Understanding Food Reactions: Allergy, Sensitivity, and Intolerance Explained

Composition with common food allergens including egg, milk, soya, peanuts, hazelnut, fish, seafood and wheat flour

If you’ve ever experienced symptoms like irritable bowel syndrome (IBS), constipation, bloating, fatigue, weight gain, muscle aches, or joint stiffness after eating certain foods, you may be dealing with more than just a simple digestive issue. These symptoms could be indicative of food reactions, which can broadly fall into three categories: food allergy, food sensitivity, and food intolerance. While these terms are often used interchangeably, they represent distinct conditions with varying causes and symptoms. Let’s discuss each to better understand what might be causing your discomfort.

Food Allergy

A food allergy is a true immune system reaction (IgE-mediated type 1 hypersensitivity) that occurs soon after eating a particular food (or beverage). These food reactions are triggered when the immune system mistakenly identifies a food protein, or allergen, as foreign. In response, the body releases chemicals such as cytokines and histamine that contribute to symptoms including:

  • Skin reactions: Itchy skin (pruritus), flushing, hives
  • Respiratory problems: Wheezing, shortness of breath, nasal congestion, sneezing
  • Digestive issues: Nausea, vomiting, diarrhea, abdominal pain, cramping
  • Neurological: Anxiety, feeling of doom, dizziness
  • Anaphylaxis: A severe allergic reaction that can cause swelling of the throat, difficulty breathing, and a sudden drop in blood pressure

Common allergenic foods include dairy, peanuts, tree nuts, shellfish, fish, eggs, wheat, and soy. It’s important to note that even a tiny amount of the allergen can trigger a reaction in susceptible individuals, meaning that the response is not dose dependent. These reactions also typically occur within minutes to hours of ingesting the antigenic food. 

Food Sensitivity

Food sensitivity is also an immune system reaction (IgA or IgG-mediated delayed hypersensitivity) that is triggered when the immune system mistakenly identifies a specific food protein as foreign. Because the immune system is activated in a different manner, there are key differences between food allergy and food sensitivity:

  • Food sensitivity reactions are delayed and can occur hours to days after ingestion
  • Sensitivity reactions may not occur with small exposures and may be cumulative (reactions are dose-dependent)
  • Sensitivity reactions typically do not result in anaphylaxis

Food Intolerance

Food intolerance is a non-immunologic reaction to food that often involves difficulty digesting certain foods. Unlike food allergies and sensitivities that involve the immune system, food intolerances typically involve the lack of a digestive enzyme or sensitivity to a specific component. Common food intolerances include:

  • Lactose intolerance: Deficiency in the enzyme lactase, which typically breaks down lactose into glucose and galactose. Lactose is poorly absorbed in the small bowel and passes into the colon, where it is fermented by bacteria causing symptoms
  • Tyramine and histamine: These are vasoactive amines that occur naturally in certain foods that cause symptoms in individuals who are intolerant. 
  • Salicylates (eicosanoid metabolism): Certain foods that contain salicylates can cause symptoms in individuals as a result of the buildup of inflammatory mediators called leukotrienes.  
  • FODMAPs (Fermentable oligosaccharides, disaccharides, monosaccharides and polyols): Short-chain carbohydrates that the small intestine absorbs poorly, resulting in the gut microbes fermenting them 

Food intolerances typically cause symptoms within hours of ingesting the food, are usually dose-dependent, and may only cause gastrointestinal symptoms.

Key Differences

Onset and Timing:

  • Food Allergy: Immediate reaction, often within minutes to a few hours after eating.
  • Food Sensitivity/Intolerance: Delayed reaction, symptoms may appear hours to days after consuming the offending food.


  • Food Allergy: Can range from mild to severe, including potentially life-threatening anaphylactic reactions.
  • Food Sensitivity/Intolerance: Generally less severe and rarely life-threatening, though symptoms can still be uncomfortable and impact daily life.


  • Food Allergy: Diagnosed through elimination diet, skin tests, blood tests (measuring specific IgE antibodies), and oral food challenges under medical supervision.
  • Food Sensitivity/Intolerance: Diagnosis is often based on elimination diets, food diaries, and symptom tracking, sometimes supported by blood tests or specialized testing for specific sensitivities (e.g., lactose intolerance).

The gold standard for diagnosing and differentiating between food allergy, sensitivity and intolerance is the elimination diet. An elimination diet removes all of the common foods triggers for 3-4 weeks, which improves symptoms and allows the gut to heal from ongoing damage. Then, specific types of food are reintroduced in a predetermined quantity to determine if the food causes any symptoms over 2-3 days.  The elimination diet is considered the gold standard because other types of testing like blood tests may not identify the specific antibody involved in the reaction. 

It is important to note that after starting an elimination diet, symptoms may worsen for a short time (Herxheimer reaction) due to a decrease in the antigens causing symptoms, which results in immune complexes joining together and potentiating the immune response, which is responsible for symptoms. This worsening of symptoms typically occurs four to seven days after starting the elimination diet and can include changes in sleep patterns, fatigue, lightheadedness, headaches, joint or muscle stiffness, and changes in digestion. 


Understanding the differences between food allergy, food sensitivity, and food intolerance is crucial for effectively managing symptoms like IBS, constipation, bloating, fatigue, and others. By identifying trigger foods and adopting appropriate management strategies, individuals can improve their quality of life and reduce discomfort associated with these conditions.

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