Food intolerance is an unpleasant reaction to a food, drink or food additive that, unlike an allergy, does not involve an immune response. The symptoms are often delayed (from 30 minutes to 1–2 days) and depend on the amount eaten, so the culprit food can be hard to pin down.
It is important not to confuse it with food allergy: an allergy is an immune (usually IgE-mediated) reaction that develops quickly and can cause hives, swelling or anaphylaxis even to a small dose. Autoimmune celiac disease stands apart — a reaction to gluten with bowel damage.
Main mechanisms and types
Enzyme deficiency. The classic example is lactose intolerance from a lack of lactase: milk sugar is not broken down and causes bloating, gas and diarrhea.
A pharmacological reaction to natural substances in food — histamine, tyramine, salicylates, sulfites and caffeine. FODMAP malabsorption — poorly absorbed fermentable carbohydrates that bacteria ferment, causing bloating; this is a common mechanism in irritable bowel syndrome. Non-celiac gluten sensitivity also belongs here. The state of the gut microbiome also affects tolerance.
How it is diagnosed and what does not work
Reliable methods are a supervised elimination diet with subsequent reintroduction of the food, the hydrogen breath test (for lactose and carbohydrate malabsorption) and, for rigorous assessment, double-blind challenge tests.
But the popular blood tests "for IgG to foods" are considered unreliable: IgG antibodies to food are found in healthy people too and merely reflect exposure to a food, not intolerance. Relying on them risks needlessly excluding many foods and developing deficiencies. With alarming symptoms (blood in the stool, weight loss), organic diseases are ruled out first, including ulcerative colitis and celiac disease.
Frequently asked questions
Are food intolerance and food allergy the same thing?
No. An allergy is an immune (usually IgE) reaction, fast and dangerous even at a small dose. Intolerance does not involve the immune system, depends on the amount of food and usually is not life-threatening, though it worsens well-being.
Is it worth taking an IgG test for foods?
No, such tests do not confirm intolerance: IgG to food is present in healthy people too. It is more reliable to keep a food diary and run a supervised elimination diet, and if needed a breath test.