Food Intolerance or Food Allergy? How to Tell the Difference
Quick answer: A food allergy is an immune reaction to a food protein. It usually starts within minutes to two hours, can be triggered by tiny amounts, and can be life-threatening. A food intolerance does not involve that allergic immune response. It is usually digestive (bloating, gas, loose stools, pain), depends on how much you eat, and often appears hours later. Allergies need medical testing by an allergist; intolerances are usually identified through careful tracking and structured food trials.
- Allergy: immune system, small amounts, fast reaction, possible hives, swelling or breathing difficulty.
- Intolerance: mostly digestive, dose-dependent, slower, uncomfortable but usually not dangerous.
- IgG "food intolerance" blood tests are not recommended by allergy specialists.
- Swelling of the lips or throat, wheezing or feeling faint after eating is an emergency: use your adrenaline auto-injector if you have one, then call 112.
"I think I'm allergic to dairy" is one of the sentences I hear most often in sessions. Sometimes it is a true allergy. Far more often it is an intolerance, or a gut that is reacting to the amount, the combination or the timing of a food rather than the food itself. The difference matters, because it changes what you should do next and who you should see.
If bloating is your main symptom, you may also want to read why you can feel bloated even when you eat healthily, and if you are wondering whether your symptoms fit a wider pattern, my free gut health quiz is a good place to start.
What is a food allergy?
A food allergy happens when your immune system mistakes a food protein for a threat. In the most common type, the body makes IgE antibodies against that protein. Next time you eat it, those antibodies trigger the release of histamine and other chemicals, and symptoms appear quickly, usually within minutes and almost always within two hours [1].
Typical signs include hives or an itchy rash, swelling of the lips, face or eyes, tingling or itching in the mouth, vomiting, stomach cramps, sneezing, wheezing or a tight throat. Even a very small amount of the food can trigger a reaction. Common culprits include peanuts, tree nuts, fish, shellfish and sesame; milk and egg allergies are more typical in childhood. Any food can be involved.
There are also non-IgE food allergies, more common in babies and young children, which mainly cause delayed gut and skin symptoms. These still involve the immune system and should be managed with a doctor.
What is a food intolerance?
A food intolerance means your body has difficulty digesting or processing a food, without that allergic immune reaction [2]. The most common examples are:
- Lactose intolerance — low levels of the enzyme lactase, so milk sugar reaches the large intestine and is fermented, causing gas, bloating and loose stools.
- Fermentable carbohydrates (FODMAPs) — fructans in wheat, onion and garlic, fructose in some fruit, polyols in sugar-free products. These are a frequent trigger in IBS.
- Histamine intolerance — difficulty breaking down histamine from foods such as aged cheese, wine and cured meats. You can read more in my article on histamine intolerance: foods, symptoms and what the evidence says.
- Reactions to food additives such as sulphites, or to caffeine and alcohol.
Intolerances are dose-dependent. A splash of milk in coffee may be fine, while a large latte and a bowl of ice cream on the same day are not. Symptoms often appear several hours later, and sometimes the following day, which is exactly why they are so hard to pin down.
Food allergy vs food intolerance: the key differences
| Food allergy | Food intolerance | |
|---|---|---|
| What is involved | Immune system (usually IgE antibodies) | Digestion or metabolism (enzymes, fermentation, sensitivity) |
| Amount needed | Tiny amounts can trigger a reaction | Usually depends on the dose |
| Timing | Minutes to 2 hours | Often several hours, sometimes next day |
| Typical symptoms | Hives, swelling, itching, vomiting, wheeze, faintness | Bloating, gas, abdominal pain, diarrhoea, nausea, sometimes headaches or fatigue |
| Can it be life-threatening? | Yes (anaphylaxis) | Usually not, although it can affect quality of life a lot (some sulphite reactions can trigger asthma) |
| Who confirms it | GP and allergist (skin prick, specific IgE, supervised food challenge) | Symptom tracking, structured elimination and reintroduction, some breath tests |
| Usual approach | Strict avoidance and an emergency plan | Finding your personal threshold, often not total avoidance |
Is coeliac disease an allergy or an intolerance?
Neither. Coeliac disease is an autoimmune condition in which gluten triggers the immune system to damage the lining of the small intestine. It can cause bloating, diarrhoea, constipation, fatigue and anaemia, and it needs to be ruled out by your doctor with a blood test. That test is only reliable while you are still eating gluten, so please do not remove gluten before speaking to your GP [3].
Are food intolerance tests reliable?
