts, peanuts, wheat, soy, and some fruits.1,6 

Pollen food allergy: Hay fever meets food allergy

Do certain raw fruits or vegetables make your tongue swell or lips itch? You could have a pollen food allergy, also called oral food allergy syndrome (OFAS). It's a type of contact allergy that affects people who are allergic to pollen, usually those who have hay fever.1,4 

Pollen food allergy occurs when proteins in fruits and vegetables are cross-reactive with allergy-triggering pollen proteins. If you have this type of allergy, then eating a raw fruit or vegetable with these pollen proteins will trigger mild symptoms in the lining of your mouth and throat, although they can lead to a severe reaction in rare cases.4,6 

Which fruit or veggie affects you will depend on what type of pollen triggers your seasonal allergy symptoms.1

Pollen allergy                   Associated food allergies

Birch tree                            Apple, pear, cherry, carrot, kiwi, raw potato

Ragweed                              Watermelon, cantaloupe, honeydew, banana

Mugwort                              Celery

Heating or cooking the food will destroy most of the allergy-causing substances in OFAS.1 

Possible food allergies

Food allergy science is constantly evolving and being studied. One debated topic is whether immunoglobulin G (IgG) antibodies are to blame for hidden food allergies. The body can produce IgG antibodies to specific foods even under normal circumstances. Some individuals have noticed they are more sensitive to the inflammatory processes caused by these substances, although their role in food allergies is controversial.

Food intolerance: When digestion goes awry

A food intolerance is not the same thing as a food allergy. If you have a food intolerance, then your body has trouble digesting or breaking down a certain food or chemical in a food. An overactive immune system response isn't to blame.1,7 Instead, your body lacks a protein needed to properly digest a food, or there is a problem with how your gut processes the food, or your symptoms are triggered by specific substances in food. Sometimes the cause of a food intolerance is unknown.4 

Most adverse food reactions are due to food intolerances.

Symptoms often only involve stomach problems, such as stomach pain, bloating, gas, and diarrhea, although more systemic symptoms, including migraines and other body aches and pains are not uncommon.

Unlike with a true food allergy, with a food intolerance, symptom severity is directly related to the amount of the food you eat. For example, if ice cream upsets your digestion, then the more you eat, the worse you will feel.1 

Several types of food intolerances are common

Lactose intolerance (lactase deficiency)

Individuals with this condition do not have the lactase enzyme needed to digest lactose, a natural sugar in milk, ice cream, and other dairy products. Symptoms include stomach cramps, gas, bloating, and diarrhea. Lactose intolerance is not the same as a milk allergy (an allergy to certain milk proteins such as casein).3

Carbohydrate intolerance

Some individuals who have irritable bowel syndrome (IBS) have more stomach pain, bloating, diarrhea, and gas when they eat foods containing one or more types of carbohydrates called FODMAPs – fermentable oligosaccharides, disaccharides, monosaccharides, and polyols. FODMAP carbs include fructose, lactose, sorbitol, fructans, and galactooligosaccharides. Fructans are found in high amounts in foods that contain gluten, which causes the mistaken belief that this stomach distress is due to a gluten sensitivity.8 This article provides information about a low FODMAP diet.

Alcohol dehydrogenase deficiency

Feeling flushed can happen to anyone after a few alcoholic drinks, because alcohol's effect on blood vessels can cause facial redness. But if the flushing is severe, then you could be missing an enzyme called alcohol dehydrogenase (ADH) or your ADH might not be working as it should. ADH is needed to break down alcohol you drink or alcohol made in the body. This type of food intolerance is more common in those of Asian descent.1,9

Gluten intolerance

Gluten is a protein found mostly in wheat, rye, and barley. There has been much focus recently on gluten and how gluten affects health. Concerns about gluten's ill effects have prompted many stores and manufacturers to label gluten-free items so they are easier to see. Current sale of gluten-free products is $2.6 billion.9,10

Gluten intolerance is not the same as celiac disease, an autoimmune disorder in which gluten can cause serious, even life-threatening complications.4,5 Celiac disease causes damage to the wall of the small intestine; whereas, gluten intolerance does not.

To differentiate between the two conditions, some practitioners refer to gluten intolerance as non-celiac gluten sensitivity.4,11

Gluten intolerance can cause major stomach discomfort, including bloating, cramping, gassiness, diarrhea, and constipation. Some individuals who are gluten intolerant also report having skin rashes, headaches, fatigue, mood disturbances, and muscle cramps after eating gluten.

