Histamine intolerance (HIT; med: Histamine Intolerance Syndrome (HIS), food-induced histaminosis, enteral-induced histaminosis) describes a non-immunologically mediated reaction intolerance to foods with higher histamine levels and/or the inability of the human body to break down (metabolise) the ingested histamine and other biogenic amines sufficiently. Other factors, such as stress, can contribute to the rise in histamine levels. This results in a higher than normal level of histamine in the body and usually several symptoms that look very much like an allergy. Since Histamine Intolerance is not immunologically mediated, it should not be mistaken for any sort of allergy. Some articles have mentioned that HIT is an allergy to histamine. There is no such thing as a histamine allergy. If someone refers to a „histamine allergy“ then they have either been misinformed or have misunderstood the information given to them. To repeat, there is no such thing as an allergy to histamine. We are dealing with an intolerance here.
Around 1-8% of the population is thought to be affected by histamine intolerance. Considering, however, that histamine intolerance is not well known among some sections of the medical profession and is therefore underdiagnosed, it is currently impossible to say how many people really do have HIT. Any numbers currently given are estimates from different sources in different countries, which means one should be careful at this point of giving any definite general results. Hopefully this will become clearer in the future. According to some authors approximately 80% of sufferers are women and most of them are around age 401). A possible cause for this lies in the decline of oestrogen levels as menopause sets in.
Histidine (a natural amino acid) in foods is metabolised by bacteria into histamine. The histamine will then be further metabolised in the human body with the help of the enzyme diamine oxidase (DAO; to be found mainly in the intestine, kidney and placenta) and histamine N-methyltransferase (HMT, HNMT (mainly found in the kidney and liver, and in smaller concentrations in other areas of the body). The DAO supposedly uses the Vitamin B6 as a co-factor. In the case of histamine intolerance the activity of either one - mostly the DAO - or both of these enzymes is reduced. This reduces the body's ability to break down the histamine that has either been ingested or produced by the body itself. When a person eats foods that are rich in histamine or release histamine from storage (mast) cells in the body then this will result in a "pseudoallergic" reaction.
Since histamine intolerance tends to appear as a consequence of other food intolerances and food allergies, it should be pointed out that the ingestion of foods that are otherwise not tolerated, e.g. lactose by those suffering from lactose intolerance, will also cause a massive release of histamine.
A healthy person continuously produces DAO that is released into the intestinal lumen. When the person eats foods that are rich in histamine, then that histamine can already be "neutralised" in the intestine. This, of course, only works up to a certain level. If too much histamine is ingested (for example, from foods that have gone bad (often bad fish)), then even a healthy person will experience the typical symptoms of "histamine toxicity". If not enough DAO is produced, then a person can already experience symptoms after eating foods with relatively low histamine levels. This is called histaminosis or histamine intolerance.
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