Food Allergy or Intolerance? How to Tell the Difference

Bloating after bread, an itchy mouth after nuts, a rash after seafood: it's natural to wonder whether you have a food allergy or an intolerance. The two are often confused, but they're very different. One involves your immune system and can be serious; the other is usually about digestion. Knowing which you're dealing with tells you which test is worth having, and which ones to avoid.

What is a food allergy?

A food allergy happens when your immune system mistakes a food protein for a threat and makes IgE antibodies against it. The next time you eat that food, those antibodies trigger a reaction, usually within minutes to two hours.

Typical signs include:

• tingling or itching in the mouth
• hives (raised, itchy rash) or flushing
• swelling of the lips, face or eyes
• vomiting or stomach cramps soon after eating
• wheezing, coughing or a runny nose

Even a tiny amount of the food can cause a reaction. Common culprits in the UK include milk, egg, peanuts, tree nuts, fish, shellfish, sesame, soya and wheat.

What is a food intolerance?

A food intolerance doesn't involve IgE antibodies. It's usually your gut having trouble digesting something, for example lactose intolerance, where the body makes too little of the enzyme that breaks down milk sugar.

Intolerance symptoms tend to:

• come on more slowly, often hours after eating
• depend on how much you eat (a little may be fine)
• be mainly digestive: bloating, wind, tummy pain, diarrhoea
• be unpleasant, but not dangerous

Allergy vs intolerance at a glance

Which tests actually help?

For a suspected food allergy: an IgE blood test

An allergy blood test measures IgE antibodies to specific foods. It needs one blood sample, and unlike skin-prick testing, you don't need to stop antihistamines beforehand. For food reactions, our Food Screen covers 20 common foods, including egg, milk, wheat, peanut, tree nuts, sesame, shrimp and fish. The Milk & Gluten Screen looks at cow's milk and its proteins, plus gluten.

A positive result shows sensitisation: your body has made IgE to that food. That isn't always the same as a true allergy, so results are best read alongside your symptoms, ideally with a doctor.

For coeliac disease: a coeliac blood test

Coeliac disease is often mistaken for a wheat intolerance, but it's an autoimmune condition triggered by gluten. Our Coeliac Profile checks the antibodies linked to it, including tissue transglutaminase (tTG) IgA. Keep eating gluten in the weeks before the test; cutting it out first can make the result falsely normal.

For a food intolerance: a food diary

There's no reliable blood test for most food intolerances. "Intolerance" tests that measure IgG antibodies to foods aren't recommended by UK allergy specialists: IgG to foods is a normal response to eating them, so these tests can wrongly flag foods you eat often. The better approach is a food and symptom diary, then cutting out one suspected food at a time, ideally with a GP or dietitian, so you don't miss out on important nutrients.

Which allergy test should I choose?

• Reactions to food: the Food Screen (20 foods)
• Milk or gluten worries: the Milk & Gluten Screen
• Not sure, or food and hay fever-type symptoms: the 30-allergen Atopic Screen
• A child with suspected allergies: the Paediatric Screen

You can compare all 12 of our allergy blood tests, with prices, on our allergy blood test page.

Frequently asked questions

Can a food allergy start in adulthood?

Yes. Many food allergies begin in childhood, but adults can develop new ones too, including to shellfish, fish and fruit.

Do I need a GP referral for an allergy blood test?

No. You can book directly at our Slough, Victoria or Neasden clinics. Results usually take about 5 days.

Can children have an allergy blood test?

Yes, at our Slough or Neasden clinic. Please call 020 3633 2371 before booking a child's appointment.

Should I stop eating the food before an allergy test?

For an IgE allergy test there's no need to change your diet. For a coeliac test, keep eating gluten until after your blood is taken.