Food Allergy or Food Intolerance? The Difference Decides Everything
People use “allergy” loosely to mean any food that disagrees with them. Medically the word has a specific meaning, and the distinction determines whether you need an adrenaline auto-injector or simply a smaller portion.
Allergy
An immune reaction, usually mediated by IgE antibodies. The immune system misidentifies a food protein as a threat and releases histamine and other mediators.
Characteristics:
- Rapid onset, usually within minutes to two hours
- Reproducible with every exposure
- Can be triggered by tiny amounts, including traces
- Can involve skin, gut, airway and circulation together
- Can be life-threatening
Symptoms: hives, swelling of lips, face or throat, itching, vomiting, abdominal pain, wheezing, difficulty breathing, hoarseness, dizziness, collapse.
Common triggers: milk, egg, peanut, tree nuts, wheat, soy, fish, shellfish, sesame.
Intolerance
Not an immune reaction. Usually an enzyme deficiency, a pharmacological effect, or a digestive issue.
Characteristics:
- Slower onset, often hours later
- Dose-dependent, so a small amount may be fine
- Symptoms are limited to the gut, or are general, such as headache or fatigue
- Unpleasant but not life-threatening
Examples: lactose intolerance from reduced lactase enzyme; fructose malabsorption; FODMAP sensitivity in IBS; histamine intolerance; reactions to caffeine, sulphites or monosodium glutamate.
Coeliac disease is neither, exactly
Coeliac disease is an autoimmune condition triggered by gluten, in which the immune system damages the small intestine lining. It is not an allergy and not an intolerance, and it causes real structural damage.
Crucially, testing must be done while still eating gluten. Cutting gluten before testing produces false negatives and leaves people on a lifelong restrictive diet without ever knowing whether they needed it. If you suspect coeliac disease, get tested first.
Non-coeliac gluten sensitivity exists as a separate entity, with symptoms but no antibodies or intestinal damage, and in many people the actual trigger turns out to be fructans in wheat rather than gluten itself.
Testing
For IgE-mediated allergy:
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- Skin prick testing, fast and sensitive
- Specific IgE blood testing
- Component-resolved diagnostics, which identify which protein within a food the antibodies target, helpful for predicting severity, particularly with peanut and nuts
- Oral food challenge under medical supervision, which remains the definitive test
A critical point: skin and blood tests show sensitisation, not necessarily clinical allergy. A positive result in someone who eats the food without problems does not mean they are allergic. This is why tests are ordered to investigate a specific history, not as a screening panel across many foods. Random panel testing produces false positives and leads to unnecessary avoidance, which can itself increase the risk of developing true allergy.
For intolerance: hydrogen breath testing for lactose and fructose, and structured elimination with reintroduction, ideally with a dietitian.
For coeliac: tissue transglutaminase IgA with total IgA, while eating gluten, followed by endoscopy with biopsy where positive.
Tests to avoid
Several commercially marketed tests have no diagnostic validity for food allergy or intolerance:
- IgG food antibody panels. IgG to a food indicates exposure, not intolerance. Major allergy organisations recommend against these. They typically return a long list of commonly eaten foods, leading to unnecessary and sometimes nutritionally harmful restriction
- Hair analysis
- Applied kinesiology, or muscle testing
- Vega or electrodermal testing
- Pulse testing
- Cytotoxic testing
These are widely sold and produce confident-looking reports. That is a presentation problem, not evidence.
Prevention in children
Guidance reversed substantially after the LEAP trial and related work. Delaying introduction of allergenic foods increases rather than decreases risk.
Current advice is to introduce common allergenic foods, including peanut and egg in age-appropriate forms, from around six months alongside other solids, and to continue them regularly. Infants with severe eczema or existing egg allergy are higher risk and should be assessed before peanut introduction.
Never give whole nuts to young children because of choking risk; use smooth peanut butter thinned with water or milk.
Living with a confirmed allergy
- Carry an adrenaline auto-injector if prescribed, and carry two
- Know how to use it, and make sure family, school and colleagues do
- Use it early in a suspected anaphylactic reaction rather than waiting. Antihistamines do not treat anaphylaxis
- Call emergency services after using it, because reactions can return hours later
- Read labels every time; formulations change
- Ask about cross-contamination when eating out
- Have a written allergy action plan
- Wear medical identification
Anaphylaxis signs: difficulty breathing, throat tightness, hoarseness, swelling of tongue or throat, persistent cough, dizziness or collapse, widespread hives with any of these. Adrenaline into the outer thigh, immediately, then emergency services, and lie flat with legs raised rather than standing up.
The short version
Allergy is immune, fast, and potentially dangerous; intolerance is dose-dependent and uncomfortable. Test for coeliac disease before removing gluten, not after. Ignore IgG food panels and the alternative tests sold alongside them, and if a child is at risk of allergy, introducing allergenic foods early is now the advice rather than avoiding them.