Food intolerance or food allergy?

A food allergy is an immune reaction. A food intolerance is not. That single difference explains everything else: allergy symptoms can start within minutes and can be life-threatening, while intolerance symptoms usually take hours and are not. With an intolerance you can often eat a small amount of the food without trouble. With an allergy you cannot.

What is the actual difference?

According to the NHS, a food allergy happens when the immune system mistakenly treats proteins in a food as a threat. A food intolerance is not caused by the immune system overreacting. It is a problem with digesting or processing something — lactose being the most familiar example.

That distinction is not academic. It decides how urgent the situation is.

Which symptoms point to which?

The NHS lists intolerance symptoms as diarrhoea, bloating and stomach cramps — digestive, uncomfortable, and slow to appear.

Allergy symptoms are different in kind, not just in degree: itchy skin, wheezing, and swelling of the lips, face and eyes. Swelling and breathing difficulty are the signs that need medical help immediately, not a diary entry.

If you are reading this because something you ate made you struggle to breathe, stop reading and get medical help.

Why is an intolerance so hard to pin down?

Three things work against you at once.

The delay. Symptoms arrive hours after the meal, so the obvious suspect — whatever you ate most recently — is often the wrong one.

The threshold. A small portion may cause nothing and a large one may cause a lot, so the same food looks innocent on Tuesday and guilty on Friday.

The overlap. A single meal contains many ingredients. Bread alone brings wheat, yeast and often milk.

This is the situation a written record is actually for. Not to diagnose anything, but to turn "something disagrees with me" into a list of candidates with dates attached.

How is an intolerance properly identified?

Through elimination and reintroduction, supervised by a dietitian. Monash University, whose researchers developed the low FODMAP diet, describes it in three phases:

1. Elimination — 2 to 6 weeks on a restricted diet.
2. Reintroduction — about 6 to 8 weeks, adding one food group back at a time while the rest of the diet stays restricted.
3. Personalisation — keeping what you tolerate, avoiding only what you do not.

Monash is explicit that phases one and two should be done with dietitian guidance, and that the restricted phase is not meant to be permanent. Cutting out food groups indefinitely, on your own, is how people end up with a narrower diet and no more answers than when they started.

What about home test kits?

The NHS does not recommend commercial food-intolerance testing kits. They promise a shortcut past the slow, methodical process above, and the process is slow for a reason: the delay between eating and symptoms is the whole problem, and no blood test resolves it.

What can you usefully do before the appointment

Keep a record for two to three weeks before you see anyone. For each entry, write down what you ate, when, and what you felt afterwards — including the days nothing happened, because those matter as much.

A food and symptom diary is the one piece of preparation that consistently makes the appointment shorter, because it replaces "I think it might be dairy" with dates, portions and timings someone can actually read. We built FoodLog to make that record easier to keep than a notes app, but a paper notebook works too — what matters is that it is written down at the time rather than remembered later.

More on this in food and symptoms.