Biogenic amines: Histamine, tyramine

Foods rich in substances called biogenic amines, nitrogen-containing compounds, might be the culprit behind several food intolerances.1,3 Amine levels naturally rise when a food ripens or spoils.

One type of amine that might be linked to food intolerance is tyramine, found in aged cheeses. It triggers migraines in people who have a food intolerance to the substance.1

Histamine is another biogenic amine. Some individuals feel bloated and have stomach pain, diarrhea, constipation, flushing, itching, and other symptoms after eating histamine-containing foods like strawberries, certain citrus fruits, shellfish, fermented foods, smoked meats, aged cheese, and some alcoholic beverages (red wine in particular). There is ongoing debate over the role of histamine from foods causing these symptoms.

Stale or spoiled fish and shellfish can have very high levels of histamine, which can cause symptoms of food poisoning within hours of eating. The symptoms include vomiting, headache, flushing, hives, and wheezing, and occasionally can mimic a bad allergic reaction. An imbalance in intestinal bacteria (gut microbiota) also might contribute to higher histamine levels in the body.12,13

The food allergy / food intolerance disease connection

Understanding and treating food allergies and food intolerances is critical to good overall health.4,14 Food allergies can make asthma and eczema worse. Some conditions that can be triggered by or made worse by food intolerances include:

  • Sleep issues
  • Irritable bowel disease
  • Migraines and other headaches
  • Mood disorders, including anxiety and depression15 
  • Arthritis, including ankylosing spondylitis, psoriatic arthritis, and rheumatoid arthritis14 
  • Vertigo

More research is needed to establish a definitive link between certain chronic diseases and foods. Diagnosing a food allergy or food intolerance is challenging and might require complete elimination of a suspected food or class of foods.1

If you often feel sick after eating or have a chronic health condition or family history of any type of allergies, then ask your health-care professional if you should be tested for food allergies. Keeping a detailed diary of when and what you eat, and any symptoms you have, is helpful.4

Avoiding foods that trigger your symptoms will help you feel better. It’s always best to discuss your symptoms, diet, and health history with your health-care professional before trying an elimination diet and food reintroduction challenge.4


  1. Commins S. Food allergy and food intolerance in adults: an overview. [Accessed April 2, 2019]
  2. Iweala O, Choudhary S, Commins S. Food allergy. Curr Gastroenterol Rep2018;20:17.
  3. DeGeeter C, Guandalini S. Food sensitivities: fact versus fiction. Gastroenterol Clin North Am 2018;47:895-908.
  4. Samson H. Food allergies. In: Feldman M, ed. Sleisenger and Fordtran's Gastrointestinal and Liver Disease: Pathophysiology, Diagnosis, Management. 10th ed. Philadelphia, Pa.: Saunders Elsevier; 2016:148-157. 
  5. Anaphylaxis. [Accessed May 17, 2019.]
  6. Divekar R (expert opinion). Mayo Clinic, Rochester, Minn. May 28, 2019. 
  7. Gluten intolerance definition. American Academy of Allergy, Asthma & Immunology. [Accessed April 11, 2019.]
  8. Gaby A. Food allergy and intolerance. In: Rakel D, ed. Integrative Medicine. 4th ed. Philadelphia, Pa.: Elsevier; 2018:310-318.
  9. Fazio S. Approach to flushing in adults. [Accessed April 28, 2019] 
  10. Celiac disease, non-celiac gluten sensitivity, and food allergy: how are they different? American Academy of Allergy, Asthma & Immunology. [Accessed April 11, 2019]
  11. Acker W, Plasek J, Blumenthal K, et al. Prevalence of food allergies and intolerances documented in electronic health records. J Allergy Clin Immunol 2017;140:1587-1591.
  12. Schink M, Konturek P, Tietz E, et al. Microbial patterns in patients with histamine intolerance. J Physiol Pharmacol 2018 Aug;69(4):579-593.
  13. Schnedl W, Lackner S, Enko D, et al. Evaluation of symptoms and symptom combinations in histamine intolerance. Intest Res 2019 Mar 7. In press.
  14. Niu Q, Wei W, Huang Z, et al. Association between food allergy and ankylosing spondylitis: an observational study. Medicine (Baltimore) 2019;98:e14421.
  15. Feng C. Beyond avoidance: the psychosocial impact of food allergies. Clin Rev Allergy Immunol 2018 Sep 1. [In press]