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Food Allergies vs. Intolerance: Diagnose Them Correctly


By Dr.Divya Goil

Table of Contents 

1. What are the most common food allergens?
2. Pathophysiology of Type 1 hypersensitivity reactions and food allergy
3. Symptoms of food allergies
4. Diagnosis and management of food allergy
5. The situation in India
6. Food intolerances
7. All about gluten: Celiac disease, Wheat allergy & Gluten sensitivity
8. References


These terminologies are often confused with each other. Food allergies are IgE-mediated immunological hypersensitivity reactions to certain food proteins, whereas food intolerance occurs in the gastrointestinal tract due to the inability to digest a certain kind of food.

What are the most common food allergens?

Eggs, milk, shellfish, peanuts, soy, sesame, tree nuts, and wheat are the most common allergens found worldwide.

In India, eggs, milk, sesame, gram/dal, fruit and vegetable proteins, wheat, and nuts have been documented to cause allergic reactions and anaphylaxis.

Pathophysiology of Type 1 Hypersensitivity Reactions and Food Allergies

Image 1. Pathophysiology of Type 1 Hypersensitivity Reactions. Kuby Immunology, Sixth edition
© 2007 W.H Freeman and Company


On first-time exposure to these foods in susceptible atopic individuals, IgE antibodies are produced that bind to mast cells and basophils (sensitization). On repeat exposure, the allergen binds to the IgE antibodies and causes mast cell degranulation and release of mediators that cause the allergic reaction.

The pathogenesis of food allergy relies on immune reactions triggered by epitopes - small amino-acid sequences that bind to antibodies or antigen-presenting cells.

These allergens encounter antigen-presenting dendritic cells in the GIT, which leads to T-cell priming. In the case of IgE-mediated allergy, the T-cells trigger the production of allergen-specific IgE by B-cells.

Symptoms of Food Allergies

Allergic reactions can have variable presentations of variable severity such as:

  • Asthma
  • Stridor
  • Rhinitis
  • Urticaria
  • Swelling of lips, tongue, and periorbital regions
  • Angioedema
  • Atopic dermatitis
  • Eosinophilic esophagitis
  • Gastrointestinal symptoms
  • Life-threatening anaphylactic shock and cardiovascular collapse. 
  • Food allergies may also present with chronic conditions such as atopic dermatitis and eosinophilic esophagitis. 

Diagnosis and Management of Food Allergies

For diagnosing food allergies, detailed clinical history, physical examination, and knowledge about local allergens is essential. 

  • History-taking questions - Temporal relationship of allergen exposure and the onset of clinical features, periodicity of allergic reactions, aggravating/relieving factors, travel history, history of atopy in the family, and occupational history can give clues to the cause. 

  • Important investigations - The investigations that can be done are the skin prick test, RAST, and food-specific IgE antibody measurement. Food elimination challenges and diet diaries are also useful for assessing the cause.
    IgG testing, though still done in some centers, does not indicate a food allergy. Specific IgG4 antibodies do not suggest hypersensitivity to the food, but are a normal physiological response and are a marker of immune tolerance due to previous exposure to the food.
  • Management - Strict elimination of the offending food is recommended. The latest in the management of food allergies is allergen-specific immunotherapy, however, this isn’t commonly available in India yet. In the case of anaphylaxis, injectable epinephrine is the drug of choice. Patients should be asked to carry a card on their person at all times with details of the food allergies, to make an anticipatory plan, and carry self-injectable epinephrine on them at all times.

The Situation in India

In India, there are no updated records of the range of allergens found, no database of people affected, and no systematic epidemiological data. Although it is estimated that food allergy is not as much of a problem here as in western countries, healthcare professionals are noticing a rising trend in practice. However, until we have systematic reports, it is hard to estimate or manage the current situation accurately. 

Allergy in India remains a relatively undefined entity and the diagnosis and management are thus affected.

Food Intolerances

Food intolerance is a differential diagnosis to food allergies due to certain overlapping gastrointestinal symptoms, however, intolerance is not immunologically mediated. 

