According to the 2003 review of allergy nomenclature by the World Allergy Organization, adverse reactions to food without an immunological basis should be referred to as nonallergic food hypersensitivity, in order to clearly differentiate them from food allergies initiated by a specific immune mechanism [
53]. Nonallergic food hypersensitivity is commonly known as food intolerance, a response triggered by a food or any of its components at a dose normally tolerated by the healthy population [
54]. While the prevalence of food allergies is estimated at 1–2% in adults, currently almost 20% of the Westernized world’s population suffers from some type of food intolerance, with lactose intolerance being the most common [
54].
The DAO enzyme was first identified back in 1929 by Charles H. Best in autolyzing lung tissue, which he called histaminase because of its ability to degrade histamine [
56]. Years later, given its ability to also degrade other diamines, as described above, the more accurate designation of DAO was proposed [
57,
58]. Beyond its role in the intestinal degradation of histamine in humans, DAO is also present in microorganisms, plants and animals, where it also catalyzes the oxidative deamination of the primary amino group of histamine into its corresponding aldehyde, concomitantly producing stoichiometric amounts of ammonia and hydrogen peroxide (
Figure 4) [
14,
59,
60].
Although the first scientific references to histamine intolerance date from more than 20 years ago, it is significant that almost 80% are from the last decade, reflecting the growing interest of researchers in this disorder (
Figure 5). In 2011, EFSA already considered histamine intolerance as one of the risks associated with histamine intake, clinically differentiating it from histamine intoxication [
1]. In a subsequent joint report, the WHO and FAO emphasized that the no observed adverse effect level (NOAEL) established for histamine was only valid for healthy people, and not for members of susceptible populations, such as those with histamine intolerance [
30]. EFSA concluded that only foods with histamine levels below the detection limits are safe for individuals with histamine intolerance [
1].
Clinical manifestations of histamine intolerance consist of a wide range of nonspecific gastrointestinal and extraintestinal symptoms, due to the ubiquitous distribution of the four histamine receptors in different organs and tissues of the body (
Figure 6) [
10,
13,
54,
61]. In a very recently published study, a team of Austrian researchers comprehensively analyzed the symptoms experienced by 133 patients diagnosed with histamine intolerance [
62]. The most frequent and severe manifestations were gastrointestinal, with abdominal distension observed in 92% of patients and postprandial fullness, diarrhea, abdominal pain and constipation in 55–73%. Impairments of the nervous and cardiovascular systems, such as dizziness, headaches and palpitations, were recorded in second place, followed by respiratory and dermatological symptoms. Highlighting the complexity of the clinical picture of histamine intolerance, combinations of three or more symptoms involving different organs were recorded in 97% of cases, with an average of 11 symptoms per patient. The low specificity and complex variability of symptoms undoubtedly contribute to the current difficulty in achieving consensus on the diagnostic criteria for histamine intolerance, as will be discussed in detail below [
13]. A lack of data also makes it difficult to determine the current incidence of this condition, although some authors have estimated that it affects 1–3% of the population, a percentage that will possibly increase as more knowledge and diagnostic tools for histamine intolerance become available [
10,
13,
63].
5.1. The Etiology of Histamine Intolerance
As mentioned in previous sections, the main barrier against exogenous histamine in the intestines is the DAO enzyme, which prevents its passage into the systemic circulation [
10,
13,
65]. Numerous clinical studies have provided data on the prevalence of low plasma DAO levels in individuals showing symptoms of histamine intolerance, mainly headaches and gastrointestinal or dermatological disorders [
66]. Although certain studies have limitations, either in the design or number of participants, the majority point to an association between symptoms and DAO deficiency, establishing a general trend that supports the key role of DAO in the etiology of these disorders. A DAO deficiency that predisposes a population subgroup to histamine intolerance may have a genetic, pathological or pharmacological origin [
1,
41].
