Food allergy and food intolerance: mechanisms, timing and where judgement ends

Is the immune system misidentifying something, or are digestive enzymes in short supply?

Food allergy is when the immune system mistakes proteins in food for something dangerous. The body makes antibodies against that protein. The next time you eat it, immune cells release substances such as histamine, causing swelling, itching, rash, and sometimes airway reactions. The problem lies in immune recognition. It has little to do with whether the food itself is nutritious or clean.

Food intolerance is a problem in digestion and does not involve the immune system. The most common is lactose intolerance: the small intestine does not have enough of the enzyme that breaks down lactose. The lactose reaches the large intestine, where bacteria ferment it, causing gas, rumbling, abdominal pain and sometimes diarrhoea. The food itself is not treated as an enemy; it just is not processed properly.

There is also a group of reactions that come from spoiled or contaminated food, or from certain additives causing allergy-like symptoms. These are neither typical allergy nor typical intolerance. They need to be looked at case by case and should not all be put into one category.

How soon symptoms appear after eating is an important clue

Food allergy symptoms often come on quickly. Most appear from a few minutes to two hours after eating. The lips, face or eyelids swell; the skin develops weals and itching; there may also be coughing, wheezing, or an itchy, runny nose. Symptoms can appear in the skin, airways and digestive tract at the same time, and can range from mild to severe.

Food intolerance is usually slower. It often appears several hours after eating. The main symptoms are bloating, abdominal pain, passing a lot of gas and diarrhoea. It usually does not involve the skin or airways. Symptoms are concentrated in the lower digestive tract, come on gradually and settle slowly.

There are exceptions. A small number of allergic reactions are delayed by several hours and show up as abdominal pain, nausea, cramps or diarrhoea. These are easily mistaken for intolerance. So timing alone cannot settle it. You also need to look at which parts of the body the symptoms affect, and whether the skin and airways are involved.

Can you judge it yourself? Start with a food diary

Reaching a conclusion entirely on your own is not reliable, but there is one useful thing you can do first: keep a record. For two to three weeks, note what you ate, roughly how much, what symptoms appeared and how long after, and how long they lasted. This record is far more useful than a vague memory, and it can help the doctor when you go for a consultation.

When judging, look at two things. First, the type of symptom: skin weals, lip swelling, wheezing and an itchy nose point more towards allergy; bloating, abdominal pain and diarrhoea alone point more towards intolerance. Second, repeatability: the clue is stronger when the same food repeatedly causes the same reaction.

Do not test it by "trying a big mouthful". If it really is an allergy, one full exposure could trigger a severe reaction. If you suspect an allergy, the doctor should decide how to confirm it and whether tests are needed.

Why "food intolerance tests" cannot be used as a reason to avoid foods

Some intolerance tests on the market measure the level of antibodies against many different foods in the blood. Raised levels of these antibodies are not uncommon in the general population. Many people who eat those foods have no symptoms at all, yet the result is positive. A positive result does not mean the food will make you unwell, and a negative result does not mean there is definitely no problem.

Using these results to draw up a long list of foods to avoid carries very real risks: a large part of your nutrition is cut out, especially for children who are still growing; family meals become stressful; and the real problem can be hidden. The longer the list of foods to avoid, the more a doctor needs to be involved.

Food allergy is diagnosed from the history plus tests arranged by a doctor, such as a skin prick test or specific IgE. The results must match the reaction that actually happened after eating. A single lab report cannot stand on its own, and you should certainly not avoid foods long term based on one alone.

Which signs mean you should see a doctor quickly

If any of the following happen, do not wait and watch at home. See a doctor as soon as possible or call 120 (the emergency number in China): sudden swelling of the lips, tongue, throat or face; a hoarse voice, tight throat or difficulty swallowing; shortness of breath, wheezing, a feeling of suffocation or of being choked; blue-grey or unusually pale skin or lips; sudden dizziness, drowsiness or confusion; a child who is floppy, less responsive, or cannot hold their head up.

If you do not have the emergency signs above, but repeatedly get a rash, itchy lips, wheezing or diarrhoea after eating a certain food, it is also worth seeing a doctor so they can judge whether it is allergy or intolerance. Children, pregnant women and people who already have asthma should ask sooner.

If it is only gas and diarrhoea after eating dairy, the symptoms are mild and you feel well, you can first keep a food diary and cut down on the suspected food, then see a doctor to confirm. You do not need to avoid a wide range of foods long term on your own. What to eat and whether to use substitutes needs to be judged by a doctor, taking your age and nutritional status into account.

This article is for general information only and is not medical advice. Please follow your doctor's instructions.