Importantly, current thinking is that HIT is not solely due to dietary histamine exposure, but exposure coupled with insufficient or impaired histamine detoxification
Elevations in histamine due to an imbalance between detoxification and dietary exposure can cause histamine receptor mediated complications with allergic type symptoms.
Dietary histamine is found at particularly high concentration in aged foods (eg cheeses, alcoholic beverages, cured meats, fermented or spoiled foods) where it is produced by bacterial or yeast fermentation of the amino acid histidine to histamine. Other foods, such as citrus for example, may have the capacity to enhance histamine release even though they contain low levels of histamine themselves.
Typical symptoms include, flushing, itching, nausea, vomiting, abdominal pain, cramps, diarrhoea, headache, migraine, dizziness, sinus issues and rhinitis.
The diagnosis of HIT typically requires the presence of two or more symptoms, improvement with a low histamine diet and/or antihistamines, and the exclusion of food allergies. Testing remains controversial as blood and urine histamine is often normal, and the usefulness of determining DAO levels or DAO and HNMT genotype is unclear.
After dietary histamine removal and symptom improvement many patients may be able to tolerate histamine-containing foods again and return to a normal diet. In addition to dietary therapy, nutritional supplementation with nutritional co-factors for DOA has been suggested, in particular zinc, copper, vitamin C and vitamin B6. Of these nutrients, both vitamin C and B6 have some evidence to suggest they would support histamine detoxification
Finally, although it has been relatively unexplored, it is plausible to suggest that differences in gut bacteria could result in increase histamine production through fermentation of histidine to histamine,or have direct affects on intestinal DOA activity. Lending support to this idea the GG strain of Lactobacillus rhamnosus (LGG) was found to reduce allergy-related immune activation by down regulation of the expression of histamine receptor genes
In summary, it appears more attention should be given to the possibility of HIT as a clinical diagnosis, namely because nutritional therapy appears to be a safe and effective treatment for symptoms that may be otherwise misdiagnosed.