Many clients arrive with a printout from an IgG food intolerance test listing twenty or thirty "reactive" foods. I understand why these tests are appealing, but the European Academy of Allergy and Clinical Immunology (EAACI) has stated clearly that IgG4 testing against foods should not be used to identify food allergy or intolerance [4], and the Canadian Society of Allergy and Clinical Immunology reached the same conclusion for IgG food testing in general [6]. IgG antibodies to foods are a normal response to eating them. A high result often simply reflects that you eat that food regularly, not that it is harming you.
The real cost of these tests is not the price. It is the long list of foods people remove unnecessarily, which can make the diet less varied, reduce the fibre that feeds your gut bacteria and increase anxiety around eating.
Tests that do have a place include:
- Skin prick tests and specific IgE blood tests, interpreted by an allergist alongside your history, for suspected allergy.
- Coeliac blood tests through your doctor.
- Hydrogen breath tests for lactose or fructose malabsorption, in some cases.
- A structured elimination and reintroduction, ideally with support, which remains the most practical way to identify intolerances [2].
How to work out what is going on
- Write down what happens and when. Note the food, the amount, the time and when symptoms start. A food and symptom diary makes patterns visible surprisingly quickly.
- Look at timing first. Reactions within minutes with skin, mouth or breathing symptoms point towards allergy and need a doctor. Digestive symptoms hours later point towards intolerance.
- Consider the dose. If a small amount is fine but a large amount is not, an intolerance is much more likely.
- Change one thing at a time, for two to four weeks, then reintroduce it deliberately. Removing many foods at once makes it impossible to know what helped.
- Look at the gut as a whole. Intolerances often improve when digestion, bowel habit, stress and the gut microbiome are supported, rather than staying fixed for life.
When it is an emergency, and when to see a doctor
If, after eating, you have swelling of the lips, tongue or throat, difficulty breathing or swallowing, wheezing, a hoarse voice, or you feel faint or collapse, this is an emergency. Use an adrenaline auto-injector straight away if you have one, then call 112 (or your local emergency number). Lie down with your legs raised (or sit up if breathing is difficult), and use a second auto-injector after 5 minutes if there is no improvement [5]. See your GP and ask about referral to an allergist if you have ever had hives, swelling or mouth itching after a food, if reactions are getting stronger, or before reintroducing any food you think you may be allergic to. Also see your doctor for weight loss, blood in your stool, anaemia or persistent diarrhoea.
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Book your free 15-minute call →Frequently asked questions
What is the main difference between a food allergy and a food intolerance?
A food allergy is an immune reaction that can be triggered by tiny amounts of a food, appears quickly and can be dangerous. A food intolerance is a digestive difficulty that depends on how much you eat, appears more slowly and is uncomfortable but usually not dangerous.
Can a food intolerance turn into an allergy?
Not usually, because they involve different mechanisms. However, new allergies can develop in adulthood, so any new reaction involving hives, swelling or breathing should be checked by a doctor.
Are IgG food intolerance tests worth doing?
Allergy specialists do not recommend them. IgG antibodies to foods are a normal response to eating those foods and do not reliably show intolerance. They often lead to removing foods unnecessarily.
Can food intolerances go away?
Often, yes, or at least improve. Many intolerances depend on the state of the gut, such as bacterial balance, digestion and gut sensitivity, so tolerance frequently increases when those are supported.
How long after eating do intolerance symptoms appear?
Usually from about 30 minutes to several hours, and sometimes the next day. Allergic reactions typically start within minutes and almost always within two hours.
Can I work with you online if I am not in Lisbon?
Yes. I see clients in person in Lisbon and Estoril and online, in English, Portuguese, Spanish and French. You can start with a free 15-minute discovery call.
References
- NHS. Food allergy. nhs.uk/conditions/food-allergy
- Tuck CJ, Biesiekierski JR, Schmid-Grendelmeier P, Pohl D. Food intolerances. Nutrients. 2019;11(7):1684. PubMed
- National Institute for Health and Care Excellence (NICE). Coeliac disease: recognition, assessment and management (NG20). 2015. nice.org.uk/guidance/ng20
- Stapel SO, Asero R, Ballmer-Weber BK, et al. Testing for IgG4 against foods is not recommended as a diagnostic tool: EAACI Task Force Report. Allergy. 2008;63(7):793–796. PubMed
- NHS. Anaphylaxis. nhs.uk/conditions/anaphylaxis
- Carr S, Chan E, Lavine E, Moote W. CSACI position statement on the use of IgG testing for food allergy. Allergy, Asthma & Clinical Immunology. 2012;8(1):12. PubMed
This article is for educational purposes and is not medical advice. Always work with your doctor for diagnosis and before making changes to your care.