Key takeaways
- A food allergy involves the immune system, while an intolerance usually affects digestion.
- Breathing difficulty, throat swelling, faintness or a rapidly worsening reaction requires emergency help.
- Diagnosis should rely on a careful history and appropriate clinical testing, not unvalidated intolerance panels.
The short answer: a food allergy involves the immune system and can sometimes become life-threatening. A food intolerance usually affects digestion and is generally not dangerous, although it can make daily life uncomfortable. “Food sensitivity” is a broad, less precise term that people often use when symptoms seem connected to food but the cause is not yet clear.
Because the symptoms can overlap, it is easy to put every unpleasant reaction under the same label. Understanding the differences helps you describe the problem accurately and choose a safer next step.
What is a food allergy?
A food allergy happens when the immune system reacts to a food protein as though it were a threat. Symptoms may begin within minutes or a few hours. They can include hives, itching, swelling of the lips or face, vomiting, wheezing, breathing difficulty, dizziness or collapse.
Even a very small amount of the food may trigger a reaction in some people. The severity of a previous reaction does not reliably predict the next one, so a suspected allergy deserves medical assessment rather than home experimentation.
Call emergency services immediately if someone develops breathing difficulty, throat or tongue swelling, sudden confusion, fainting or other signs of a severe allergic reaction.
What is a food intolerance?
A food intolerance does not usually involve the immune system. It often occurs because the body has difficulty digesting or processing a particular component of food. Lactose intolerance, for example, is caused by insufficient lactase, the enzyme that helps digest the natural sugar in milk.
Typical symptoms include bloating, gas, abdominal discomfort, diarrhoea, nausea or changes in bowel habits. They often appear several hours after eating and may depend on the amount consumed. Some people can tolerate a small portion without symptoms but react to a larger serving.
What does “food sensitivity” mean?
Food sensitivity is widely used in everyday conversation, but it is not one single medical diagnosis. It may describe digestive symptoms, headaches, fatigue or other concerns that a person associates with a food. Those symptoms are real, but the label alone does not explain the cause.
Several conditions can look like a food reaction, including coeliac disease, irritable bowel syndrome, reflux, migraine triggers and reactions to food additives. This is one reason why a careful history is more useful than guessing from a long list of foods.
A safer way to investigate recurring symptoms
- Keep a simple food-and-symptom diary. Record meals, portions, timing, symptoms, medicines, sleep and stress for two to four weeks.
- Look for repeatable patterns. One uncomfortable day does not prove that the last food you ate was responsible.
- Speak with a qualified clinician. A doctor, allergy specialist or registered dietitian can decide whether testing or a supervised elimination and reintroduction plan is appropriate.
- Avoid cutting out many foods at once. Broad restriction can make the cause harder to identify and may lead to nutritional gaps.
Be cautious with commercial home tests
Many mail-order “intolerance” tests measure food-specific IgG antibodies. Major allergy organisations advise that these tests have not been shown to diagnose food intolerance or food allergy. A positive result may simply reflect exposure to a food, not a harmful reaction. Hair analysis and similar alternative tests also lack good evidence for diagnosing intolerance.
When should you ask for help?
Arrange a medical review if symptoms keep returning, affect your weight or nutrition, wake you at night, or are accompanied by blood in the stool, persistent vomiting, fever or severe pain. Seek urgent help for any possible severe allergic reaction.
Sources and further reading
NHS: Food intolerance, AAAAI: Food intolerance and allergy, and AAAAI: The myth of IgG food testing.
This article provides general information and is not a diagnosis or a substitute for individual medical advice.