Regarding the genetic background of histamine intolerance, several studies have analyzed in depth the polymorphisms in genes encoding the enzymes L-histidine decarboxylase, DAO and HNMT, as well as the different histamine receptors. More than 50 nonsynonymous single-nucleotide polymorphisms (SNPs) in the DAO-encoding gene have been identified, some of which can produce a protein with altered activity and lead to symptoms of histamine intolerance [
67,
68,
69,
70,
71,
72]. Specifically, the most relevant SNPs affecting DAO enzyme functionality in Caucasian individuals are rs10156191, rs1049742, rs2268999 and especially rs1049793 [
69,
71]. On the other hand, an SNP in the promoter region of the gene has also been identified that causes a lower transcriptional activity of the DAO-encoding gene (rs2052129), as well as several genetic variations responsible for enzyme deficiency in people of Asian or African origin (rs45558339 and rs35070995, respectively) [
67,
72]. In most cases, the effect of these genetic variations on DAO functionality is through changes in enzyme kinetics, the resulting increase in K
M causing a reduction in the rate of histamine degradation [
69]. In parallel, three SNPs have been identified as being responsible for enhanced DAO enzyme activity (rs2071514, rs1049748 and rs2071517) [
72]. There is also evidence of DAO mutations in patients with certain cardiovascular, gastrointestinal and nervous system pathologies, although with contradictory results regarding positive/negative effects [
68].
DAO deficiency can also be an acquired condition, caused by certain pathologies or interaction with drugs. Several inflammatory bowel pathologies affecting mucosal integrity are known to result in impaired DAO activity, the degree of which can be correlated with the severity of mucosal damage [
73,
74,
75]. Thus, DAO activity has been proposed as a marker of integrity of the intestinal mucosa. Miyoshi et al. demonstrated that DAO activity can be a useful predictor of intestinal mucosal damage in patients receiving chemotherapy [
76]. Additionally, DAO deficiency has also been linked to certain functional gastrointestinal disorders, such as carbohydrate malabsorption and nonceliac gluten sensitivity (NCGS) [
63,
73,
77,
78,
79]. Enko et al. found that a concomitant reduction in DAO and lactase enzyme activities could be a consequence of mucosal damage in the small intestine due to gastrointestinal disorders (e.g., gastroenteritis, irritable bowel syndrome, short bowel syndrome and gastrointestinal surgery) [
73]. Moreover, patients with lactose intolerance and plasma DAO deficit showed higher end-expiratory H
2 levels and the appearance of more symptoms during the H
2 breath test in comparison with lactose-intolerant individuals with normal DAO activity [
79]. More recently, two works have suggested a potential relationship between a reduced DAO activity and the presence of NCGS. Schnedl et al. based this relationship on the broad parallelism between the symptomatology of NCGS and histamine intolerance, while the pilot study conducted by Griauzdaite et al. reported a strong association between reduced DAO activity and the presence of NCGS, although with a reduced number of patients [
77,
78]. In fact, Griauzdaite et al. found out that nine of 10 patients with NCGS had decreased serum DAO activity levels [
78]. This recently indicated relationship between both disorders, NCGS and histamine intolerance, should be further explored as it may be of interest for the correct clinical management of affected patients.
Finally, DAO deficiency can be a temporary and reversible condition, caused by the inhibitory effect of substances such as biogenic amines and alcohol, as discussed above, as well as several widely used drugs (
Table 2) [
1,
10]. It has been estimated that approximately 20% of the European population regularly take DAO-inhibiting drugs, which significantly increases the number of people susceptible to the adverse effects of dietary histamine [
28]. In vitro experimental results show a potent inhibitory effect (greater than 90%) of chloroquine, a historical antimalarial active ingredient, and clavulanic acid, a β-lactam antibiotic widely used in combination with amoxicillin [
80]. A significant inhibition of the enzymatic activity has also been observed with the antihypertensive drug verapamil and the histamine H2 receptor antagonist cimetidine, although the clinical use of the latter is currently anecdotal [
23,
80]. Other substances have also shown an inhibitory effect, albeit to a lesser extent (
Table 2) [
23,
80,
81]. In most cases, the structural similarity of the cited drugs with histamine could explain their potential to bind to the active site of DAO and reduce its enzymatic activity [
23]. Along the same lines, substances with an inhibitory effect on other enzymes involved in any of the metabolic pathways of histamine in the body (i.e., HNMT, ALDH and MAO) may act as a trigger of histamine hypersensitivity [
82].
