The human immune system is always involved in an allergic reaction (more information here). A reaction resulting from an intolerance is different from an allergic reaction.
Many intolerances are due to an enzyme defect. An enzyme deficiency in the gut causes symptoms such as bloating, cramps, diarrhoea or constipation (as is the case in lactose intolerance). Because the enzyme is missing, the carbohydrates in food cannot be properly digested. Some patients only experience symptoms once a certain amount of the offending substance is ingested.
Some intolerances, such as histamine intolerance, may be accompanied by what are called pseudoallergic reactions. The symptoms are very similar to an allergic reaction (e.g. difficulty breathing or skin rash). They are caused by cells in the body that play a key role in allergies. However, the activation of these cells is not controlled by the antibodies in the immune system. They are therefore referred to as ‘pseudoallergic’ reactions (from the Greek ‘pseudes’ meaning ‘lying’ or ‘false’). Pseudoallergic reactions can be severe and can occur as a reaction to food additives (E numbers) and other substances.
Research on intolerances remains inconclusive at this time. The number of people with an intolerance is also unclear, since widely varying results have emerged from clinically controlled studies and self-report studies.
A number of diagnostic methods are available for diagnosing food intolerances; some of these are different than the tests used to diagnose allergies. An in-depth anamesis, food diary, a breath test or provocation test are the methods of choice for determining whether specific symptoms are due to an intolerance. In the breath test, substances are measured in the breath, such as hydrogen, that remain as by-products of the digestive process when enzymes are missing.
There are additional tests whose diagnostic value is unclear or refuted (for example, IgG tests, hair analysis and kinesiological tests). Given what we know today, self-diagnosis or self-treatment based on these tests is not recommended. Anyone with a suspected food intolerance should consult a doctor.
Some of the known intolerances are: