Food allergies and intolerances explained

The key difference

A food allergy happens when the immune system wrongly treats a harmless food as a threat and reacts against it. This can happen very quickly, sometimes within minutes, and can affect the skin, breathing and circulation. A food intolerance is different: it does not involve the immune system in the same way and is usually a problem with digesting or processing a food. Intolerances tend to cause uncomfortable gut symptoms that come on more slowly and depend partly on how much of the food is eaten. The crucial point is that a true allergy can be dangerous and even life-threatening, whereas an intolerance, though unpleasant, does not put life at risk in the same way.

Common food allergies

Some foods cause allergies more often than others. In the UK, common triggers include peanuts, tree nuts, milk, eggs, fish, shellfish, soya, wheat and sesame. Allergic reactions can range from mild to severe. Milder signs include an itchy or tingling mouth, raised itchy bumps on the skin (hives), and swelling of the lips or face. More serious reactions affect breathing and circulation. Some allergies, such as to milk and egg, are common in childhood and are often outgrown, while others, like nut and shellfish allergies, tend to last for life. Because reactions can vary and sometimes worsen, anyone with a known food allergy is advised to avoid the food carefully and have an action plan.

Common food intolerances

Intolerances are common and usually centre on the gut. Lactose intolerance, where the body struggles to digest the sugar in milk, causes bloating, wind, tummy pain and diarrhoea after dairy. Some people react to certain food additives, caffeine, or naturally occurring substances in foods. Coeliac disease is often confused with intolerance but is actually an autoimmune condition where gluten damages the gut lining, and it needs proper diagnosis before cutting gluten out. Intolerance symptoms are genuinely unpleasant and can affect quality of life, but they build up more gradually, are dose-related, and do not cause the sudden, dangerous whole-body reactions seen in severe allergy. Keeping a food and symptom diary often helps identify the culprit.

Getting a diagnosis

It is best not to self-diagnose or cut out major food groups without advice, as this can lead to poor nutrition and miss the real cause. For suspected allergy, a GP can refer to an allergy clinic, where tests such as skin-prick tests and blood tests, guided by a careful history, help confirm the trigger. For coeliac disease, specific blood tests and sometimes a gut biopsy are used — and you need to keep eating gluten until tested, or the results can be misleading. For many intolerances, there is no single reliable test, so diagnosis relies on a food diary and carefully removing then reintroducing the suspected food under guidance. Beware commercial tests claiming to diagnose intolerances, as many are not reliable.

Managing reactions and staying safe

The main treatment for food allergy is strict avoidance of the trigger, which means reading labels carefully and asking about ingredients when eating out; UK law requires allergens to be highlighted on food labels. People at risk of severe reactions may be prescribed adrenaline auto-injectors and taught how and when to use them. The most serious reaction, anaphylaxis, is a medical emergency: signs include difficulty breathing, swelling of the throat or tongue, a sudden drop in blood pressure, feeling faint and collapse. If anaphylaxis is suspected, use an adrenaline auto-injector if available and call 999 immediately. Intolerances are managed by limiting the offending food, sometimes with support from a dietitian to keep the diet balanced and healthy.