5.2. Prevalence of DAO Deficit in Persons with Symptoms Related to Histamine Intolerance
Several studies have evaluated the prevalence of DAO deficit in plasma of individuals with symptoms of histamine intolerance and/or diagnosis with certain chronic disorders.
Mušič et al. found DAO deficiency in 80% of 316 adult patients showing various symptoms associated with histamine intolerance (e.g., urticaria, pruritus, diarrhea, abdominal pain, vomiting, constipation, cough, rhinitis and headache), as well as significantly lower plasma DAO activity compared to the control group [
83]. Similarly, in a retrospective study, Manzotti et al. evaluated DAO activity in 14 patients with a confirmed diagnosis of histamine intolerance who showed mainly gastrointestinal and dermatological symptoms, but also headaches [
84]. In this case, patients showed a high prevalence of DAO deficit (71%) and a significantly lower mean DAO activity compared to healthy volunteers. A lower percentage of DAO deficiency in histamine-intolerant patients (24%) was reported by Pinzer et al. [
63]. Those patients featured elevated histamine levels and constantly reduced DAO activities throughout the day.
In a study focused only on headache symptoms, Steinbrecher and Jarisch reported DAO deficiency in 23 of 27 patients (85%) [
85]. In parallel, the authors described a significant increase in DAO activity after patients followed a low-histamine diet for four weeks, along with a remission or reduction in frequency of headaches in almost 90% of individuals. More recently, Izquierdo et al. studied the prevalence of DAO deficit in 137 patients diagnosed with a confirmed migraine diagnosis and in a control group of 61 nonmigraine individuals [
66]. In this study, a high prevalence of DAO deficiency was observed in the migraine group (87%) and with a mean DAO activity significantly lower in comparison with that obtained from control volunteers. However, the prevalence of DAO deficiency in the control population amounted up to 44%, which was attributed to the fact that certain individuals could present other symptoms associated with histamine intolerance or DAO deficiency other than migraines. Another study with 44 migraine patients reported a 60% prevalence of DAO deficiency and a significant copresence of certain gastrointestinal disorders, such as celiac disease and NCGS [
78].
In the field of dermatological symptomatology, several studies have monitored plasma DAO activity in patients with eczema, chronic idiopathic urticaria and atopic dermatitis. Overall, the reported prevalence of DAO deficiency ranges from 19 to 57%, with the exception of the study by Worm et al., who did not detect statistically significant differences in plasma DAO activity between control patients and those with atopic dermatitis [
86,
87,
88,
89].
Finally, regarding gastrointestinal symptoms, Honzawa et al. assessed the clinical significance of plasma DAO activity levels in 98 patients suffering inflammatory bowel disease [
90]. This study showed that DAO activity in blood was significantly lower in patients with Crohn’s disease and ulcerative colitis compared to the control population, suggesting its potential importance as a marker of intestinal permeability. In a pediatric population under 15 years of age, Rosell-Camps et al. determined DAO deficiency in 88% of patients with abdominal pain, diarrhea and vomiting [
91]. In contrast, in a more recent study by a group of Austrian researchers, DAO deficiency was found in only 8% of 394 children with chronic abdominal pain [
92].
To date, little data is available on the prevalence of this enzymatic deficiency related to gender, and it is inconclusive. Klockler et al. found no differences in plasma DAO activity between men and women, although the number of individuals considered was scarce (n = 28) [
93]. Likewise, the study performed by Izquierdo et al. reported similar percentages of DAO deficiency in migraine-suffering women (83%) and men (90%) [
66]. On the contrary, García-Martín et al. did describe differences in plasma DAO activity by gender, with the prevalence of this enzyme deficiency being higher in women [
94]. Significant fluctuations in DAO activity values have also been reported in women associated with different stages of the menstrual cycle [
94,
95].
