Understanding Food Reactions: Allergy, Sensitivity, and Intolerance

Many people who experience adverse reactions to food don’t realize that a specific food is causing symptoms. Food reactions are often overlooked as a contributor to chronic health issues. Adverse reactions to food can be broken down into three categories: allergies, intolerances, and sensitivities.

Hand rejecting a glass of milk, symbolizing intolerance or allergy to dairy products

Food Allergies

True food allergies involve immune system reactions triggered by certain foods. These reactions typically occur immediately after consuming the trigger food and are mediated by immunoglobulin E (IgE) antibodies. When the immune system mistakes a harmless food protein for a threat, it launches an IgE-mediated response, releasing histamine and other chemicals that cause allergic symptoms.

Testing for Food Allergies

Skin prick tests or blood tests to measure the presence of specific IgE antibodies to common allergens. In some cases, an oral food challenge may be necessary to confirm the diagnosis.

Common Symptoms of Food Allergies

  • Rash
  • Swollen or itchy tongue
  • Runny nose
  • Hives
  • Abdominal pain
  • Vomiting
  • Difficulty breathing
  • Coughing
  • Wheezing
  • Closed airway

Common Food Allergens

Peanuts, tree nuts, wheat, soy, milk, fish, and shellfish.
However, any food has the potential to cause allergic reactions.

Food Sensitivities

Food sensitivities, also known as delayed food hypersensitivities, involve immune reactions that occur hours or even days after consuming a trigger food. These reactions are often linked to imbalances in the gastrointestinal system, such as intestinal permeability or “leaky gut syndrome.” Unlike allergies, which involve IgE antibodies, sensitivities may involve IgG or IgA antibodies or other immune mechanisms.

Testing for Food Sensitivities

Blood tests to measure levels of specific antibodies, such as ELISA IgG or IgA, against various food proteins. Elimination diets followed by food challenges may also be used to identify trigger foods and confirm sensitivities.

Common Symptoms of Food Sensitivities

  • Migraines
  • Headaches
  • Dizziness
  • Difficulty sleeping
  • Mood swings
  • Depression
  • Anxiety
  • Unintentional weight loss or gain
  • Dark under-eye circles
  • Asthma
  • Irregular heartbeat
  • Irritable bowels
  • Bloating
  • Excessive sweating
  • Acne
  • Wheezing
  • Runny nose
  • Sinus problems
  • Ear infections
  • Food cravings
  • Muscle or joint pain
  • Indigestion
  • Nausea
  • Vomiting
  • Bladder control issues
  • Fatigue
  • Hyperactivity
  • Hives
  • Rashes
  • Dry skin

Common Sources of Food Sensitivities

Cow’s milk and dairy products, eggs, gluten (found in wheat, rye, spelt, and barley), soy, shellfish, and tree nuts and potentially any food.

Food Intolerances

Food intolerances are non-immune reactions to specific food components, often due to a lack of digestive enzymes or nutrients necessary for breaking down those components. Unlike allergies, which involve the immune system, intolerances do not trigger an immune response. Instead, they result in digestive symptoms or other discomforts due to the body’s inability to properly process certain foods.

Testing for Food Intolerances

Elimination diets or breath tests to identify specific intolerances. Keeping a food diary and noting symptoms after eating certain foods can also help pinpoint intolerances.

Common Symptoms of Food Intolerances

  • Flushing
  • Cold or flu-like symptoms
  • Digestive issues like bloating and gas
  • Inflammation
  • General discomfort

Common Triggers for Food Intolerances

FODMAPs, dairy products, sulfites, histamines, lectins, preservatives, artificial colors, fillers, flavorings, chocolate, citrus fruits, and acidic foods.

Detailed diagram distinguishing between immune-mediated and non-immune food reactions, useful for educational purposes on food sensitivities

Image source: Food Allergy, Hypersensitivity, and Intolerances: Diagnosis and Treatment – IFM lecture 02/2020

Sensitivity or intolerance?