It occurs in the gastrointestinal system when a person isn’t able to digest a certain food due to enzyme deficiencies, sensitivity to food additives, or reactions to naturally occurring chemicals in foods. 

Taking the classic example of milk protein allergy vs lactose intolerance, the differences between the allergy and intolerance have been examined below.


Milk Protein Allergy

Lactose Intolerance


IgE mediated immunological reaction

Deficiency of lactase enzyme required to digest lactose sugar – either genetic or post intestinal infection.

Offending Agent

Bovine milk protein

Mammalian milk carbohydrate



High; It is estimated that two-thirds of the world’s population and 58-64% of Indians are lactose intolerant.

Age of Onset

Usually in infancy

Lactose enzyme declines as we grow older and intolerance is thus usually observed in adults.


Respiratory, GI, Skin, Anaphylaxis

Only GI – abdominal pain, bloating, nausea, diarrhea, flatulence, borborygmi

Onset of symptoms

Usually within 2 hours of exposure

30 minutes to 2 hours post-exposure




Serum IgE



Other tests

Skin Prick Test, RAST, Challenge Test, Food Specific IgE

Reducing substances in stool test, Hydrogen Breath Test, Jejunal Biopsy, Lactose Tolerance Test

Table 1. Milk Protein Allergy vs. Lactose Intolerance.

Lactose intolerance can be managed by avoiding lactose-containing foods like milk. Fermented products like yoghurt tend to be better tolerated. Alternatives like pre-hydrolysed milk and plant-based milk alternatives like soy, oat and almond milk can be consumed, though since their nutrition profile varies, appropriate dietary modifications should be made. A preparation of lactase enzyme, usually in pill form, may be used as replacement therapy. Calcium and Vitamin D supplements are also generally recommended. 

All About Gluten

The disorders associated with gluten are – Celiac disease, Wheat Allergy and Gluten Sensitivity. They are currently rising in prevalence worldwide, either being diagnosed more due to increasing awareness and increasing in prevalence due to global changes in diet. 

Gluten is a family of proteins contained in foods such as wheat, barley and rye.

Celiac disease is an autoimmune disease, not an allergy, to gliadin – the alcohol-soluble fraction of gluten, manifesting with both intestinal and extraintestinal symptoms. The antibodies found are IgA anti-EMA, IgA anti-tTG, IgG anti-DGP, IgA anti-gliadin and duodenal villi atrophy on biopsy.

This is a classic IgE-mediated food allergy, induced by wheat (not only the gluten component) intake, that leads to type I and type IV hypersensitivity reactions. The diagnosis is made as with all other food allergies. Specific serum IgE antibodies against wheat and gliadin are usually seen and duodenal villi atrophy may or may not be present. Patients should be recommended complete avoidance of wheat and wheat-containing foods to avoid adverse effects.

NCGS is a diagnosis of great debate as not enough studies have been performed on it to firmly establish it as an entity of its own. Patients usually present with intestinal symptoms such as abdominal pain, bloating, diarrhea, and flatulence – however, these are common to other conditions such as IBS and Crohn’s as well. 50% of these patients have anti-gliadin antibodies and duodenal biopsy shows either normal mucosa or mild inflammatory changes without villus atrophy.
The Salermo diagnostic guidelines recommend a 6-week course of a gluten-free diet—with continuous assessment of symptoms, followed by measuring the effect of reintroducing gluten into the patient’s diet. Eliminating foods that cause or exacerbate symptoms is recommended. 


Doctors need to be aware of these food allergies and intolerances as the symptoms are usually non-specific and common to many other conditions, but with variable treatment. More research and systematic databasing are required so that food allergies receive due importance in India to ensure better diagnostic and treatment outcomes.

Med Train in collaboration with the International Asthma Society, University of Colorado, American Academy of Allergy Asthma and Immunology, and Bharati Vidyapeeth has started the 6-month distance learning “Allergy Asthma Certificate Course” for 100 ambitious doctors who want to take forward the field of allergic disorders.




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