One factor that could explain the discordance among the prevalence data of DAO deficit in patients with disorders associated with histamine intolerance is that the parameter considered in all of them was serum DAO activity, which, a priori, would not reflect an enzymatic deficiency derived from certain intestinal pathologies. Overall, in spite of the varying percentages in DAO deficiency, the currently available studies seem to indicate an etiological relationship between DAO deficiency and certain symptoms or disorders related to histamine intolerance. Nevertheless, more studies are needed to assess the clinical significance of the determination of plasma DAO activity, as well as to develop new diagnostic methods aimed at identifying individuals with histamine intolerance due to DAO deficiency.
5.3. Diagnosis of Histamine Intolerance
Despite significant advances in the understanding of histamine intolerance, reaching a consensus on a diagnostic algorithm remains a pending challenge. The nonspecificity of symptoms and lack of validated diagnostic tools prompts many affected individuals to go “doctor shopping”; that is, to consult several medical specialists in search of an explanation and solution for their varied symptomatology [
13,
63]. In the absence of a consensual and clinically validated diagnosis,
Figure 7 shows a schematic summary of the diagnostic algorithm for histamine intolerance based on the available scientific evidence reviewed below.
The combination of diagnostic criteria currently in use includes the appearance of typical clinical manifestations and the exclusion of other related disorders [
10,
13,
54]. All the authors who have proposed a diagnostic algorithm for histamine intolerance emphasize the need to initially rule out other potential causes of symptoms associated with an increase in plasma histamine [
10,
13,
54]. For this purpose, it is advisable to carry out an intradermal skin allergy test (i.e., skin prick test) to discard IgE sensitization caused by food allergy, and to measure plasma tryptase to exclude an underlying systemic mastocytosis [
10]. It is also important to know whether the patient is taking any medication with a possible inhibitory effect on DAO activity [
55]. If these conditions are negative, the appearance of two or more typical symptoms of histamine intolerance and their improvement or remission after the following of a low-histamine diet (i.e., a diet excluding foods that, a priori, contain high histamine levels) will confirm the diagnosis of histamine intolerance [
10,
54,
96,
97]. In the diet follow-up, a thorough 24-h record of all the foods consumed and symptoms experienced is recommended in order to establish a relationship, if any, between a food and the onset of symptoms [
10,
13]. The duration of the low-histamine diet to confirm the diagnosis is not clearly stipulated, although some studies suggest a period of 4 to 8 weeks [
54,
97]. In addition to the diet, testing the effect of antihistamine treatment on symptoms has also been proposed, although its usefulness once dietary histamine is removed is unclear [
10,
54].
Once it has been established that dietary histamine is responsible for the intolerance-associated symptoms, the diagnosis of this disorder is virtually confirmed. A range of nonvalidated complementary tests have also been proposed by several authors with the aim of obtaining a marker to confirm the diagnosis [
97]. However, it has to be taken into account that not all of the tests consider the different origins of DAO deficiency (i.e., genetic, pathological or pharmacological). Thus, a genetic origin would lead to a reduction of the DAO enzymatic activity in the whole organism. Likewise, the pharmacological blockade of DAO would take place in all tissues where the drug is distributed after entering the systemic circulation, although in a punctual manner upon the substance’s introduction. Lastly, the scope of a DAO deficit due to intestinal pathologies would be limited to the local intestinal environment.
Due to the genetic background of DAO deficiency, one of the strategies for the diagnosis could be the determination of genetic polymorphisms (SNPs) that characterize the population as genetically susceptible to histamine [
54]. Currently, there is already the possibility of performing a noninvasive genetic analysis capable of identifying three of the SNPs associated with reduced DAO activity (i.e., rs10156191, rs1049742 and rs1049793) from a sample of the oral mucosa, although evidence-based studies on the diagnosis potential of this test are still needed. It is important to note that this test will only reflect the existence of a genetic DAO deficiency.