It’s imperative to understand that the distinction between food sensitivities and intolerances primarily lies in the nature of the body’s reaction to different components of food.

Food sensitivities are predominantly immune system reactions against proteins in foods. When the immune system identifies a specific food protein as a threat, such as gluten in wheat or casein in dairy, it mounts an immune response, which can manifest in various symptoms beyond the gastrointestinal tract. This response is indicative of the body’s defense mechanism inappropriately reacting to what it perceives as harmful substances.

On the other hand, food intolerances often pertain to the gut’s reaction to certain carbohydrates, including FODMAPs (fermentable oligo-, di-, monosaccharides, and polyols). Unlike sensitivities, intolerances do not involve the immune system but rather a difficulty in digesting certain carbohydrates due to the lack of specific enzymes or the gut’s inability to absorb them efficiently. This distinction highlights the complexity of dietary reactions and underscores the importance of targeted approaches for management and treatment.


  • Sensitivity: Non-celiac gluten sensitivity (NCGS) involves an immune response to gluten in individuals without celiac disease or wheat allergy. This condition underscores the role of the immune system in reacting to gluten, leading to a broad spectrum of symptoms that improve with a gluten-free diet. Sometimes symptoms may persist even on a gluten-free diet.
  • Intolerance: Insights suggest that symptoms attributed to gluten sensitivity might be due to fructans, a type of FODMAP present in wheat. Fructan intolerance involves digestive issues due to the body’s struggle to break down fructans, leading to IBS-like symptoms.


  • Sensitivity: Similar to gluten sensitivity, casein sensitivity involves an immune response to a protein in milk. Individuals may experience symptoms that are immune-mediated.
  • Intolerance: A condition where the body lacks lactase, necessary for breaking down lactose in milk. This deficiency leads to gastrointestinal symptoms, highlighting a non-immune reaction.


Understanding the differences between food allergies, intolerances, and sensitivities is crucial for managing dietary choices and addressing potential health issues related to food reactions. By identifying trigger foods and making appropriate adjustments to the diet, individuals can experience relief from symptoms and improve overall well-being. Consulting with a functional medicine practitioner can provide guidance on testing and managing food-related issues.

  1. American Gastroenterological Association. Food Allergies and Intolerances: Food Allergy vs. Food Intolerance. https://aga-cms-assets.s3.amazonaws.com/201821420832—All_FAI_2017.pdf. Updated July 2017. Accessed April 2, 2020.
  2. American Academy of Allergy Asthma and Immunology. Food Intolerance versus Food Allergy. https://www.aaaai.org/conditions-and-treatments/library/allergy-library/food-intolerance. Accessed April 2, 2020.
  3. Sapone, A., Bai, J.C., Ciacci, C., Dolinsek, J., Green, P.H.R., Hadjivassiliou, M., … & Fasano, A. (2012). Spectrum of gluten-related disorders: consensus on new nomenclature and classification. BMC Medicine, 10(1), 13. https://doi.org/10.1186/1741-7015-10-13
  4. Skodje, G.I., Sarna, V.K., Minelle, I.H., Rolfsen, K.L., Muir, J.G., Gibson, P.R., … & Lundin, K.E.A. (2018). Fructan, Rather Than Gluten, Induces Symptoms in Patients With Self-reported Non-celiac Gluten Sensitivity. Gastroenterology, 154(3), 529-539.e2. https://doi.org/10.1053/j.gastro.2017.10.040
  5. Misselwitz, B., Pohl, D., Frühauf, H., Fried, M., Vavricka, S.R., & Fox, M. (2013). Lactose malabsorption and intolerance: pathogenesis, diagnosis and treatment. United European Gastroenterology Journal, 1(3), 151-159. https://doi.org/10.1177/2050640613484463
  6. Vojdani, A. (2015). Immune response to dietary proteins, gliadin and cerebellar peptides in children with autism. Nutritional Neuroscience, 8(2), 88-100. https://doi.org/10.1080/10284150500069118

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