The most studied, and possibly also the most controversial, is the determination of plasma DAO activity. This analytical test consists of measuring the amount of histamine degraded in a blood sample in a given time interval. Two types of commercial testing kits are currently available on the market, one consisting of an ELISA-type immunoassay, and the other a radioimmunoassay using radioactively labeled putrescine [
83,
84]. The evidence for the validity of blood DAO activity measurements for the diagnosis of histamine intolerance is neither abundant nor conclusive. Some studies have proposed that determining blood DAO activity may be helpful in identifying subjects with symptoms associated with histamine intolerance [
63,
83,
84]. In contrast, three studies did not find a significant relationship between the clinical history of patients with typical symptoms of histamine intolerance and blood DAO activity values, concluding that this technique cannot be recommended as a diagnostic tool in routine clinical practice until studies have validated its effectiveness [
98,
99,
100]. Moreover, the work performed by Schnoor et al. also reported a high interassay variation in DAO activity values that made the proper classification of histamine-intolerant subjects impossible [
100]. This controversy is described in a joint article published in 2017 by the German and Swiss allergology societies, which emphasizes the need for more research before giving plasma DAO activity a definitive diagnostic value for histamine intolerance [
97].
A variant of the intradermal skin allergy test called the histamine 50-skin-prick test was also proposed by Kofler et al. to diagnose histamine intolerance [
101]. In this technique, the results were read after 50 min (as opposed to the usual 20 min) and showed that, although the size of the wheal did not differ between the histamine intolerant and control groups, the time course was significantly different. Patients with symptoms of intolerance showed a delayed remission of the wheal induced by cutaneous administration of histamine, signaling a reduced degradation ability. The same results were obtained in a study recently published by Wagner et al., who re-evaluated this skin test as a diagnostic tool of histamine intolerance, also observing a correlation between the delay in wheal disappearance and a lower plasma DAO activity [
102].
Both the determination of plasma DAO activity and the histamine 50-skin-prick test could be suitable tests to identify a DAO deficiency from genetic or pharmacological origin, but they would not be useful to determine a deficit secondary to certain intestinal diseases.
On the contrary, there are certain alternatives, such as the intestinal biopsy, the histamine provocation test or the histamine metabolomics in urine, that could make it possible to diagnose histamine intolerance due to DAO deficiency without excluding any of the possible etiological causes.
The measurement of intestinal DAO activity by a colon biopsy during endoscopic procedures has been studied as a possible diagnostic marker. The few available studies have shown a reduced intestinal DAO catabolic activity in patients with recurrent urticaria, food allergy and colon adenoma, accompanied by an increase in histamine levels [
103,
104,
105,
106]. Although this test has interesting diagnostic potential, more studies are needed to validate its clinical significance and its relationship with the symptoms of histamine intolerance [
97]. If proven, this diagnostic test would be very adequate since this disorder originates from a reduced ability of the intestinal DAO enzyme to cope with dietary histamine.
Histamine challenge/provocation test has also been proposed by some authors as a diagnostic tool for intolerance, which would, at the same time, establish the individual tolerance threshold. This double-blind, placebo-controlled test involves oral administration of histamine and requires patient medical supervision and hospitalization. In the study by Wöhrl et al., half of the healthy volunteers developed symptoms after the administration of a solution containing 75 mg of histamine [
107]. In contrast, the results of a multicenter study by Komericki et al. using the same oral dose of histamine indicated the challenge test was unreliable for diagnosing histamine intolerance due to a lack of intraindividual reproducibility of symptoms after two different provocation tests [
108]. The application of this procedure is still limited because of the risk of serious adverse side effects and the absence of a standardized dose of histamine and properly established protocol [
97].
Finally, in recent years, efforts have been made to identify a noninvasive marker to establish a solid and clinically irrefutable diagnostic criterion for histamine intolerance due to DAO deficiency. Currently, the application of metabolomics as a tool for the identification of biomarkers of histamine metabolism in urine is also being challenged as a possible new diagnostic strategy [
11]. The hypothesis is that individuals with histamine intolerance could have a different excretion profile of histamine and its metabolites in urine than normal individuals. For this purpose, Comas-Basté et al. have recently proposed a chromatographic approach that allows for determining in a fast and unequivocal manner the urinary levels of histamine and its methylated metabolite, methylhistamine [
11]. It is still necessary to validate the potential diagnostic utility of this approach in patients with histamine intolerance, as well as complementing the excretion profile with other histamine metabolites to obtain a more accurate image of the possible alterations produced in this intolerance.